Optimize isPlainObject.

Former-commit-id: 37754e27d03929927d8b8653bd7e44ad3ce2b23c

lodash.js

@@ -798,51 +798,6 @@
     return htmlEscapes[match];
   }
 
-  /**
-   * Checks if a given `value` is an object created by the `Object` constructor
-   * assuming objects created by the `Object` constructor have no inherited
-   * enumerable properties and that there are no `Object.prototype` extensions.
-   *
-   * @private
-   * @param {Mixed} value The value to check.
-   * @param {Boolean} [skipArgsCheck=false] Internally used to skip checks for
-   *  `arguments` objects.
-   * @returns {Boolean} Returns `true` if the `value` is a plain `Object` object,
-   *  else `false`.
-   */
-  function isPlainObject(value, skipArgsCheck) {
-    // avoid non-objects and false positives for `arguments` objects
-    var result = false;
-    if (!(value && typeof value == 'object') || (!skipArgsCheck && isArguments(value))) {
-      return result;
-    }
-    // IE < 9 presents DOM nodes as `Object` objects except they have `toString`
-    // methods that are `typeof` "string" and still can coerce nodes to strings.
-    // Also check that the constructor is `Object` (i.e. `Object instanceof Object`)
-    var ctor = value.constructor;
-    if ((!noNodeClass || !(typeof value.toString != 'function' && typeof (value + '') == 'string')) &&
-        (!isFunction(ctor) || ctor instanceof ctor)) {
-      // IE < 9 iterates inherited properties before own properties. If the first
-      // iterated property is an object's own property then there are no inherited
-      // enumerable properties.
-      if (iteratesOwnLast) {
-        forIn(value, function(objValue, objKey) {
-          result = !hasOwnProperty.call(value, objKey);
-          return false;
-        });
-        return result === false;
-      }
-      // In most environments an object's own properties are iterated before
-      // its inherited properties. If the last iterated property is an object's
-      // own property then there are no inherited enumerable properties.
-      forIn(value, function(objValue, objKey) {
-        result = objKey;
-      });
-      return result === false || hasOwnProperty.call(value, result);
-    }
-    return result;
-  }
-
   /**
    * Creates a new function that, when called, invokes `func` with the `this`
    * binding of `thisArg` and the arguments (value, index, object).
@@ -1005,6 +960,59 @@
     };
   }
 
+  /**
+   * Checks if a given `value` is an object created by the `Object` constructor
+   * assuming objects created by the `Object` constructor have no inherited
+   * enumerable properties and that there are no `Object.prototype` extensions.
+   *
+   * @private
+   * @param {Mixed} value The value to check.
+   * @param {Boolean} [skipArgsCheck=false] Internally used to skip checks for
+   *  `arguments` objects.
+   * @returns {Boolean} Returns `true` if the `value` is a plain `Object` object,
+   *  else `false`.
+   */
+  function isPlainObject(value, skipArgsCheck) {
+    return value
+      ? value == ObjectProto || (value.__proto__ == ObjectProto && (skipArgsCheck || !isArguments(value)))
+      : false;
+  }
+  // fallback for IE
+  if (!isPlainObject(objectTypes)) {
+    isPlainObject = function(value, skipArgsCheck) {
+      // avoid non-objects and false positives for `arguments` objects
+      var result = false;
+      if (!(value && typeof value == 'object') || (!skipArgsCheck && isArguments(value))) {
+        return result;
+      }
+      // IE < 9 presents DOM nodes as `Object` objects except they have `toString`
+      // methods that are `typeof` "string" and still can coerce nodes to strings.
+      // Also check that the constructor is `Object` (i.e. `Object instanceof Object`)
+      var ctor = value.constructor;
+      if ((!noNodeClass || !(typeof value.toString != 'function' && typeof (value + '') == 'string')) &&
+          (!isFunction(ctor) || ctor instanceof ctor)) {
+        // IE < 9 iterates inherited properties before own properties. If the first
+        // iterated property is an object's own property then there are no inherited
+        // enumerable properties.
+        if (iteratesOwnLast) {
+          forIn(value, function(objValue, objKey) {
+            result = !hasOwnProperty.call(value, objKey);
+            return false;
+          });
+          return result === false;
+        }
+        // In most environments an object's own properties are iterated before
+        // its inherited properties. If the last iterated property is an object's
+        // own property then there are no inherited enumerable properties.
+        forIn(value, function(objValue, objKey) {
+          result = objKey;
+        });
+        return result === false || hasOwnProperty.call(value, result);
+      }
+      return result;
+    };
+  }
+
   /**
    * A shim implementation of `Object.keys` that produces an array of the given
    * object's own enumerable property names.