Merge "resourceloader: Use null-objects for 'sources' and 'registry'"

[lhc/web/wiklou.git] / resources / src / startup / mediawiki.js

/**
 * Base library for MediaWiki.
 *
 * Exposed globally as `mw`, with `mediaWiki` as alias.
 *
 * @class mw
 * @alternateClassName mediaWiki
 * @singleton
 */
/* global $VARS, $CODE */

( function () {
        'use strict';

        var mw, StringSet, log,
                trackQueue = [];

        /**
         * FNV132 hash function
         *
         * This function implements the 32-bit version of FNV-1.
         * It is equivalent to hash( 'fnv132', ... ) in PHP, except
         * its output is base 36 rather than hex.
         * See <https://en.wikipedia.org/wiki/FNV_hash_function>
         *
         * @private
         * @param {string} str String to hash
         * @return {string} hash as an seven-character base 36 string
         */
        function fnv132( str ) {
                /* eslint-disable no-bitwise */
                var hash = 0x811C9DC5,
                        i;

                for ( i = 0; i < str.length; i++ ) {
                        hash += ( hash << 1 ) + ( hash << 4 ) + ( hash << 7 ) + ( hash << 8 ) + ( hash << 24 );
                        hash ^= str.charCodeAt( i );
                }

                hash = ( hash >>> 0 ).toString( 36 );
                while ( hash.length < 7 ) {
                        hash = '0' + hash;
                }

                return hash;
                /* eslint-enable no-bitwise */
        }

        function defineFallbacks() {
                // <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set>
                StringSet = window.Set || ( function () {
                        /**
                         * @private
                         * @class
                         */
                        function StringSet() {
                                this.set = Object.create( null );
                        }
                        StringSet.prototype.add = function ( value ) {
                                this.set[ value ] = true;
                        };
                        StringSet.prototype.has = function ( value ) {
                                return value in this.set;
                        };
                        return StringSet;
                }() );
        }

        /**
         * Alias property to the global object.
         *
         * @private
         * @static
         * @member mw.Map
         * @param {mw.Map} map
         * @param {string} key
         * @param {Mixed} value
         */
        function setGlobalMapValue( map, key, value ) {
                map.values[ key ] = value;
                log.deprecate(
                        window,
                        key,
                        value,
                        // Deprecation notice for mw.config globals (T58550, T72470)
                        map === mw.config && 'Use mw.config instead.'
                );
        }

        /**
         * Log a message to window.console, if possible.
         *
         * Useful to force logging of some errors that are otherwise hard to detect (i.e., this logs
         * also in production mode). Gets console references in each invocation instead of caching the
         * reference, so that debugging tools loaded later are supported (e.g. Firebug Lite in IE).
         *
         * @private
         * @param {string} topic Stream name passed by mw.track
         * @param {Object} data Data passed by mw.track
         * @param {Error} [data.exception]
         * @param {string} data.source Error source
         * @param {string} [data.module] Name of module which caused the error
         */
        function logError( topic, data ) {
                /* eslint-disable no-console */
                var msg,
                        e = data.exception,
                        source = data.source,
                        module = data.module,
                        console = window.console;

                if ( console && console.log ) {
                        msg = ( e ? 'Exception' : 'Error' ) + ' in ' + source;
                        if ( module ) {
                                msg += ' in module ' + module;
                        }
                        msg += ( e ? ':' : '.' );
                        console.log( msg );

                        // If we have an exception object, log it to the warning channel to trigger
                        // proper stacktraces in browsers that support it.
                        if ( e && console.warn ) {
                                console.warn( e );
                        }
                }
                /* eslint-enable no-console */
        }

        /**
         * Create an object that can be read from or written to via methods that allow
         * interaction both with single and multiple properties at once.
         *
         * @private
         * @class mw.Map
         *
         * @constructor
         * @param {boolean} [global=false] Whether to synchronise =values to the global
         *  window object (for backwards-compatibility with mw.config; T72470). Values are
         *  copied in one direction only. Changes to globals do not reflect in the map.
         */
        function Map( global ) {
                this.values = Object.create( null );
                if ( global === true ) {
                        // Override #set to also set the global variable
                        this.set = function ( selection, value ) {
                                var s;
                                if ( arguments.length > 1 ) {
                                        if ( typeof selection !== 'string' ) {
                                                return false;
                                        }
                                        setGlobalMapValue( this, selection, value );
                                        return true;
                                }
                                if ( typeof selection === 'object' ) {
                                        for ( s in selection ) {
                                                setGlobalMapValue( this, s, selection[ s ] );
                                        }
                                        return true;
                                }
                                return false;
                        };
                }
        }

        Map.prototype = {
                constructor: Map,

                /**
                 * Get the value of one or more keys.
                 *
                 * If called with no arguments, all values are returned.
                 *
                 * @param {string|Array} [selection] Key or array of keys to retrieve values for.
                 * @param {Mixed} [fallback=null] Value for keys that don't exist.
                 * @return {Mixed|Object|null} If selection was a string, returns the value,
                 *  If selection was an array, returns an object of key/values.
                 *  If no selection is passed, a new object with all key/values is returned.
                 */
                get: function ( selection, fallback ) {
                        var results, i;
                        fallback = arguments.length > 1 ? fallback : null;

                        if ( Array.isArray( selection ) ) {
                                results = {};
                                for ( i = 0; i < selection.length; i++ ) {
                                        if ( typeof selection[ i ] === 'string' ) {
                                                results[ selection[ i ] ] = selection[ i ] in this.values ?
                                                        this.values[ selection[ i ] ] :
                                                        fallback;
                                        }
                                }
                                return results;
                        }

                        if ( typeof selection === 'string' ) {
                                return selection in this.values ?
                                        this.values[ selection ] :
                                        fallback;
                        }

                        if ( selection === undefined ) {
                                results = {};
                                for ( i in this.values ) {
                                        results[ i ] = this.values[ i ];
                                }
                                return results;
                        }

                        // Invalid selection key
                        return fallback;
                },

                /**
                 * Set one or more key/value pairs.
                 *
                 * @param {string|Object} selection Key to set value for, or object mapping keys to values
                 * @param {Mixed} [value] Value to set (optional, only in use when key is a string)
                 * @return {boolean} True on success, false on failure
                 */
                set: function ( selection, value ) {
                        var s;
                        // Use `arguments.length` because `undefined` is also a valid value.
                        if ( arguments.length > 1 ) {
                                if ( typeof selection !== 'string' ) {
                                        return false;
                                }
                                this.values[ selection ] = value;
                                return true;
                        }
                        if ( typeof selection === 'object' ) {
                                for ( s in selection ) {
                                        this.values[ s ] = selection[ s ];
                                }
                                return true;
                        }
                        return false;
                },

                /**
                 * Check if one or more keys exist.
                 *
                 * @param {Mixed} selection Key or array of keys to check
                 * @return {boolean} True if the key(s) exist
                 */
                exists: function ( selection ) {
                        var i;
                        if ( Array.isArray( selection ) ) {
                                for ( i = 0; i < selection.length; i++ ) {
                                        if ( typeof selection[ i ] !== 'string' || !( selection[ i ] in this.values ) ) {
                                                return false;
                                        }
                                }
                                return true;
                        }
                        return typeof selection === 'string' && selection in this.values;
                }
        };

        defineFallbacks();

        log = ( function () {
                /**
                 * Write a verbose message to the browser's console in debug mode.
                 *
                 * This method is mainly intended for verbose logging. It is a no-op in production mode.
                 * In ResourceLoader debug mode, it will use the browser's console if available, with
                 * fallback to creating a console interface in the DOM and logging messages there.
                 *
                 * See {@link mw.log} for other logging methods.
                 *
                 * @member mw
                 * @param {...string} msg Messages to output to console.
                 */
                var log = function () {},
                        console = window.console;

                // Note: Keep list of methods in sync with restoration in mediawiki.log.js
                // when adding or removing mw.log methods below!

                /**
                 * Collection of methods to help log messages to the console.
                 *
                 * @class mw.log
                 * @singleton
                 */

                /**
                 * Write a message to the browser console's warning channel.
                 *
                 * This method is a no-op in browsers that don't implement the Console API.
                 *
                 * @param {...string} msg Messages to output to console
                 */
                log.warn = console && console.warn ?
                        Function.prototype.bind.call( console.warn, console ) :
                        function () {};

                /**
                 * Write a message to the browser console's error channel.
                 *
                 * Most browsers also print a stacktrace when calling this method if the
                 * argument is an Error object.
                 *
                 * This method is a no-op in browsers that don't implement the Console API.
                 *
                 * @since 1.26
                 * @param {...Mixed} msg Messages to output to console
                 */
                log.error = console && console.error ?
                        Function.prototype.bind.call( console.error, console ) :
                        function () {};

                /**
                 * Create a property on a host object that, when accessed, will produce
                 * a deprecation warning in the console.
                 *
                 * @param {Object} obj Host object of deprecated property
                 * @param {string} key Name of property to create in `obj`
                 * @param {Mixed} val The value this property should return when accessed
                 * @param {string} [msg] Optional text to include in the deprecation message
                 * @param {string} [logName=key] Optional custom name for the feature.
                 *  This is used instead of `key` in the message and `mw.deprecate` tracking.
                 */
                log.deprecate = function ( obj, key, val, msg, logName ) {
                        var stacks;
                        function maybeLog() {
                                var name,
                                        trace = new Error().stack;
                                if ( !stacks ) {
                                        stacks = new StringSet();
                                }
                                if ( !stacks.has( trace ) ) {
                                        stacks.add( trace );
                                        name = logName || key;
                                        mw.track( 'mw.deprecate', name );
                                        mw.log.warn(
                                                'Use of "' + name + '" is deprecated.' + ( msg ? ( ' ' + msg ) : '' )
                                        );
                                }
                        }
                        // Support: Safari 5.0
                        // Throws "not supported on DOM Objects" for Node or Element objects (incl. document)
                        // Safari 4.0 doesn't have this method, and it was fixed in Safari 5.1.
                        try {
                                Object.defineProperty( obj, key, {
                                        configurable: true,
                                        enumerable: true,
                                        get: function () {
                                                maybeLog();
                                                return val;
                                        },
                                        set: function ( newVal ) {
                                                maybeLog();
                                                val = newVal;
                                        }
                                } );
                        } catch ( err ) {
                                obj[ key ] = val;
                        }
                };

                return log;
        }() );

        /**
         * @class mw
         */
        mw = {
                redefineFallbacksForTest: function () {
                        if ( !window.QUnit ) {
                                throw new Error( 'Reset not allowed outside unit tests' );
                        }
                        defineFallbacks();
                },

                /**
                 * Get the current time, measured in milliseconds since January 1, 1970 (UTC).
                 *
                 * On browsers that implement the Navigation Timing API, this function will produce
                 * floating-point values with microsecond precision that are guaranteed to be monotonic.
                 * On all other browsers, it will fall back to using `Date`.
                 *
                 * @return {number} Current time
                 */
                now: function () {
                        // Optimisation: Define the shortcut on first call, not at module definition.
                        var perf = window.performance,
                                navStart = perf && perf.timing && perf.timing.navigationStart;

                        // Define the relevant shortcut
                        mw.now = navStart && typeof perf.now === 'function' ?
                                function () { return navStart + perf.now(); } :
                                Date.now;

                        return mw.now();
                },

                /**
                 * List of all analytic events emitted so far.
                 *
                 * @private
                 * @property {Array}
                 */
                trackQueue: trackQueue,

                track: function ( topic, data ) {
                        trackQueue.push( { topic: topic, timeStamp: mw.now(), data: data } );
                        // The base module extends this method to fire events here
                },

                /**
                 * Track an early error event via mw.track and send it to the window console.
                 *
                 * @private
                 * @param {string} topic Topic name
                 * @param {Object} data Data describing the event, encoded as an object; see mw#logError
                 */
                trackError: function ( topic, data ) {
                        mw.track( topic, data );
                        logError( topic, data );
                },

                // Expose Map constructor
                Map: Map,

                /**
                 * Map of configuration values.
                 *
                 * Check out [the complete list of configuration values](https://www.mediawiki.org/wiki/Manual:Interface/JavaScript#mw.config)
                 * on mediawiki.org.
                 *
                 * If `$wgLegacyJavaScriptGlobals` is true, this Map will add its values to the
                 * global `window` object.
                 *
                 * @property {mw.Map} config
                 */
                // Dummy placeholder later assigned in ResourceLoaderStartUpModule
                config: null,

                /**
                 * Empty object for third-party libraries, for cases where you don't
                 * want to add a new global, or the global is bad and needs containment
                 * or wrapping.
                 *
                 * @property
                 */
                libs: {},

                /**
                 * Access container for deprecated functionality that can be moved from
                 * from their legacy location and attached to this object (e.g. a global
                 * function that is deprecated and as stop-gap can be exposed through here).
                 *
                 * This was reserved for future use but never ended up being used.
                 *
                 * @deprecated since 1.22 Let deprecated identifiers keep their original name
                 *  and use mw.log#deprecate to create an access container for tracking.
                 * @property
                 */
                legacy: {},

                /**
                 * Store for messages.
                 *
                 * @property {mw.Map}
                 */
                messages: new Map(),

                /**
                 * Store for templates associated with a module.
                 *
                 * @property {mw.Map}
                 */
                templates: new Map(),

                // Expose mw.log
                log: log,

                /**
                 * Client for ResourceLoader server end point.
                 *
                 * This client is in charge of maintaining the module registry and state
                 * machine, initiating network (batch) requests for loading modules, as
                 * well as dependency resolution and execution of source code.
                 *
                 * For more information, refer to
                 * <https://www.mediawiki.org/wiki/ResourceLoader/Features>
                 *
                 * @class mw.loader
                 * @singleton
                 */
                loader: ( function () {

                        /**
                         * Fired via mw.track on various resource loading errors.
                         *
                         * @event resourceloader_exception
                         * @param {Error|Mixed} e The error that was thrown. Almost always an Error
                         *   object, but in theory module code could manually throw something else, and that
                         *   might also end up here.
                         * @param {string} [module] Name of the module which caused the error. Omitted if the
                         *   error is not module-related or the module cannot be easily identified due to
                         *   batched handling.
                         * @param {string} source Source of the error. Possible values:
                         *
                         *   - style: stylesheet error (only affects old IE where a special style loading method
                         *     is used)
                         *   - load-callback: exception thrown by user callback
                         *   - module-execute: exception thrown by module code
                         *   - resolve: failed to sort dependencies for a module in mw.loader.load
                         *   - store-eval: could not evaluate module code cached in localStorage
                         *   - store-localstorage-init: localStorage or JSON parse error in mw.loader.store.init
                         *   - store-localstorage-json: JSON conversion error in mw.loader.store
                         *   - store-localstorage-update: localStorage conversion error in mw.loader.store.
                         */

                        /**
                         * Fired via mw.track on resource loading error conditions.
                         *
                         * @event resourceloader_assert
                         * @param {string} source Source of the error. Possible values:
                         *
                         *   - bug-T59567: failed to cache script due to an Opera function -> string conversion
                         *     bug; see <https://phabricator.wikimedia.org/T59567> for details
                         */

                        /**
                         * Mapping of registered modules.
                         *
                         * See #implement and #execute for exact details on support for script, style and messages.
                         *
                         * Format:
                         *
                         *     {
                         *         'moduleName': {
                         *             // From mw.loader.register()
                         *             'version': '########' (hash)
                         *             'dependencies': ['required.foo', 'bar.also', ...]
                         *             'group': 'somegroup', (or) null
                         *             'source': 'local', (or) 'anotherwiki'
                         *             'skip': 'return !!window.Example', (or) null
                         *             'module': export Object
                         *
                         *             // Set from execute() or mw.loader.state()
                         *             'state': 'registered', 'loaded', 'loading', 'ready', 'error', or 'missing'
                         *
                         *             // Optionally added at run-time by mw.loader.implement()
                         *             'skipped': true
                         *             'script': closure, array of urls, or string
                         *             'style': { ... } (see #execute)
                         *             'messages': { 'key': 'value', ... }
                         *         }
                         *     }
                         *
                         * State machine:
                         *
                         * - `registered`:
                         *    The module is known to the system but not yet required.
                         *    Meta data is registered via mw.loader#register. Calls to that method are
                         *    generated server-side by the startup module.
                         * - `loading`:
                         *    The module was required through mw.loader (either directly or as dependency of
                         *    another module). The client will fetch module contents from the server.
                         *    The contents are then stashed in the registry via mw.loader#implement.
                         * - `loaded`:
                         *    The module has been loaded from the server and stashed via mw.loader#implement.
                         *    Once the module has no more dependencies in-flight, the module will be executed,
                         *    controlled via #requestPropagation and #doPropagation.
                         * - `executing`:
                         *    The module is being executed.
                         * - `ready`:
                         *    The module has been successfully executed.
                         * - `error`:
                         *    The module (or one of its dependencies) produced an error during execution.
                         * - `missing`:
                         *    The module was registered client-side and requested, but the server denied knowledge
                         *    of the module's existence.
                         *
                         * @property
                         * @private
                         */
                        var registry = Object.create( null ),
                                // Mapping of sources, keyed by source-id, values are strings.
                                //
                                // Format:
                                //
                                //     {
                                //         'sourceId': 'http://example.org/w/load.php'
                                //     }
                                //
                                sources = Object.create( null ),

                                // For queueModuleScript()
                                handlingPendingRequests = false,
                                pendingRequests = [],

                                // List of modules to be loaded
                                queue = [],

                                /**
                                 * List of callback jobs waiting for modules to be ready.
                                 *
                                 * Jobs are created by #enqueue() and run by #doPropagation().
                                 * Typically when a job is created for a module, the job's dependencies contain
                                 * both the required module and all its recursive dependencies.
                                 *
                                 * Format:
                                 *
                                 *     {
                                 *         'dependencies': [ module names ],
                                 *         'ready': Function callback
                                 *         'error': Function callback
                                 *     }
                                 *
                                 * @property {Object[]} jobs
                                 * @private
                                 */
                                jobs = [],

                                // For #requestPropagation() and #doPropagation()
                                willPropagate = false,
                                errorModules = [],

                                /**
                                 * @private
                                 * @property {Array} baseModules
                                 */
                                baseModules = $VARS.baseModules,

                                /**
                                 * For #addEmbeddedCSS() and #addLink()
                                 *
                                 * @private
                                 * @property {HTMLElement|null} marker
                                 */
                                marker = document.querySelector( 'meta[name="ResourceLoaderDynamicStyles"]' ),

                                // For #addEmbeddedCSS()
                                nextCssBuffer,
                                rAF = window.requestAnimationFrame || setTimeout;

                        /**
                         * Create a new style element and add it to the DOM.
                         *
                         * @private
                         * @param {string} text CSS text
                         * @param {Node|null} [nextNode] The element where the style tag
                         *  should be inserted before
                         * @return {HTMLElement} Reference to the created style element
                         */
                        function newStyleTag( text, nextNode ) {
                                var el = document.createElement( 'style' );
                                el.appendChild( document.createTextNode( text ) );
                                if ( nextNode && nextNode.parentNode ) {
                                        nextNode.parentNode.insertBefore( el, nextNode );
                                } else {
                                        document.head.appendChild( el );
                                }
                                return el;
                        }

                        /**
                         * @private
                         * @param {Object} cssBuffer
                         */
                        function flushCssBuffer( cssBuffer ) {
                                var i;
                                // Mark this object as inactive now so that further calls to addEmbeddedCSS() from
                                // the callbacks go to a new buffer instead of this one (T105973)
                                cssBuffer.active = false;
                                newStyleTag( cssBuffer.cssText, marker );
                                for ( i = 0; i < cssBuffer.callbacks.length; i++ ) {
                                        cssBuffer.callbacks[ i ]();
                                }
                        }

                        /**
                         * Add a bit of CSS text to the current browser page.
                         *
                         * The creation and insertion of the `<style>` element is debounced for two reasons:
                         *
                         * - Performing the insertion before the next paint round via requestAnimationFrame
                         *   avoids forced or wasted style recomputations, which are expensive in browsers.
                         * - Reduce how often new stylesheets are inserted by letting additional calls to this
                         *   function accumulate into a buffer for at least one JavaScript tick. Modules are
                         *   received from the server in batches, which means there is likely going to be many
                         *   calls to this function in a row within the same tick / the same call stack.
                         *   See also T47810.
                         *
                         * @private
                         * @param {string} cssText CSS text to be added in a `<style>` tag.
                         * @param {Function} callback Called after the insertion has occurred
                         */
                        function addEmbeddedCSS( cssText, callback ) {
                                // Create a buffer if:
                                // - We don't have one yet.
                                // - The previous one is closed.
                                // - The next CSS chunk syntactically needs to be at the start of a stylesheet (T37562).
                                if ( !nextCssBuffer || nextCssBuffer.active === false || cssText.slice( 0, '@import'.length ) === '@import' ) {
                                        nextCssBuffer = {
                                                cssText: '',
                                                callbacks: [],
                                                active: null
                                        };
                                }

                                // Linebreak for somewhat distinguishable sections
                                nextCssBuffer.cssText += '\n' + cssText;
                                nextCssBuffer.callbacks.push( callback );

                                if ( nextCssBuffer.active === null ) {
                                        nextCssBuffer.active = true;
                                        // The flushCssBuffer callback has its parameter bound by reference, which means
                                        // 1) We can still extend the buffer from our object reference after this point.
                                        // 2) We can safely re-assign the variable (not the object) to start a new buffer.
                                        rAF( flushCssBuffer.bind( null, nextCssBuffer ) );
                                }
                        }

                        /**
                         * @private
                         * @param {Array} modules List of module names
                         * @return {string} Hash of concatenated version hashes.
                         */
                        function getCombinedVersion( modules ) {
                                var hashes = modules.reduce( function ( result, module ) {
                                        return result + registry[ module ].version;
                                }, '' );
                                return fnv132( hashes );
                        }

                        /**
                         * Determine whether all dependencies are in state 'ready', which means we may
                         * execute the module or job now.
                         *
                         * @private
                         * @param {Array} modules Names of modules to be checked
                         * @return {boolean} True if all modules are in state 'ready', false otherwise
                         */
                        function allReady( modules ) {
                                var i;
                                for ( i = 0; i < modules.length; i++ ) {
                                        if ( mw.loader.getState( modules[ i ] ) !== 'ready' ) {
                                                return false;
                                        }
                                }
                                return true;
                        }

                        /**
                         * Determine whether all direct and base dependencies are in state 'ready'
                         *
                         * @private
                         * @param {string} module Name of the module to be checked
                         * @return {boolean} True if all direct/base dependencies are in state 'ready'; false otherwise
                         */
                        function allWithImplicitReady( module ) {
                                return allReady( registry[ module ].dependencies ) &&
                                        ( baseModules.indexOf( module ) !== -1 || allReady( baseModules ) );
                        }

                        /**
                         * Determine whether all dependencies are in state 'ready', which means we may
                         * execute the module or job now.
                         *
                         * @private
                         * @param {Array} modules Names of modules to be checked
                         * @return {boolean} True if no modules are in state 'error' or 'missing', false otherwise
                         */
                        function anyFailed( modules ) {
                                var i, state;
                                for ( i = 0; i < modules.length; i++ ) {
                                        state = mw.loader.getState( modules[ i ] );
                                        if ( state === 'error' || state === 'missing' ) {
                                                return true;
                                        }
                                }
                                return false;
                        }

                        /**
                         * Handle propagation of module state changes and reactions to them.
                         *
                         * - When a module reaches a failure state, this should be propagated to
                         *   modules that depend on the failed module.
                         * - When a module reaches a final state, pending job callbacks for the
                         *   module from mw.loader.using() should be called.
                         * - When a module reaches the 'ready' state from #execute(), consider
                         *   executing dependant modules now having their dependencies satisfied.
                         * - When a module reaches the 'loaded' state from mw.loader.implement,
                         *   consider executing it, if it has no unsatisfied dependencies.
                         *
                         * @private
                         */
                        function doPropagation() {
                                var errorModule, baseModuleError, module, i, failed, job,
                                        didPropagate = true;

                                // Keep going until the last iteration performed no actions.
                                do {
                                        didPropagate = false;

                                        // Stage 1: Propagate failures
                                        while ( errorModules.length ) {
                                                errorModule = errorModules.shift();
                                                baseModuleError = baseModules.indexOf( errorModule ) !== -1;
                                                for ( module in registry ) {
                                                        if ( registry[ module ].state !== 'error' && registry[ module ].state !== 'missing' ) {
                                                                if ( baseModuleError && baseModules.indexOf( module ) === -1 ) {
                                                                        // Propate error from base module to all regular (non-base) modules
                                                                        registry[ module ].state = 'error';
                                                                        didPropagate = true;
                                                                } else if ( registry[ module ].dependencies.indexOf( errorModule ) !== -1 ) {
                                                                        // Propagate error from dependency to depending module
                                                                        registry[ module ].state = 'error';
                                                                        // .. and propagate it further
                                                                        errorModules.push( module );
                                                                        didPropagate = true;
                                                                }
                                                        }
                                                }
                                        }

                                        // Stage 2: Execute 'loaded' modules with no unsatisfied dependencies
                                        for ( module in registry ) {
                                                if ( registry[ module ].state === 'loaded' && allWithImplicitReady( module ) ) {
                                                        // Recursively execute all dependent modules that were already loaded
                                                        // (waiting for execution) and no longer have unsatisfied dependencies.
                                                        // Base modules may have dependencies amongst eachother to ensure correct
                                                        // execution order. Regular modules wait for all base modules.
                                                        // eslint-disable-next-line no-use-before-define
                                                        execute( module );
                                                        didPropagate = true;
                                                }
                                        }

                                        // Stage 3: Invoke job callbacks that are no longer blocked
                                        for ( i = 0; i < jobs.length; i++ ) {
                                                job = jobs[ i ];
                                                failed = anyFailed( job.dependencies );
                                                if ( failed || allReady( job.dependencies ) ) {
                                                        jobs.splice( i, 1 );
                                                        i -= 1;
                                                        try {
                                                                if ( failed && job.error ) {
                                                                        job.error( new Error( 'Module has failed dependencies' ), job.dependencies );
                                                                } else if ( !failed && job.ready ) {
                                                                        job.ready();
                                                                }
                                                        } catch ( e ) {
                                                                // A user-defined callback raised an exception.
                                                                // Swallow it to protect our state machine!
                                                                mw.trackError( 'resourceloader.exception', {
                                                                        exception: e,
                                                                        source: 'load-callback'
                                                                } );
                                                        }
                                                        didPropagate = true;
                                                }
                                        }
                                } while ( didPropagate );

                                willPropagate = false;
                        }

                        /**
                         * Request a (debounced) call to doPropagation().
                         *
                         * @private
                         */
                        function requestPropagation() {
                                if ( willPropagate ) {
                                        // Already scheduled, or, we're already in a doPropagation stack.
                                        return;
                                }
                                willPropagate = true;
                                // Yield for two reasons:
                                // * Allow successive calls to mw.loader.implement() from the same
                                //   load.php response, or from the same asyncEval() to be in the
                                //   propagation batch.
                                // * Allow the browser to breathe between the reception of
                                //   module source code and the execution of it.
                                //
                                // Use a high priority because the user may be waiting for interactions
                                // to start being possible. But, first provide a moment (up to 'timeout')
                                // for native input event handling (e.g. scrolling/typing/clicking).
                                mw.requestIdleCallback( doPropagation, { timeout: 1 } );
                        }

                        /**
                         * Update a module's state in the registry and make sure any neccesary
                         * propagation will occur. See #doPropagation for more about propagation.
                         * See #registry for more about how states are used.
                         *
                         * @private
                         * @param {string} module
                         * @param {string} state
                         */
                        function setAndPropagate( module, state ) {
                                registry[ module ].state = state;
                                if ( state === 'loaded' || state === 'ready' || state === 'error' || state === 'missing' ) {
                                        if ( state === 'ready' ) {
                                                // Queue to later be synced to the local module store.
                                                mw.loader.store.add( module );
                                        } else if ( state === 'error' || state === 'missing' ) {
                                                errorModules.push( module );
                                        }
                                        requestPropagation();
                                }
                        }

                        /**
                         * Resolve dependencies and detect circular references.
                         *
                         * @private
                         * @param {string} module Name of the top-level module whose dependencies shall be
                         *  resolved and sorted.
                         * @param {Array} resolved Returns a topological sort of the given module and its
                         *  dependencies, such that later modules depend on earlier modules. The array
                         *  contains the module names. If the array contains already some module names,
                         *  this function appends its result to the pre-existing array.
                         * @param {StringSet} [unresolved] Used to track the current dependency
                         *  chain, and to report loops in the dependency graph.
                         * @throws {Error} If any unregistered module or a dependency loop is encountered
                         */
                        function sortDependencies( module, resolved, unresolved ) {
                                var i, deps, skip;

                                if ( !( module in registry ) ) {
                                        throw new Error( 'Unknown dependency: ' + module );
                                }

                                if ( registry[ module ].skip !== null ) {
                                        // eslint-disable-next-line no-new-func
                                        skip = new Function( registry[ module ].skip );
                                        registry[ module ].skip = null;
                                        if ( skip() ) {
                                                registry[ module ].skipped = true;
                                                registry[ module ].dependencies = [];
                                                setAndPropagate( module, 'ready' );
                                                return;
                                        }
                                }

                                if ( resolved.indexOf( module ) !== -1 ) {
                                        // Module already resolved; nothing to do
                                        return;
                                }
                                // Create unresolved if not passed in
                                if ( !unresolved ) {
                                        unresolved = new StringSet();
                                }

                                // Add base modules
                                if ( baseModules.indexOf( module ) === -1 ) {
                                        baseModules.forEach( function ( baseModule ) {
                                                if ( resolved.indexOf( baseModule ) === -1 ) {
                                                        resolved.push( baseModule );
                                                }
                                        } );
                                }

                                // Tracks down dependencies
                                deps = registry[ module ].dependencies;
                                unresolved.add( module );
                                for ( i = 0; i < deps.length; i++ ) {
                                        if ( resolved.indexOf( deps[ i ] ) === -1 ) {
                                                if ( unresolved.has( deps[ i ] ) ) {
                                                        throw new Error(
                                                                'Circular reference detected: ' + module + ' -> ' + deps[ i ]
                                                        );
                                                }

                                                sortDependencies( deps[ i ], resolved, unresolved );
                                        }
                                }
                                resolved.push( module );
                        }

                        /**
                         * Get names of module that a module depends on, in their proper dependency order.
                         *
                         * @private
                         * @param {string[]} modules Array of string module names
                         * @return {Array} List of dependencies, including 'module'.
                         * @throws {Error} If an unregistered module or a dependency loop is encountered
                         */
                        function resolve( modules ) {
                                var i, resolved = [];
                                for ( i = 0; i < modules.length; i++ ) {
                                        sortDependencies( modules[ i ], resolved );
                                }
                                return resolved;
                        }

                        /**
                         * Like #resolve(), except it will silently ignore modules that
                         * are missing or have missing dependencies.
                         *
                         * @private
                         * @param {string[]} modules Array of string module names
                         * @return {Array} List of dependencies.
                         */
                        function resolveStubbornly( modules ) {
                                var i, saved, resolved = [];
                                for ( i = 0; i < modules.length; i++ ) {
                                        saved = resolved.slice();
                                        try {
                                                sortDependencies( modules[ i ], resolved );
                                        } catch ( err ) {
                                                // This module is unknown or has unknown dependencies.
                                                // Undo any incomplete resolutions made and keep going.
                                                resolved = saved;
                                                mw.trackError( 'resourceloader.exception', {
                                                        exception: err,
                                                        source: 'resolve'
                                                } );
                                        }
                                }
                                return resolved;
                        }

                        /**
                         * Load and execute a script.
                         *
                         * @private
                         * @param {string} src URL to script, will be used as the src attribute in the script tag
                         * @param {Function} [callback] Callback to run after request resolution
                         */
                        function addScript( src, callback ) {
                                var script = document.createElement( 'script' );
                                script.src = src;
                                script.onload = script.onerror = function () {
                                        if ( script.parentNode ) {
                                                script.parentNode.removeChild( script );
                                        }
                                        script = null;
                                        if ( callback ) {
                                                callback();
                                                callback = null;
                                        }
                                };
                                document.head.appendChild( script );
                        }

                        /**
                         * Queue the loading and execution of a script for a particular module.
                         *
                         * This does for debug mode what runScript() does for production.
                         *
                         * @private
                         * @param {string} src URL of the script
                         * @param {string} moduleName Name of currently executing module
                         * @param {Function} callback Callback to run after addScript() resolution
                         */
                        function queueModuleScript( src, moduleName, callback ) {
                                pendingRequests.push( function () {
                                        // Keep in sync with execute()/runScript().
                                        if ( moduleName !== 'jquery' ) {
                                                window.require = mw.loader.require;
                                                window.module = registry[ moduleName ].module;
                                        }
                                        addScript( src, function () {
                                                // 'module.exports' should not persist after the file is executed to
                                                // avoid leakage to unrelated code. 'require' should be kept, however,
                                                // as asynchronous access to 'require' is allowed and expected. (T144879)
                                                delete window.module;
                                                callback();
                                                // Start the next one (if any)
                                                if ( pendingRequests[ 0 ] ) {
                                                        pendingRequests.shift()();
                                                } else {
                                                        handlingPendingRequests = false;
                                                }
                                        } );
                                } );
                                if ( !handlingPendingRequests && pendingRequests[ 0 ] ) {
                                        handlingPendingRequests = true;
                                        pendingRequests.shift()();
                                }
                        }

                        /**
                         * Utility function for execute()
                         *
                         * @ignore
                         * @param {string} [media] Media attribute
                         * @param {string} url URL
                         */
                        function addLink( media, url ) {
                                var el = document.createElement( 'link' );

                                el.rel = 'stylesheet';
                                if ( media && media !== 'all' ) {
                                        el.media = media;
                                }
                                // If you end up here from an IE exception "SCRIPT: Invalid property value.",
                                // see #addEmbeddedCSS, T33676, T43331, and T49277 for details.
                                el.href = url;

                                if ( marker && marker.parentNode ) {
                                        marker.parentNode.insertBefore( el, marker );
                                } else {
                                        document.head.appendChild( el );
                                }
                        }

                        /**
                         * @private
                         * @param {string} code JavaScript code
                         */
                        function domEval( code ) {
                                var script = document.createElement( 'script' );
                                if ( mw.config.get( 'wgCSPNonce' ) !== false ) {
                                        script.nonce = mw.config.get( 'wgCSPNonce' );
                                }
                                script.text = code;
                                document.head.appendChild( script );
                                script.parentNode.removeChild( script );
                        }

                        /**
                         * Add one or more modules to the module load queue.
                         *
                         * See also #work().
                         *
                         * @private
                         * @param {string[]} dependencies Array of module names in the registry
                         * @param {Function} [ready] Callback to execute when all dependencies are ready
                         * @param {Function} [error] Callback to execute when any dependency fails
                         */
                        function enqueue( dependencies, ready, error ) {
                                if ( allReady( dependencies ) ) {
                                        // Run ready immediately
                                        if ( ready !== undefined ) {
                                                ready();
                                        }
                                        return;
                                }

                                if ( anyFailed( dependencies ) ) {
                                        if ( error !== undefined ) {
                                                // Execute error immediately if any dependencies have errors
                                                error(
                                                        new Error( 'One or more dependencies failed to load' ),
                                                        dependencies
                                                );
                                        }
                                        return;
                                }

                                // Not all dependencies are ready, add to the load queue...

                                // Add ready and error callbacks if they were given
                                if ( ready !== undefined || error !== undefined ) {
                                        jobs.push( {
                                                // Narrow down the list to modules that are worth waiting for
                                                dependencies: dependencies.filter( function ( module ) {
                                                        var state = registry[ module ].state;
                                                        return state === 'registered' || state === 'loaded' || state === 'loading' || state === 'executing';
                                                } ),
                                                ready: ready,
                                                error: error
                                        } );
                                }

                                dependencies.forEach( function ( module ) {
                                        // Only queue modules that are still in the initial 'registered' state
                                        // (not ones already loading, ready or error).
                                        if ( registry[ module ].state === 'registered' && queue.indexOf( module ) === -1 ) {
                                                // Private modules must be embedded in the page. Don't bother queuing
                                                // these as the server will deny them anyway (T101806).
                                                if ( registry[ module ].group === 'private' ) {
                                                        setAndPropagate( module, 'error' );
                                                        return;
                                                }
                                                queue.push( module );
                                        }
                                } );

                                mw.loader.work();
                        }

                        /**
                         * Executes a loaded module, making it ready to use
                         *
                         * @private
                         * @param {string} module Module name to execute
                         */
                        function execute( module ) {
                                var key, value, media, i, urls, cssHandle, siteDeps, siteDepErr, runScript,
                                        cssPending = 0;

                                if ( registry[ module ].state !== 'loaded' ) {
                                        throw new Error( 'Module in state "' + registry[ module ].state + '" may not be executed: ' + module );
                                }

                                registry[ module ].state = 'executing';
                                $CODE.profileExecuteStart();

                                runScript = function () {
                                        var script, markModuleReady, nestedAddScript;

                                        $CODE.profileScriptStart();
                                        script = registry[ module ].script;
                                        markModuleReady = function () {
                                                $CODE.profileScriptEnd();
                                                setAndPropagate( module, 'ready' );
                                        };
                                        nestedAddScript = function ( arr, callback, i ) {
                                                // Recursively call queueModuleScript() in its own callback
                                                // for each element of arr.
                                                if ( i >= arr.length ) {
                                                        // We're at the end of the array
                                                        callback();
                                                        return;
                                                }

                                                queueModuleScript( arr[ i ], module, function () {
                                                        nestedAddScript( arr, callback, i + 1 );
                                                } );
                                        };

                                        try {
                                                if ( Array.isArray( script ) ) {
                                                        nestedAddScript( script, markModuleReady, 0 );
                                                } else if ( typeof script === 'function' ) {
                                                        // Keep in sync with queueModuleScript() for debug mode
                                                        if ( module === 'jquery' ) {
                                                                // This is a special case for when 'jquery' itself is being loaded.
                                                                // - The standard jquery.js distribution does not set `window.jQuery`
                                                                //   in CommonJS-compatible environments (Node.js, AMD, RequireJS, etc.).
                                                                // - MediaWiki's 'jquery' module also bundles jquery.migrate.js, which
                                                                //   in a CommonJS-compatible environment, will use require('jquery'),
                                                                //   but that can't work when we're still inside that module.
                                                                script();
                                                        } else {
                                                                // Pass jQuery twice so that the signature of the closure which wraps
                                                                // the script can bind both '$' and 'jQuery'.
                                                                script( window.$, window.$, mw.loader.require, registry[ module ].module );
                                                        }
                                                        markModuleReady();

                                                } else if ( typeof script === 'string' ) {
                                                        // Site and user modules are legacy scripts that run in the global scope.
                                                        // This is transported as a string instead of a function to avoid needing
                                                        // to use string manipulation to undo the function wrapper.
                                                        domEval( script );
                                                        markModuleReady();

                                                } else {
                                                        // Module without script
                                                        markModuleReady();
                                                }
                                        } catch ( e ) {
                                                // Use mw.track instead of mw.log because these errors are common in production mode
                                                // (e.g. undefined variable), and mw.log is only enabled in debug mode.
                                                setAndPropagate( module, 'error' );
                                                $CODE.profileScriptEnd();
                                                mw.trackError( 'resourceloader.exception', {
                                                        exception: e,
                                                        module: module,
                                                        source: 'module-execute'
                                                } );
                                        }
                                };

                                // Add localizations to message system
                                if ( registry[ module ].messages ) {
                                        mw.messages.set( registry[ module ].messages );
                                }

                                // Initialise templates
                                if ( registry[ module ].templates ) {
                                        mw.templates.set( module, registry[ module ].templates );
                                }

                                // Adding of stylesheets is asynchronous via addEmbeddedCSS().
                                // The below function uses a counting semaphore to make sure we don't call
                                // runScript() until after this module's stylesheets have been inserted
                                // into the DOM.
                                cssHandle = function () {
                                        // Increase semaphore, when creating a callback for addEmbeddedCSS.
                                        cssPending++;
                                        return function () {
                                                var runScriptCopy;
                                                // Decrease semaphore, when said callback is invoked.
                                                cssPending--;
                                                if ( cssPending === 0 ) {
                                                        // Paranoia:
                                                        // This callback is exposed to addEmbeddedCSS, which is outside the execute()
                                                        // function and is not concerned with state-machine integrity. In turn,
                                                        // addEmbeddedCSS() actually exposes stuff further into the browser (rAF).
                                                        // If increment and decrement callbacks happen in the wrong order, or start
                                                        // again afterwards, then this branch could be reached multiple times.
                                                        // To protect the integrity of the state-machine, prevent that from happening
                                                        // by making runScript() cannot be called more than once.  We store a private
                                                        // reference when we first reach this branch, then deference the original, and
                                                        // call our reference to it.
                                                        runScriptCopy = runScript;
                                                        runScript = undefined;
                                                        runScriptCopy();
                                                }
                                        };
                                };

                                // Process styles (see also mw.loader.implement)
                                // * back-compat: { <media>: css }
                                // * back-compat: { <media>: [url, ..] }
                                // * { "css": [css, ..] }
                                // * { "url": { <media>: [url, ..] } }
                                if ( registry[ module ].style ) {
                                        for ( key in registry[ module ].style ) {
                                                value = registry[ module ].style[ key ];
                                                media = undefined;

                                                if ( key !== 'url' && key !== 'css' ) {
                                                        // Backwards compatibility, key is a media-type
                                                        if ( typeof value === 'string' ) {
                                                                // back-compat: { <media>: css }
                                                                // Ignore 'media' because it isn't supported (nor was it used).
                                                                // Strings are pre-wrapped in "@media". The media-type was just ""
                                                                // (because it had to be set to something).
                                                                // This is one of the reasons why this format is no longer used.
                                                                addEmbeddedCSS( value, cssHandle() );
                                                        } else {
                                                                // back-compat: { <media>: [url, ..] }
                                                                media = key;
                                                                key = 'bc-url';
                                                        }
                                                }

                                                // Array of css strings in key 'css',
                                                // or back-compat array of urls from media-type
                                                if ( Array.isArray( value ) ) {
                                                        for ( i = 0; i < value.length; i++ ) {
                                                                if ( key === 'bc-url' ) {
                                                                        // back-compat: { <media>: [url, ..] }
                                                                        addLink( media, value[ i ] );
                                                                } else if ( key === 'css' ) {
                                                                        // { "css": [css, ..] }
                                                                        addEmbeddedCSS( value[ i ], cssHandle() );
                                                                }
                                                        }
                                                // Not an array, but a regular object
                                                // Array of urls inside media-type key
                                                } else if ( typeof value === 'object' ) {
                                                        // { "url": { <media>: [url, ..] } }
                                                        for ( media in value ) {
                                                                urls = value[ media ];
                                                                for ( i = 0; i < urls.length; i++ ) {
                                                                        addLink( media, urls[ i ] );
                                                                }
                                                        }
                                                }
                                        }
                                }

                                // End profiling of execute()-self before we call runScript(),
                                // which we want to measure separately without overlap.
                                $CODE.profileExecuteEnd();

                                if ( module === 'user' ) {
                                        // Implicit dependency on the site module. Not a real dependency because it should
                                        // run after 'site' regardless of whether it succeeds or fails.
                                        // Note: This is a simplified version of mw.loader.using(), inlined here because
                                        // mw.loader.using() is part of mediawiki.base (depends on jQuery; T192623).
                                        try {
                                                siteDeps = resolve( [ 'site' ] );
                                        } catch ( e ) {
                                                siteDepErr = e;
                                                runScript();
                                        }
                                        if ( siteDepErr === undefined ) {
                                                enqueue( siteDeps, runScript, runScript );
                                        }
                                } else if ( cssPending === 0 ) {
                                        // Regular module without styles
                                        runScript();
                                }
                                // else: runScript will get called via cssHandle()
                        }

                        function sortQuery( o ) {
                                var key,
                                        sorted = {},
                                        a = [];

                                for ( key in o ) {
                                        a.push( key );
                                }
                                a.sort();
                                for ( key = 0; key < a.length; key++ ) {
                                        sorted[ a[ key ] ] = o[ a[ key ] ];
                                }
                                return sorted;
                        }

                        /**
                         * Converts a module map of the form `{ foo: [ 'bar', 'baz' ], bar: [ 'baz, 'quux' ] }`
                         * to a query string of the form `foo.bar,baz|bar.baz,quux`.
                         *
                         * See `ResourceLoader::makePackedModulesString()` in PHP, of which this is a port.
                         * On the server, unpacking is done by `ResourceLoaderContext::expandModuleNames()`.
                         *
                         * Note: This is only half of the logic, the other half has to be in #batchRequest(),
                         * because its implementation needs to keep track of potential string size in order
                         * to decide when to split the requests due to url size.
                         *
                         * @private
                         * @param {Object} moduleMap Module map
                         * @return {Object}
                         * @return {string} return.str Module query string
                         * @return {Array} return.list List of module names in matching order
                         */
                        function buildModulesString( moduleMap ) {
                                var p, prefix,
                                        str = [],
                                        list = [];

                                function restore( suffix ) {
                                        return p + suffix;
                                }

                                for ( prefix in moduleMap ) {
                                        p = prefix === '' ? '' : prefix + '.';
                                        str.push( p + moduleMap[ prefix ].join( ',' ) );
                                        list.push.apply( list, moduleMap[ prefix ].map( restore ) );
                                }
                                return {
                                        str: str.join( '|' ),
                                        list: list
                                };
                        }

                        /**
                         * Resolve indexed dependencies.
                         *
                         * ResourceLoader uses an optimisation to save space which replaces module names in
                         * dependency lists with the index of that module within the array of module
                         * registration data if it exists. The benefit is a significant reduction in the data
                         * size of the startup module. This function changes those dependency lists back to
                         * arrays of strings.
                         *
                         * @private
                         * @param {Array} modules Modules array
                         */
                        function resolveIndexedDependencies( modules ) {
                                var i, j, deps;
                                function resolveIndex( dep ) {
                                        return typeof dep === 'number' ? modules[ dep ][ 0 ] : dep;
                                }
                                for ( i = 0; i < modules.length; i++ ) {
                                        deps = modules[ i ][ 2 ];
                                        if ( deps ) {
                                                for ( j = 0; j < deps.length; j++ ) {
                                                        deps[ j ] = resolveIndex( deps[ j ] );
                                                }
                                        }
                                }
                        }

                        /**
                         * @private
                         * @param {Object} params Map of parameter names to values
                         * @return {string}
                         */
                        function makeQueryString( params ) {
                                return Object.keys( params ).map( function ( key ) {
                                        return encodeURIComponent( key ) + '=' + encodeURIComponent( params[ key ] );
                                } ).join( '&' );
                        }

                        /**
                         * Create network requests for a batch of modules.
                         *
                         * This is an internal method for #work(). This must not be called directly
                         * unless the modules are already registered, and no request is in progress,
                         * and the module state has already been set to `loading`.
                         *
                         * @private
                         * @param {string[]} batch
                         */
                        function batchRequest( batch ) {
                                var reqBase, splits, maxQueryLength, b, bSource, bGroup,
                                        source, group, i, modules, sourceLoadScript,
                                        currReqBase, currReqBaseLength, moduleMap, currReqModules, l,
                                        lastDotIndex, prefix, suffix, bytesAdded;

                                /**
                                 * Start the currently drafted request to the server.
                                 *
                                 * @ignore
                                 */
                                function doRequest() {
                                        // Optimisation: Inherit (Object.create), not copy ($.extend)
                                        var query = Object.create( currReqBase ),
                                                packed = buildModulesString( moduleMap );
                                        query.modules = packed.str;
                                        // The packing logic can change the effective order, even if the input was
                                        // sorted. As such, the call to getCombinedVersion() must use this
                                        // effective order, instead of currReqModules, as otherwise the combined
                                        // version will not match the hash expected by the server based on
                                        // combining versions from the module query string in-order. (T188076)
                                        query.version = getCombinedVersion( packed.list );
                                        query = sortQuery( query );
                                        addScript( sourceLoadScript + '?' + makeQueryString( query ) );
                                }

                                if ( !batch.length ) {
                                        return;
                                }

                                // Always order modules alphabetically to help reduce cache
                                // misses for otherwise identical content.
                                batch.sort();

                                // Query parameters common to all requests
                                reqBase = {
                                        skin: mw.config.get( 'skin' ),
                                        lang: mw.config.get( 'wgUserLanguage' ),
                                        debug: mw.config.get( 'debug' )
                                };
                                maxQueryLength = mw.config.get( 'wgResourceLoaderMaxQueryLength', 2000 );

                                // Split module list by source and by group.
                                splits = Object.create( null );
                                for ( b = 0; b < batch.length; b++ ) {
                                        bSource = registry[ batch[ b ] ].source;
                                        bGroup = registry[ batch[ b ] ].group;
                                        if ( !splits[ bSource ] ) {
                                                splits[ bSource ] = Object.create( null );
                                        }
                                        if ( !splits[ bSource ][ bGroup ] ) {
                                                splits[ bSource ][ bGroup ] = [];
                                        }
                                        splits[ bSource ][ bGroup ].push( batch[ b ] );
                                }

                                for ( source in splits ) {
                                        sourceLoadScript = sources[ source ];

                                        for ( group in splits[ source ] ) {

                                                // Cache access to currently selected list of
                                                // modules for this group from this source.
                                                modules = splits[ source ][ group ];

                                                // Query parameters common to requests for this module group
                                                // Optimisation: Inherit (Object.create), not copy ($.extend)
                                                currReqBase = Object.create( reqBase );
                                                // User modules require a user name in the query string.
                                                if ( group === 'user' && mw.config.get( 'wgUserName' ) !== null ) {
                                                        currReqBase.user = mw.config.get( 'wgUserName' );
                                                }

                                                // In addition to currReqBase, doRequest() will also add 'modules' and 'version'.
                                                // > '&modules='.length === 9
                                                // > '&version=1234567'.length === 16
                                                // > 9 + 16 = 25
                                                currReqBaseLength = makeQueryString( currReqBase ).length + 25;

                                                // We may need to split up the request to honor the query string length limit,
                                                // so build it piece by piece.
                                                l = currReqBaseLength;
                                                moduleMap = Object.create( null ); // { prefix: [ suffixes ] }
                                                currReqModules = [];

                                                for ( i = 0; i < modules.length; i++ ) {
                                                        // Determine how many bytes this module would add to the query string
                                                        lastDotIndex = modules[ i ].lastIndexOf( '.' );
                                                        // If lastDotIndex is -1, substr() returns an empty string
                                                        prefix = modules[ i ].substr( 0, lastDotIndex );
                                                        suffix = modules[ i ].slice( lastDotIndex + 1 );
                                                        bytesAdded = moduleMap[ prefix ] ?
                                                                suffix.length + 3 : // '%2C'.length == 3
                                                                modules[ i ].length + 3; // '%7C'.length == 3

                                                        // If the url would become too long, create a new one, but don't create empty requests
                                                        if ( maxQueryLength > 0 && currReqModules.length && l + bytesAdded > maxQueryLength ) {
                                                                // Dispatch what we've got...
                                                                doRequest();
                                                                // .. and start again.
                                                                l = currReqBaseLength;
                                                                moduleMap = Object.create( null );
                                                                currReqModules = [];

                                                                mw.track( 'resourceloader.splitRequest', { maxQueryLength: maxQueryLength } );
                                                        }
                                                        if ( !moduleMap[ prefix ] ) {
                                                                moduleMap[ prefix ] = [];
                                                        }
                                                        l += bytesAdded;
                                                        moduleMap[ prefix ].push( suffix );
                                                        currReqModules.push( modules[ i ] );
                                                }
                                                // If there's anything left in moduleMap, request that too
                                                if ( currReqModules.length ) {
                                                        doRequest();
                                                }
                                        }
                                }
                        }

                        /**
                         * @private
                         * @param {string[]} implementations Array containing pieces of JavaScript code in the
                         *  form of calls to mw.loader#implement().
                         * @param {Function} cb Callback in case of failure
                         * @param {Error} cb.err
                         */
                        function asyncEval( implementations, cb ) {
                                if ( !implementations.length ) {
                                        return;
                                }
                                mw.requestIdleCallback( function () {
                                        try {
                                                domEval( implementations.join( ';' ) );
                                        } catch ( err ) {
                                                cb( err );
                                        }
                                } );
                        }

                        /**
                         * Make a versioned key for a specific module.
                         *
                         * @private
                         * @param {string} module Module name
                         * @return {string|null} Module key in format '`[name]@[version]`',
                         *  or null if the module does not exist
                         */
                        function getModuleKey( module ) {
                                return module in registry ? ( module + '@' + registry[ module ].version ) : null;
                        }

                        /**
                         * @private
                         * @param {string} key Module name or '`[name]@[version]`'
                         * @return {Object}
                         */
                        function splitModuleKey( key ) {
                                var index = key.indexOf( '@' );
                                if ( index === -1 ) {
                                        return {
                                                name: key,
                                                version: ''
                                        };
                                }
                                return {
                                        name: key.slice( 0, index ),
                                        version: key.slice( index + 1 )
                                };
                        }

                        /**
                         * @private
                         * @param {string} module
                         * @param {string|number} [version]
                         * @param {string[]} [dependencies]
                         * @param {string} [group]
                         * @param {string} [source]
                         * @param {string} [skip]
                         */
                        function registerOne( module, version, dependencies, group, source, skip ) {
                                if ( module in registry ) {
                                        throw new Error( 'module already registered: ' + module );
                                }
                                registry[ module ] = {
                                        // Exposed to execute() for mw.loader.implement() closures.
                                        // Import happens via require().
                                        module: {
                                                exports: {}
                                        },
                                        version: String( version || '' ),
                                        dependencies: dependencies || [],
                                        group: typeof group === 'string' ? group : null,
                                        source: typeof source === 'string' ? source : 'local',
                                        state: 'registered',
                                        skip: typeof skip === 'string' ? skip : null
                                };
                        }

                        /* Public Members */
                        return {
                                /**
                                 * The module registry is exposed as an aid for debugging and inspecting page
                                 * state; it is not a public interface for modifying the registry.
                                 *
                                 * @see #registry
                                 * @property
                                 * @private
                                 */
                                moduleRegistry: registry,

                                /**
                                 * @inheritdoc #newStyleTag
                                 * @method
                                 */
                                addStyleTag: newStyleTag,

                                enqueue: enqueue,

                                resolve: resolve,

                                /**
                                 * Start loading of all queued module dependencies.
                                 *
                                 * @private
                                 */
                                work: function () {
                                        var q, batch, implementations, sourceModules;

                                        batch = [];

                                        // Appends a list of modules from the queue to the batch
                                        for ( q = 0; q < queue.length; q++ ) {
                                                // Only load modules which are registered
                                                if ( queue[ q ] in registry && registry[ queue[ q ] ].state === 'registered' ) {
                                                        // Prevent duplicate entries
                                                        if ( batch.indexOf( queue[ q ] ) === -1 ) {
                                                                batch.push( queue[ q ] );
                                                                // Mark registered modules as loading
                                                                registry[ queue[ q ] ].state = 'loading';
                                                        }
                                                }
                                        }

                                        // Now that the queue has been processed into a batch, clear the queue.
                                        // This MUST happen before we initiate any eval or network request. Otherwise,
                                        // it is possible for a cached script to instantly trigger the same work queue
                                        // again; all before we've cleared it causing each request to include modules
                                        // which are already loaded.
                                        queue = [];

                                        if ( !batch.length ) {
                                                return;
                                        }

                                        mw.loader.store.init();
                                        if ( mw.loader.store.enabled ) {
                                                implementations = [];
                                                sourceModules = [];
                                                batch = batch.filter( function ( module ) {
                                                        var implementation = mw.loader.store.get( module );
                                                        if ( implementation ) {
                                                                implementations.push( implementation );
                                                                sourceModules.push( module );
                                                                return false;
                                                        }
                                                        return true;
                                                } );
                                                asyncEval( implementations, function ( err ) {
                                                        var failed;
                                                        // Not good, the cached mw.loader.implement calls failed! This should
                                                        // never happen, barring ResourceLoader bugs, browser bugs and PEBKACs.
                                                        // Depending on how corrupt the string is, it is likely that some
                                                        // modules' implement() succeeded while the ones after the error will
                                                        // never run and leave their modules in the 'loading' state forever.
                                                        mw.loader.store.stats.failed++;

                                                        // Since this is an error not caused by an individual module but by
                                                        // something that infected the implement call itself, don't take any
                                                        // risks and clear everything in this cache.
                                                        mw.loader.store.clear();

                                                        mw.trackError( 'resourceloader.exception', {
                                                                exception: err,
                                                                source: 'store-eval'
                                                        } );
                                                        // Re-add the failed ones that are still pending back to the batch
                                                        failed = sourceModules.filter( function ( module ) {
                                                                return registry[ module ].state === 'loading';
                                                        } );
                                                        batchRequest( failed );
                                                } );
                                        }

                                        batchRequest( batch );
                                },

                                /**
                                 * Register a source.
                                 *
                                 * The #work() method will use this information to split up requests by source.
                                 *
                                 *     mw.loader.addSource( { mediawikiwiki: 'https://www.mediawiki.org/w/load.php' } );
                                 *
                                 * @private
                                 * @param {Object} ids An object mapping ids to load.php end point urls
                                 * @throws {Error} If source id is already registered
                                 */
                                addSource: function ( ids ) {
                                        var id;
                                        for ( id in ids ) {
                                                if ( id in sources ) {
                                                        throw new Error( 'source already registered: ' + id );
                                                }
                                                sources[ id ] = ids[ id ];
                                        }
                                },

                                /**
                                 * Register a module, letting the system know about it and its properties.
                                 *
                                 * The startup module calls this method.
                                 *
                                 * When using multiple module registration by passing an array, dependencies that
                                 * are specified as references to modules within the array will be resolved before
                                 * the modules are registered.
                                 *
                                 * @param {string|Array} modules Module name or array of arrays, each containing
                                 *  a list of arguments compatible with this method
                                 * @param {string|number} [version] Module version hash (falls backs to empty string)
                                 *  Can also be a number (timestamp) for compatibility with MediaWiki 1.25 and earlier.
                                 * @param {string[]} [dependencies] Array of module names on which this module depends.
                                 * @param {string} [group=null] Group which the module is in
                                 * @param {string} [source='local'] Name of the source
                                 * @param {string} [skip=null] Script body of the skip function
                                 */
                                register: function ( modules ) {
                                        var i;
                                        if ( typeof modules === 'object' ) {
                                                resolveIndexedDependencies( modules );
                                                // Optimisation: Up to 55% faster.
                                                // Typically called only once, and with a batch.
                                                // See <https://gist.github.com/Krinkle/f06fdb3de62824c6c16f02a0e6ce0e66>
                                                // Benchmarks taught us that the code for adding an object to `registry`
                                                // should actually be inline, or in a simple function that does no
                                                // arguments manipulation, and isn't also the caller itself.
                                                // JS semantics make it hard to optimise recursion to a different
                                                // signature of itself.
                                                for ( i = 0; i < modules.length; i++ ) {
                                                        registerOne.apply( null, modules[ i ] );
                                                }
                                        } else {
                                                registerOne.apply( null, arguments );
                                        }
                                },

                                /**
                                 * Implement a module given the components that make up the module.
                                 *
                                 * When #load() or #using() requests one or more modules, the server
                                 * response contain calls to this function.
                                 *
                                 * @param {string} module Name of module and current module version. Formatted
                                 *  as '`[name]@[version]`". This version should match the requested version
                                 *  (from #batchRequest and #registry). This avoids race conditions (T117587).
                                 *  For back-compat with MediaWiki 1.27 and earlier, the version may be omitted.
                                 * @param {Function|Array|string} [script] Function with module code, list of URLs
                                 *  to load via `<script src>`, or string of module code for `$.globalEval()`.
                                 * @param {Object} [style] Should follow one of the following patterns:
                                 *
                                 *     { "css": [css, ..] }
                                 *     { "url": { <media>: [url, ..] } }
                                 *
                                 * And for backwards compatibility (needs to be supported forever due to caching):
                                 *
                                 *     { <media>: css }
                                 *     { <media>: [url, ..] }
                                 *
                                 * The reason css strings are not concatenated anymore is T33676. We now check
                                 * whether it's safe to extend the stylesheet.
                                 *
                                 * @private
                                 * @param {Object} [messages] List of key/value pairs to be added to mw#messages.
                                 * @param {Object} [templates] List of key/value pairs to be added to mw#templates.
                                 */
                                implement: function ( module, script, style, messages, templates ) {
                                        var split = splitModuleKey( module ),
                                                name = split.name,
                                                version = split.version;
                                        // Automatically register module
                                        if ( !( name in registry ) ) {
                                                mw.loader.register( name );
                                        }
                                        // Check for duplicate implementation
                                        if ( registry[ name ].script !== undefined ) {
                                                throw new Error( 'module already implemented: ' + name );
                                        }
                                        if ( version ) {
                                                // Without this reset, if there is a version mismatch between the
                                                // requested and received module version, then mw.loader.store would
                                                // cache the response under the requested key. Thus poisoning the cache
                                                // indefinitely with a stale value. (T117587)
                                                registry[ name ].version = version;
                                        }
                                        // Attach components
                                        registry[ name ].script = script || null;
                                        registry[ name ].style = style || null;
                                        registry[ name ].messages = messages || null;
                                        registry[ name ].templates = templates || null;
                                        // The module may already have been marked as erroneous
                                        if ( registry[ name ].state !== 'error' && registry[ name ].state !== 'missing' ) {
                                                setAndPropagate( name, 'loaded' );
                                        }
                                },

                                /**
                                 * Load an external script or one or more modules.
                                 *
                                 * This method takes a list of unrelated modules. Use cases:
                                 *
                                 * - A web page will be composed of many different widgets. These widgets independently
                                 *   queue their ResourceLoader modules (`OutputPage::addModules()`). If any of them
                                 *   have problems, or are no longer known (e.g. cached HTML), the other modules
                                 *   should still be loaded.
                                 * - This method is used for preloading, which must not throw. Later code that
                                 *   calls #using() will handle the error.
                                 *
                                 * @param {string|Array} modules Either the name of a module, array of modules,
                                 *  or a URL of an external script or style
                                 * @param {string} [type='text/javascript'] MIME type to use if calling with a URL of an
                                 *  external script or style; acceptable values are "text/css" and
                                 *  "text/javascript"; if no type is provided, text/javascript is assumed.
                                 */
                                load: function ( modules, type ) {
                                        var filtered, l;

                                        // Allow calling with a url or single dependency as a string
                                        if ( typeof modules === 'string' ) {
                                                // "https://example.org/x.js", "http://example.org/x.js", "//example.org/x.js", "/x.js"
                                                if ( /^(https?:)?\/?\//.test( modules ) ) {
                                                        if ( type === 'text/css' ) {
                                                                l = document.createElement( 'link' );
                                                                l.rel = 'stylesheet';
                                                                l.href = modules;
                                                                document.head.appendChild( l );
                                                                return;
                                                        }
                                                        if ( type === 'text/javascript' || type === undefined ) {
                                                                addScript( modules );
                                                                return;
                                                        }
                                                        // Unknown type
                                                        throw new Error( 'invalid type for external url, must be text/css or text/javascript. not ' + type );
                                                }
                                                // Called with single module
                                                modules = [ modules ];
                                        }

                                        // Filter out top-level modules that are unknown or failed to load before.
                                        filtered = modules.filter( function ( module ) {
                                                var state = mw.loader.getState( module );
                                                return state !== 'error' && state !== 'missing';
                                        } );
                                        // Resolve remaining list using the known dependency tree.
                                        // This also filters out modules with unknown dependencies. (T36853)
                                        filtered = resolveStubbornly( filtered );
                                        // Some modules are not yet ready, add to module load queue.
                                        enqueue( filtered, undefined, undefined );
                                },

                                /**
                                 * Change the state of one or more modules.
                                 *
                                 * @param {Object} states Object of module name/state pairs
                                 */
                                state: function ( states ) {
                                        var module, state;
                                        for ( module in states ) {
                                                state = states[ module ];
                                                if ( !( module in registry ) ) {
                                                        mw.loader.register( module );
                                                }
                                                setAndPropagate( module, state );
                                        }
                                },

                                /**
                                 * Get the version of a module.
                                 *
                                 * @param {string} module Name of module
                                 * @return {string|null} The version, or null if the module (or its version) is not
                                 *  in the registry.
                                 */
                                getVersion: function ( module ) {
                                        return module in registry ? registry[ module ].version : null;
                                },

                                /**
                                 * Get the state of a module.
                                 *
                                 * @param {string} module Name of module
                                 * @return {string|null} The state, or null if the module (or its state) is not
                                 *  in the registry.
                                 */
                                getState: function ( module ) {
                                        return module in registry ? registry[ module ].state : null;
                                },

                                /**
                                 * Get the names of all registered modules.
                                 *
                                 * @return {Array}
                                 */
                                getModuleNames: function () {
                                        return Object.keys( registry );
                                },

                                /**
                                 * Get the exported value of a module.
                                 *
                                 * This static method is publicly exposed for debugging purposes
                                 * only and must not be used in production code. In production code,
                                 * please use the dynamically provided `require()` function instead.
                                 *
                                 * In case of lazy-loaded modules via mw.loader#using(), the returned
                                 * Promise provides the function, see #using() for examples.
                                 *
                                 * @private
                                 * @since 1.27
                                 * @param {string} moduleName Module name
                                 * @return {Mixed} Exported value
                                 */
                                require: function ( moduleName ) {
                                        var state = mw.loader.getState( moduleName );

                                        // Only ready modules can be required
                                        if ( state !== 'ready' ) {
                                                // Module may've forgotten to declare a dependency
                                                throw new Error( 'Module "' + moduleName + '" is not loaded.' );
                                        }

                                        return registry[ moduleName ].module.exports;
                                },

                                /**
                                 * On browsers that implement the localStorage API, the module store serves as a
                                 * smart complement to the browser cache. Unlike the browser cache, the module store
                                 * can slice a concatenated response from ResourceLoader into its constituent
                                 * modules and cache each of them separately, using each module's versioning scheme
                                 * to determine when the cache should be invalidated.
                                 *
                                 * @private
                                 * @singleton
                                 * @class mw.loader.store
                                 */
                                store: {
                                        // Whether the store is in use on this page.
                                        enabled: null,

                                        // Modules whose string representation exceeds 100 kB are
                                        // ineligible for storage. See bug T66721.
                                        MODULE_SIZE_MAX: 100 * 1000,

                                        // The contents of the store, mapping '[name]@[version]' keys
                                        // to module implementations.
                                        items: {},

                                        // Names of modules to be stored during the next update.
                                        // See add() and update().
                                        queue: [],

                                        // Cache hit stats
                                        stats: { hits: 0, misses: 0, expired: 0, failed: 0 },

                                        /**
                                         * Construct a JSON-serializable object representing the content of the store.
                                         *
                                         * @return {Object} Module store contents.
                                         */
                                        toJSON: function () {
                                                return { items: mw.loader.store.items, vary: mw.loader.store.getVary() };
                                        },

                                        /**
                                         * Get the localStorage key for the entire module store. The key references
                                         * $wgDBname to prevent clashes between wikis which share a common host.
                                         *
                                         * @return {string} localStorage item key
                                         */
                                        getStoreKey: function () {
                                                return 'MediaWikiModuleStore:' + mw.config.get( 'wgDBname' );
                                        },

                                        /**
                                         * Get a key on which to vary the module cache.
                                         *
                                         * @return {string} String of concatenated vary conditions.
                                         */
                                        getVary: function () {
                                                return mw.config.get( 'skin' ) + ':' +
                                                        mw.config.get( 'wgResourceLoaderStorageVersion' ) + ':' +
                                                        mw.config.get( 'wgUserLanguage' );
                                        },

                                        /**
                                         * Initialize the store.
                                         *
                                         * Retrieves store from localStorage and (if successfully retrieved) decoding
                                         * the stored JSON value to a plain object.
                                         *
                                         * The try / catch block is used for JSON & localStorage feature detection.
                                         * See the in-line documentation for Modernizr's localStorage feature detection
                                         * code for a full account of why we need a try / catch:
                                         * <https://github.com/Modernizr/Modernizr/blob/v2.7.1/modernizr.js#L771-L796>.
                                         */
                                        init: function () {
                                                var raw, data;

                                                if ( this.enabled !== null ) {
                                                        // Init already ran
                                                        return;
                                                }

                                                if (
                                                        // Disabled because localStorage quotas are tight and (in Firefox's case)
                                                        // shared by multiple origins.
                                                        // See T66721, and <https://bugzilla.mozilla.org/show_bug.cgi?id=1064466>.
                                                        /Firefox/.test( navigator.userAgent ) ||

                                                        // Disabled by configuration.
                                                        !mw.config.get( 'wgResourceLoaderStorageEnabled' )
                                                ) {
                                                        // Clear any previous store to free up space. (T66721)
                                                        this.clear();
                                                        this.enabled = false;
                                                        return;
                                                }
                                                if ( mw.config.get( 'debug' ) ) {
                                                        // Disable module store in debug mode
                                                        this.enabled = false;
                                                        return;
                                                }

                                                try {
                                                        // This a string we stored, or `null` if the key does not (yet) exist.
                                                        raw = localStorage.getItem( this.getStoreKey() );
                                                        // If we get here, localStorage is available; mark enabled
                                                        this.enabled = true;
                                                        // If null, JSON.parse() will cast to string and re-parse, still null.
                                                        data = JSON.parse( raw );
                                                        if ( data && typeof data.items === 'object' && data.vary === this.getVary() ) {
                                                                this.items = data.items;
                                                                return;
                                                        }
                                                } catch ( e ) {
                                                        mw.trackError( 'resourceloader.exception', {
                                                                exception: e,
                                                                source: 'store-localstorage-init'
                                                        } );
                                                }

                                                // If we get here, one of four things happened:
                                                //
                                                // 1. localStorage did not contain our store key.
                                                //    This means `raw` is `null`, and we're on a fresh page view (cold cache).
                                                //    The store was enabled, and `items` starts fresh.
                                                //
                                                // 2. localStorage contained parseable data under our store key,
                                                //    but it's not applicable to our current context (see getVary).
                                                //    The store was enabled, and `items` starts fresh.
                                                //
                                                // 3. JSON.parse threw (localStorage contained corrupt data).
                                                //    This means `raw` contains a string.
                                                //    The store was enabled, and `items` starts fresh.
                                                //
                                                // 4. localStorage threw (disabled or otherwise unavailable).
                                                //    This means `raw` was never assigned.
                                                //    We will disable the store below.
                                                if ( raw === undefined ) {
                                                        // localStorage failed; disable store
                                                        this.enabled = false;
                                                }
                                        },

                                        /**
                                         * Retrieve a module from the store and update cache hit stats.
                                         *
                                         * @param {string} module Module name
                                         * @return {string|boolean} Module implementation or false if unavailable
                                         */
                                        get: function ( module ) {
                                                var key;

                                                if ( !this.enabled ) {
                                                        return false;
                                                }

                                                key = getModuleKey( module );
                                                if ( key in this.items ) {
                                                        this.stats.hits++;
                                                        return this.items[ key ];
                                                }
                                                this.stats.misses++;
                                                return false;
                                        },

                                        /**
                                         * Queue the name of a module that the next update should consider storing.
                                         *
                                         * @since 1.32
                                         * @param {string} module Module name
                                         */
                                        add: function ( module ) {
                                                if ( !this.enabled ) {
                                                        return;
                                                }
                                                this.queue.push( module );
                                                this.requestUpdate();
                                        },

                                        /**
                                         * Add the contents of the named module to the in-memory store.
                                         *
                                         * This method does not guarantee that the module will be stored.
                                         * Inspection of the module's meta data and size will ultimately decide that.
                                         *
                                         * This method is considered internal to mw.loader.store and must only
                                         * be called if the store is enabled.
                                         *
                                         * @private
                                         * @param {string} module Module name
                                         */
                                        set: function ( module ) {
                                                var key, args, src,
                                                        descriptor = mw.loader.moduleRegistry[ module ];

                                                key = getModuleKey( module );

                                                if (
                                                        // Already stored a copy of this exact version
                                                        key in this.items ||
                                                        // Module failed to load
                                                        !descriptor ||
                                                        descriptor.state !== 'ready' ||
                                                        // Unversioned, private, or site-/user-specific
                                                        !descriptor.version ||
                                                        descriptor.group === 'private' ||
                                                        descriptor.group === 'user' ||
                                                        // Partial descriptor
                                                        // (e.g. skipped module, or style module with state=ready)
                                                        [ descriptor.script, descriptor.style, descriptor.messages,
                                                                descriptor.templates ].indexOf( undefined ) !== -1
                                                ) {
                                                        // Decline to store
                                                        return;
                                                }

                                                try {
                                                        args = [
                                                                JSON.stringify( key ),
                                                                typeof descriptor.script === 'function' ?
                                                                        String( descriptor.script ) :
                                                                        JSON.stringify( descriptor.script ),
                                                                JSON.stringify( descriptor.style ),
                                                                JSON.stringify( descriptor.messages ),
                                                                JSON.stringify( descriptor.templates )
                                                        ];
                                                } catch ( e ) {
                                                        mw.trackError( 'resourceloader.exception', {
                                                                exception: e,
                                                                source: 'store-localstorage-json'
                                                        } );
                                                        return;
                                                }

                                                src = 'mw.loader.implement(' + args.join( ',' ) + ');';
                                                if ( src.length > this.MODULE_SIZE_MAX ) {
                                                        return;
                                                }
                                                this.items[ key ] = src;
                                        },

                                        /**
                                         * Iterate through the module store, removing any item that does not correspond
                                         * (in name and version) to an item in the module registry.
                                         */
                                        prune: function () {
                                                var key, module;

                                                for ( key in this.items ) {
                                                        module = key.slice( 0, key.indexOf( '@' ) );
                                                        if ( getModuleKey( module ) !== key ) {
                                                                this.stats.expired++;
                                                                delete this.items[ key ];
                                                        } else if ( this.items[ key ].length > this.MODULE_SIZE_MAX ) {
                                                                // This value predates the enforcement of a size limit on cached modules.
                                                                delete this.items[ key ];
                                                        }
                                                }
                                        },

                                        /**
                                         * Clear the entire module store right now.
                                         */
                                        clear: function () {
                                                this.items = {};
                                                try {
                                                        localStorage.removeItem( this.getStoreKey() );
                                                } catch ( e ) {}
                                        },

                                        /**
                                         * Request a sync of the in-memory store back to persisted localStorage.
                                         *
                                         * This function debounces updates. The debouncing logic should account
                                         * for the following factors:
                                         *
                                         * - Writing to localStorage is an expensive operation that must not happen
                                         *   during the critical path of initialising and executing module code.
                                         *   Instead, it should happen at a later time after modules have been given
                                         *   time and priority to do their thing first.
                                         *
                                         * - This method is called from mw.loader.store.add(), which will be called
                                         *   hundreds of times on a typical page, including within the same call-stack
                                         *   and eventloop-tick. This is because responses from load.php happen in
                                         *   batches. As such, we want to allow all modules from the same load.php
                                         *   response to be written to disk with a single flush, not many.
                                         *
                                         * - Repeatedly deleting and creating timers is non-trivial.
                                         *
                                         * - localStorage is shared by all pages from the same origin, if multiple
                                         *   pages are loaded with different module sets, the possibility exists that
                                         *   modules saved by one page will be clobbered by another. The impact of
                                         *   this is minor, it merely causes a less efficient cache use, and the
                                         *   problem would be corrected by subsequent page views.
                                         *
                                         * This method is considered internal to mw.loader.store and must only
                                         * be called if the store is enabled.
                                         *
                                         * @private
                                         * @method
                                         */
                                        requestUpdate: ( function () {
                                                var hasPendingWrites = false;

                                                function flushWrites() {
                                                        var data, key;

                                                        // Remove anything from the in-memory store that came from previous page
                                                        // loads that no longer corresponds with current module names and versions.
                                                        mw.loader.store.prune();
                                                        // Process queued module names, serialise their contents to the in-memory store.
                                                        while ( mw.loader.store.queue.length ) {
                                                                mw.loader.store.set( mw.loader.store.queue.shift() );
                                                        }

                                                        key = mw.loader.store.getStoreKey();
                                                        try {
                                                                // Replacing the content of the module store might fail if the new
                                                                // contents would exceed the browser's localStorage size limit. To
                                                                // avoid clogging the browser with stale data, always remove the old
                                                                // value before attempting to set the new one.
                                                                localStorage.removeItem( key );
                                                                data = JSON.stringify( mw.loader.store );
                                                                localStorage.setItem( key, data );
                                                        } catch ( e ) {
                                                                mw.trackError( 'resourceloader.exception', {
                                                                        exception: e,
                                                                        source: 'store-localstorage-update'
                                                                } );
                                                        }

                                                        // Let the next call to requestUpdate() create a new timer.
                                                        hasPendingWrites = false;
                                                }

                                                function onTimeout() {
                                                        // Defer the actual write via requestIdleCallback
                                                        mw.requestIdleCallback( flushWrites );
                                                }

                                                return function () {
                                                        // On the first call to requestUpdate(), create a timer that
                                                        // waits at least two seconds, then calls onTimeout.
                                                        // The main purpose is to allow the current batch of load.php
                                                        // responses to complete before we do anything. This batch can
                                                        // trigger many hundreds of calls to requestUpdate().
                                                        if ( !hasPendingWrites ) {
                                                                hasPendingWrites = true;
                                                                setTimeout( onTimeout, 2000 );
                                                        }
                                                };
                                        }() )
                                }
                        };
                }() ),

                // Skeleton user object, extended by the 'mediawiki.user' module.
                /**
                 * @class mw.user
                 * @singleton
                 */
                user: {
                        /**
                         * @property {mw.Map}
                         */
                        options: new Map(),
                        /**
                         * @property {mw.Map}
                         */
                        tokens: new Map()
                },

                // OOUI widgets specific to MediaWiki
                widgets: {}

        };

        // Attach to window and globally alias
        window.mw = window.mediaWiki = mw;
}() );