diff --git a/resources/src/mediawiki/mediawiki.js b/resources/src/mediawiki/mediawiki.js
index 94b64b9f9f3..6bf93f49acd 100644
--- a/resources/src/mediawiki/mediawiki.js
+++ b/resources/src/mediawiki/mediawiki.js
@@ -79,12 +79,56 @@
 	 * @class mw.Map
 	 *
 	 * @constructor
-	 * @param {Object|boolean} [values] Value-bearing object to map, or boolean
-	 *  true to map over the global object. Defaults to an empty object.
+	 * @param {Object|boolean} [values] Value-bearing object to map, defaults to an empty object.
+	 *  For backwards-compatibility with mw.config, this can also be `true` in which case values
+	 *  will be copied to the Window object as global variables (T72470). Values are copied in one
+	 *  direction only. Changes to globals are not reflected in the map.
 	 */
 	function Map( values ) {
-		this.values = values === true ? window : ( values || {} );
-		return this;
+		if ( values === true ) {
+			this.values = {};
+
+			// Override #set to also set the global variable
+			this.set = function ( selection, value ) {
+				var s;
+
+				if ( $.isPlainObject( selection ) ) {
+					for ( s in selection ) {
+						setGlobalMapValue( this, s, selection[s] );
+					}
+					return true;
+				}
+				if ( typeof selection === 'string' && arguments.length ) {
+					setGlobalMapValue( this, selection, value );
+					return true;
+				}
+				return false;
+			};
+
+			return;
+		}
+
+		this.values = values || {};
+	}
+
+	/**
+	 * Alias property to the global object.
+	 *
+	 * @private
+	 * @static
+	 * @param {mw.Map} map
+	 * @param {string} key
+	 * @param {Mixed} value
+	 */
+	function setGlobalMapValue( map, key, value ) {
+		map.values[key] = value;
+		mw.log.deprecate(
+			window,
+			key,
+			value,
+			// Deprecation notice for mw.config globals (T58550, T72470)
+			map === mw.config && 'Use mw.config instead.'
+		);
 	}
 
 	Map.prototype = {
@@ -136,7 +180,7 @@
 		 *
 		 * @param {string|Object} selection String 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} This returns true on success, false on failure.
+		 * @return {boolean} This returns true on success, false on failure.
 		 */
 		set: function ( selection, value ) {
 			var s;
@@ -147,7 +191,7 @@
 				}
 				return true;
 			}
-			if ( typeof selection === 'string' && arguments.length > 1 ) {
+			if ( typeof selection === 'string' && arguments.length ) {
 				this.values[selection] = value;
 				return true;
 			}
@@ -582,6 +626,7 @@
 					} );
 				} catch ( err ) {
 					// IE8 can throw on Object.defineProperty
+					// Create a copy of the value to the object.
 					obj[key] = val;
 				}
 			};
diff --git a/tests/qunit/suites/resources/mediawiki/mediawiki.test.js b/tests/qunit/suites/resources/mediawiki/mediawiki.test.js
index 87520bd5686..6c8c62f0d5b 100644
--- a/tests/qunit/suites/resources/mediawiki/mediawiki.test.js
+++ b/tests/qunit/suites/resources/mediawiki/mediawiki.test.js
@@ -55,7 +55,7 @@
 		this.restoreWarnings();
 	} );
 
-	QUnit.test( 'mw.Map', 28, function ( assert ) {
+	QUnit.test( 'mw.Map', 34, function ( assert ) {
 		var arry, conf, funky, globalConf, nummy, someValues;
 
 		conf = new mw.Map();
@@ -126,12 +126,31 @@
 
 		conf.set( 'globalMapChecker', 'Hi' );
 
-		assert.ok( 'globalMapChecker' in window === false, 'new mw.Map did not store its values in the global window object by default' );
+		assert.ok( ( 'globalMapChecker' in window ) === false, 'Map does not its store values in the window object by default' );
 
 		globalConf = new mw.Map( true );
 		globalConf.set( 'anotherGlobalMapChecker', 'Hello' );
 
-		assert.ok( 'anotherGlobalMapChecker' in window, 'new mw.Map( true ) did store its values in the global window object' );
+		assert.ok( 'anotherGlobalMapChecker' in window, 'global Map stores its values in the window object' );
+
+		assert.equal( globalConf.get( 'anotherGlobalMapChecker' ), 'Hello', 'get value from global Map via get()' );
+		this.suppressWarnings();
+		assert.equal( window.anotherGlobalMapChecker, 'Hello', 'get value from global Map via window object' );
+		this.restoreWarnings();
+
+		// Change value via global Map
+		globalConf.set('anotherGlobalMapChecker', 'Again');
+		assert.equal( globalConf.get( 'anotherGlobalMapChecker' ), 'Again', 'Change in global Map reflected via get()' );
+		this.suppressWarnings();
+		assert.equal( window.anotherGlobalMapChecker, 'Again', 'Change in global Map reflected window object' );
+		this.restoreWarnings();
+
+		// Change value via window object
+		this.suppressWarnings();
+		window.anotherGlobalMapChecker = 'World';
+		assert.equal( window.anotherGlobalMapChecker, 'World', 'Change in window object works' );
+		this.restoreWarnings();
+		assert.equal( globalConf.get( 'anotherGlobalMapChecker' ), 'Again', 'Change in window object not reflected in global Map' );
 
 		// Whitelist this global variable for QUnit's 'noglobal' mode
 		if ( QUnit.config.noglobals ) {