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

ui/webui/resources/js/cr.js

Index: ui/webui/resources/js/cr.js
diff --git a/ui/webui/resources/js/cr.js b/ui/webui/resources/js/cr.js
index ceb1eb9707474fb9d91f9bc6b12ad5b13858569f..f9b2c799b76357ac59341bf84db7ae6033c3acf6 100644
--- a/ui/webui/resources/js/cr.js
+++ b/ui/webui/resources/js/cr.js
@@ -318,9 +318,9 @@ var cr = function() {
    * 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
@@ -335,9 +335,9 @@ var cr = function() {
    *    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];
   }
 
   /**
@@ -353,16 +353,16 @@ var cr = function() {
    */
   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.
@@ -372,7 +372,7 @@ var cr = function() {
    * @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));
@@ -386,10 +386,10 @@ var cr = function() {
    * @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 {