Respect original enumerability/writability when polyfilling globals

Summary:
Reuses the original property descriptor when overwriting / polyfilling globals to ensure enumerability and writability are the same
Closes facebook#7704

Differential Revision: D3338119

Pulled By: davidaurelio

fbshipit-source-id: ab456324a3346cd3ec8b2c3e3a2378408c92087c

Libraries/JavaScriptAppEngine/Initialization/InitializeJavaScriptAppEngine.js

@@ -64,40 +64,47 @@ function setUpConsole() {
  * https://github.com/facebook/react-native/issues/934
  */
 function polyfillGlobal(name, newValue, scope = global) {
-  const descriptor = Object.getOwnPropertyDescriptor(scope, name) || {
-    // jest for some bad reasons runs the polyfill code multiple times. In jest
-    // environment, XmlHttpRequest doesn't exist so getOwnPropertyDescriptor
-    // returns undefined and defineProperty default for writable is false.
-    // Therefore, the second time it runs, defineProperty will fatal :(
-    writable: true,
-  };
-
-  if (scope[name] !== undefined) {
+  const descriptor = Object.getOwnPropertyDescriptor(scope, name);
+  if (descriptor) {
     const backupName = `original${name[0].toUpperCase()}${name.substr(1)}`;
     Object.defineProperty(scope, backupName, {...descriptor, value: scope[name]});
   }
 
-  Object.defineProperty(scope, name, {...descriptor, value: newValue});
+  const {enumerable, writable} = descriptor || {};
+
+  // jest for some bad reasons runs the polyfill code multiple times. In jest
+  // environment, XmlHttpRequest doesn't exist so getOwnPropertyDescriptor
+  // returns undefined and defineProperty default for writable is false.
+  // Therefore, the second time it runs, defineProperty will fatal :(
+
+  Object.defineProperty(scope, name, {
+    configurable: true,
+    enumerable: enumerable !== false,
+    writable: writable !== false,
+    value: newValue,
+  });
 }
 
 function polyfillLazyGlobal(name, valueFn, scope = global) {
-  if (scope[name] !== undefined) {
-    const descriptor = Object.getOwnPropertyDescriptor(scope, name);
+  const descriptor = getPropertyDescriptor(scope, name);
+  if (descriptor) {
     const backupName = `original${name[0].toUpperCase()}${name.substr(1)}`;
-    Object.defineProperty(scope, backupName, {...descriptor, value: scope[name]});
+    Object.defineProperty(scope, backupName, descriptor);
   }
 
+  const {enumerable, writable} = descriptor || {};
   Object.defineProperty(scope, name, {
     configurable: true,
-    enumerable: true,
+    enumerable: enumerable !== false,
     get() {
       return (this[name] = valueFn());
     },
     set(value) {
       Object.defineProperty(this, name, {
         configurable: true,
-        enumerable: true,
-        value
+        enumerable: enumerable !== false,
+        writable: writable !== false,
+        value,
       });
     }
   });
@@ -211,6 +218,16 @@ function setUpDevTools() {
   }
 }
 
+function getPropertyDescriptor(object, name) {
+  while (object) {
+    const descriptor = Object.getOwnPropertyDescriptor(object, name);
+    if (descriptor) {
+      return descriptor;
+    }
+    object = Object.getPrototypeOf(object);
+  }
+}
+
 setUpProcess();
 setUpConsole();
 setUpTimers();