cr.js: Replace usage of ES6 Map with Object.

ui/webui/resources/js/cr.js

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * The global object.
  * @type {!Object}
  * @const
  */
 var global = this;
@@ skipping 300 matching lines @@
       };
     });
   }
 
   /**
    * The mapping used by the sendWithCallback mechanism to tie the callback
    * supplied to an invocation of sendWithCallback with the WebUI response
    * sent by the browser in response to the chrome.send call. The mapping is
    * from ID to callback function; the ID is generated by sendWithCallback and
    * is unique across all invocations of said method.
-   * @type {!Map<string, Function>}
+   * @type {!Object<string, Function>}

[Dan Beam, 2015/07/09 22:11:15]

nit: Object<string is redundant (just Object<Function> is fine).  there's no
different.  keys can only be strings in JS

[James Hawkins, 2015/07/09 22:19:50]

Done.

    */
-  var chromeSendCallbackMap = new Map();
+  var chromeSendCallbackMap = Object.create(null);

[Dan Beam, 2015/07/09 22:11:15]

nit: I don't see any Object.prototype.* modification in cs.chromium.org and this
should be basically equivalent to {} which is probably far more common (and
shorter).

[James Hawkins, 2015/07/09 22:19:50]

According to MDN, {} is equivalent to Object.create(Object.prototype), and
inheriting the prototype is what I was avoiding here.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Obje...

[Dan Beam, 2015/07/09 22:21:41]

I get what you're saying, we just don't do that currently (which is why looked
through chrome for prototype modification of Object) and we hopefully never
will.

at this level (in cr.js) it's probably the most likely of anywhere, so it's fine
I just think it's unnecessary

 
   /**
    * The named method the WebUI handler calls directly in response to a
    * chrome.send call that expects a callback. The handler requires no knowledge
    * of the specific name of this method, as the name is passed to the handler
    * as the first argument in the arguments list of chrome.send. The handler
    * must pass the ID, also sent via the chrome.send arguments list, as the
    * first argument of the JS invocation; additionally, the handler may
    * supply any number of other arguments that will be forwarded to the
    * callback.
    * @param {string} id The unique ID identifying the callback method this
    *    response is tied to.
    */
   function webUIResponse(id) {
-    chromeSendCallbackMap.get(id).apply(
+    chromeSendCallbackMap[id].apply(
         null, Array.prototype.slice.call(arguments, 1));
-    chromeSendCallbackMap.delete(id);
+    delete chromeSendCallbackMap[id];
   }
 
   /**
    * A variation of chrome.send which allows the client to receive a direct
    * callback without requiring the handler to have specific knowledge of any
    * JS internal method names or state. The callback will be removed from the
    * mapping once it has fired.
    * @param {string} methodName The name of the WebUI handler API.
    * @param {Array|undefined} args Arguments for the method call sent to the
    *     WebUI handler. Pass undefined if no args should be sent to the handler.
    * @param {Function} callback A callback function which is called (indirectly)
    *     by the WebUI handler.
    */
   function sendWithCallback(methodName, args, callback) {
     var id = methodName + createUid();
-    chromeSendCallbackMap.set(id, callback);
+    chromeSendCallbackMap[id] = callback;
     chrome.send(methodName, ['cr.webUIResponse', id].concat(args || []));
   }
 
   /**
    * A registry of callbacks keyed by event name. Used by addWebUIListener to
    * register listeners.
-   * @type {!Map<string, Array<Function>>}
+   * @type {!Object<string, Array<Function>>}

[Dan Beam, 2015/07/09 22:11:15]

same nit re: Object<string, -> Object<

[James Hawkins, 2015/07/09 22:19:50]

Done.

    */
-  var webUIListenerMap = new Map();
+  var webUIListenerMap = Object.create(null);
 
   /**
    * The named method the WebUI handler calls directly when an event occurs.
    * The WebUI handler must supply the name of the event as the first argument
    * of the JS invocation; additionally, the handler may supply any number of
    * other arguments that will be forwarded to the listener callbacks.
    * @param {string} event The name of the event that has occurred.
    */
   function webUIListenerCallback(event) {
-    var listenerCallbacks = webUIListenerMap.get(event);
+    var listenerCallbacks = webUIListenerMap[event];
     for (var i = 0; i < listenerCallbacks.length; i++) {
       var callback = listenerCallbacks[i];
       callback.apply(null, Array.prototype.slice.call(arguments, 1));
     }
   }
 
   /**
    * Registers a listener for an event fired from WebUI handlers. Any number of
    * listeners may register for a single event.
    * @param {string} event The event to listen to.
    * @param {Function} callback The callback run when the event is fired.
    */
   function addWebUIListener(event, callback) {
-    if (!webUIListenerMap.has(event))
-      webUIListenerMap.set(event, [callback]);
+    if (event in webUIListenerMap)
+      webUIListenerMap[event].push(callback);
     else
-      webUIListenerMap.get(event).push(callback);
+      webUIListenerMap[event] = [callback];
   }
 
   return {
     addSingletonGetter: addSingletonGetter,
     createUid: createUid,
     define: define,
     defineProperty: defineProperty,
     dispatchPropertyChange: dispatchPropertyChange,
     dispatchSimpleEvent: dispatchSimpleEvent,
     exportPath: exportPath,
@@ skipping 23 matching lines @@
     get isChromeOS() {
       return /CrOS/.test(navigator.userAgent);
     },
 
     /** Whether this is on vanilla Linux (not chromeOS). */
     get isLinux() {
       return /Linux/.test(navigator.userAgent);
     },
   };
 }();