将此对象绑定到高阶函数中的原始对象

Question

In JavaScript, is it possible to bind the this object to a function returned by a higher-order function? The code sample below is essentially what I am using:

var restrict = function(restrictOn, fn) {
   var that = this;

   return function() {
      if (arguments[0] === restrictOn) {
         throw new Error("Condition not met");
      }

      return fn.apply(/*original object*/that, arguments);
   };
};

var MyConstr = function(name) {
   this.name = name;
};
MyConstr.prototype.sayNameWhenNotThree = restrict(3, function() {
   return this.name;
});

var myObj = new MyConstr("Fido");
myObj.sayNameWhenNotThree(3); // Throws error - OK
myObj.sayNameWhenNotThree(5); // SHOULD return "Fido" - does not

In this example, the restrict() function correctly passes through to the function that it is wrapping, but it is not executing in the context of the myObj function. I have tried various this bindings in the apply call, but I cannot figure out how to retain the binding to the original object. Can this be done cleanly?

Answer 1

You need to use this within your inner function:

var restrict = function(restrictOn, fn) {
   /* at this point this refers to whatever context restrict 
   is called in, in this case - it's window */

   return function() {
      if (arguments[0] === restrictOn) {
         throw new Error("Condition not met");
      }
      /* at this point this refers to the proper target that the returned
      function is being assigned to */
      return fn.apply(this, arguments);
   };
};