Add hasThis to avoid repeatedly checking for this.

Former-commit-id: 15ce8566364ddac60ac01f3a36343c33d9739b77

build.js

@@ -1360,6 +1360,18 @@
     return result ? result[0] : '';
   }
 
+  /**
+   * Gets the `hasThis` fork from `source`.
+   *
+   * @private
+   * @param {String} source The source to inspect.
+   * @returns {String} Returns the `isFunction` fork.
+   */
+  function getHasThisFork(source) {
+    var result = source.match(/(?:\s*\/\/.*)*\n( *)if *\(!defineProperty\s*\|\|\s*!reThis\)[\s\S]+?\n\1}/);
+    return result ? result[0] : '';
+  }
+
   /**
    * Gets the Lo-Dash method assignments snippet from `source`.
    *
@@ -1784,21 +1796,34 @@
   }
 
   /**
-   * Removes the binding optimization from `source`.
+   * Removes `hasThis` and its binding optimizations from `source`.
    *
    * @private
    * @param {String} source The source to process.
    * @returns {String} Returns the modified source.
    */
-  function removeBindingOptimization(source) {
-    // remove `reThis` from `createCallback`
+  function removeHasThis(source) {
+    source = removeFunction(source, 'hasThis');
+
+    // remove `hasThis` use `_.createCallback`
     source = source.replace(matchFunction(source, 'createCallback'), function(match) {
-      return match.replace(/\s*\|\|\s*\(reThis[\s\S]+?\)\)\)/, '');
+      return match.replace(/\s*\|\|\s*!hasThis\(.+?\)/, '');
     });
 
     return source;
   }
 
+  /**
+   * Removes the `hasThis` fork from `source`.
+   *
+   * @private
+   * @param {String} source The source to process.
+   * @returns {String} Returns the modified source.
+   */
+  function removeHasThisFork(source) {
+    return source.replace(getHasThisFork(source), '');
+  }
+
   /**
    * Removes the `Object.keys` object iteration optimization from `source`.
    *
@@ -2982,8 +3007,9 @@
         );
       }
       if (isModern) {
-        source = removeSupportSpliceObjects(source);
         source = removeIsArgumentsFork(source);
+        source = removeHasThisFork(source);
+        source = removeSupportSpliceObjects(source);
 
         if (isMobile) {
           source = replaceSupportProp(source, 'enumPrototypes', 'true');
@@ -3001,7 +3027,7 @@
         }
       }
       if ((isLegacy || isMobile || isUnderscore) && !isLodash('createCallback')) {
-        source = removeBindingOptimization(source);
+        source = removeHasThis(source);
       }
       if (isLegacy || isMobile || isUnderscore) {
         if (isMobile || (!isLodash('assign') && !isLodash('defaults') && !isLodash('forIn') && !isLodash('forOwn'))) {
@@ -4080,7 +4106,7 @@
       });
 
       // remove forks of removed functions
-      _.each(['createObject', 'defer', 'isArguments', 'isArray', 'isFunction'], function(funcName) {
+      _.each(['createObject', 'defer', 'hasThis', 'isArguments', 'isArray', 'isFunction'], function(funcName) {
         if (isExcluded(funcName)) {
           source = eval('remove' + capitalize(funcName) + 'Fork')(source);
         }

lodash.js

@@ -1204,6 +1204,34 @@
       return result;
     }
 
+    /**
+     * Checks if `func` references the `this` keyword.
+     *
+     * @private
+     * @param {Function} func The function to inspect.
+     * @returns {Boolean} Returns `true` if `this` is referenced, else `false`.
+     */
+    function hasThis(func) {
+      var result = func.__hasThis__;
+      if (typeof result == 'boolean') {
+        return result;
+      }
+      result = reThis.test(fnToString.call(func));
+      defineProperty(func, '__hasThis__', {
+        'configurable': true,
+        'enumerable': false,
+        'writable': true,
+        'value': result
+      });
+      return result;
+    }
+    // fallback for older browsers
+    if (!defineProperty || !reThis) {
+      hasThis = function() {
+        return true;
+      };
+    }
+
     /**
      * A fallback implementation of `isPlainObject` which checks if a given `value`
      * is an object created by the `Object` constructor, assuming objects created
@@ -4784,7 +4812,7 @@
           return result;
         };
       }
-      if (typeof thisArg == 'undefined' || (reThis && !reThis.test(fnToString.call(func)))) {
+      if (typeof thisArg == 'undefined' || !hasThis(func)) {
         return func;
       }
       if (argCount === 1) {