Make Set and Map initialization lazy

Reviewed By: javache

Differential Revision: D5461810

fbshipit-source-id: 6f22528bac4dd6e073e98a093e02d84114d0706a

Author: fromcelticpark

Libraries/Core/InitializeCore.js

@@ -128,10 +128,13 @@ if (!global.__fbDisableExceptionsManager) {
 }
 
 // Set up collections
-// We can't make these lazy because `Map` checks for `global.Map` (which wouldc
-// not exist if it were lazily defined).
-defineProperty(global, 'Map', () => require('Map'), true);
-defineProperty(global, 'Set', () => require('Set'), true);
+const _shouldPolyfillCollection = require('_shouldPolyfillES6Collection');
+if (_shouldPolyfillCollection('Map')) {
+  defineProperty(global, 'Map', () => require('Map'));
+}
+if (_shouldPolyfillCollection('Set')) {
+  defineProperty(global, 'Set', () => require('Set'));
+}
 
 // Set up Promise
 // The native Promise implementation throws the following error:

Libraries/vendor/core/_shouldPolyfillES6Collection.js

@@ -16,7 +16,7 @@
  * Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
  * that is safe to be used.
  */
-function shouldPolyfillES6Collection(collectionName: string): boolean {
+function _shouldActuallyPolyfillES6Collection(collectionName: string): boolean {
   var Collection = global[collectionName];
   if (Collection == null) {
     return true;
@@ -43,4 +43,23 @@ function shouldPolyfillES6Collection(collectionName: string): boolean {
     typeof proto.forEach !== 'function';
 }
 
-module.exports = shouldPolyfillES6Collection;
+const cache: { [name: string]: bool } = {};
+
+  /**
+   * Checks whether a collection name (e.g. "Map" or "Set") has a native polyfill
+   * that is safe to be used and caches this result.
+   * Make sure to make a first call to this function before a corresponding
+   * property on global was overriden in any way.
+   */
+function _shouldPolyfillES6Collection(collectionName: string) {
+    let result = cache[collectionName];
+    if (result !== undefined) {
+      return result;
+    }
+
+    result = _shouldActuallyPolyfillES6Collection(collectionName);
+    cache[collectionName] = result;
+    return result;
+}
+
+module.exports = _shouldPolyfillES6Collection;