Javascript scope referencing outer object

Question

Basically, I use a meta-class framework called Joose for Javascript that allows me to make use of a more elegant class syntax - but I don't know how I might go about referencing the scope of the object from within deeper methods of the class declaration. I also use require.js for dependemcy management...

Here's an example class definition:

   define([
      'jquery',
      'handlebars',
   ], function($, Handlebars){

      var MyClass = Class("MyClass", {

         //inheritance         
         isa: SuperClass,

         //instance vars
         has: {

            hello:{
               is: 'r',
               init: 'Hi There!',
            },

            someVarToBeSetUsingAjax:{
               is: 'rw',
               init: false,
            },
         },

         //methods
         methods: {

            init: function () {

               var self = this;
               self.getAjaxVar();

            },

            getAjaxVar: function() {

               var self = this;

               //HOW CAN I REFERENCE 'self' THROUGHOUT MY OBJECT?

               $.get('ajax/test.html', function(response) {
                 self.someVarToBeSetUsingAjax = response.value;
               });
            },

            //lots more methods...

         }
    });

   return MyClass;

});

Ok, so my issue is - in the AJAX function I have to write var self = this to get my object into the scope of the AJAX call - no problem. But, I find myself doing this for almost every single method in my class declaration! How can I reference self in all of the methods in a clean and efficient way? I know you can use scope in AJAX by setting a parameter, assume it's not just AJAX but other functions that close the scope to the outside.

Thanks.

Answer 1

Everytime you nest a function, you have to think about this. But if you dont nest a function, or that function doesn't use this you don't need to think about it.

        init: function () {
           var self = this;
           self.getAjaxVar();
        },

So in this case it's not necessary. This is exactly the same:

        init: function () {
           this.getAjaxVar();
        },

But here:

        getAjaxVar: function() {
           var self = this;

           $.get('ajax/test.html', function(response) {
             self.someVarToBeSetUsingAjax = response.value;
           });
        },

You create an inner function, and you want a reference to the original value of this, so you do have to alias this to self to make it accessible.

There isn't a way to fix this to a value from everywhere in your class.

---

That said, you do have some options.

Function.prototype.bind() can help.

var func = function() { return this.name };
var obj = { name: 'Bob' };
var boundFunc = func.bind(obj);
boundFunc(); // 'Bob'

bind will return a new function with this always set to a specific object.

So:

        getAjaxVar: function() {
           $.get('ajax/test.html', function(response) {
             this.someVarToBeSetUsingAjax = response.value;
           }.bind(this));
        },

Note this isn't supported in all browsers, you may need a shim for the old ones.

Or just get used to self = this.

---

I want to give a minor nod to coffeescript as well, because it supports declaration of functions that dont change the context when run.

obj = {
  name: "bob"
  sayHello: ->
    doSomeAjax({
      success: =>
        alert "successfully made " + this.name + " say hello!"
    })
}

obj.sayHello()

-> makes a normal function. But the fat arrow => will instead preserve the value of this inside and outside the function. It's very very handy in callbacks within instance methods. When compiled to JS, it basically does a self = this alias for you, using self within the inner function everytime to reference this. It's pretty slick.

In plain JS though, the most common pattern is simply self = this, stick to it.

Answer 2

The answer is to use this. You only need to create a closure (which you do with the var self = this; when you have a function which will be called external to the object. (as you do here with the return from the ajax call).

There is no other way to create a closure.

---

To be clear (since some will jump on any slight technical hand waving), you don't "need" to create a closure. But I think you should -- JavaScript was designed to work with closures, they work well and they are well understood by other JavaScript programmers.