Understanding Javascript scope with "var that = this"

Question

Say I have the following property method in an object:

  onReady: function FlashUpload_onReady()
  {
     Alfresco.util.Ajax.jsonGet({
       url: Alfresco.constants.PROXY_URI + "org/app/classification",
       successCallback: {
         fn: function (o) {
           var classButtonMenu = [],
               menuLabel, that = this;

           var selectButtonClick = function (p_sType, p_aArgs, p_oItem) {
               var sText = p_oItem.cfg.getProperty("text");
               that.classificationSelectButton.set("label", sText);
           };

           for (var i in o.json.items) {
             classButtonMenu.push({
               text: o.json.items[i].classification,
               value: o.json.items[i].filename,
               onClick: {fn: selectButtonClick}
             });
           }

           this.classificationSelectButton = new YAHOO.widget.Button({
             id: this.id + "-appClassification",
             type: "menu",
             label: classButtonMenu[0].text,
             name: "appClassification",
             menu: classButtonMenu,
             container: this.id + "-appClassificationSection-div"
           });
         },
         scope: this
       },
       failureMessage: "Failed to retrieve classifications!"
     });

It took me some guess work to figure out that in the selectButtonClick function that I needed to reference that instead of this in order to gain access to this.classificationSelectButton (otherwise it comes up undefined), but I'm uncertain as to why I can't use this. My best guess is that any properties in the overall object that gets referenced within new YAHOO.widget.Button somehow looses scope once the constructor function is called.

Could someone please explain why it is that I have to reference classificationSelectButton with var that = this instead of just calling `this.classificationSelectButton'?

Answer 1

The most important thing to understand is that a function object does not have a fixed this value -- the value of this changes depending on how the function is called. We say that a function is invoked with some a particular this value -- the this value is determined at invocation time, not definition time.

• If the function is called as a "raw" function (e.g., just do someFunc()), this will be the global object (window in a browser) (or undefined if the function runs in strict mode).
• If it is called as a method on an object, this will be the calling object.
• If you call a function with call or apply, this is specified as the first argument to call or apply.
• If it is called as an event listener (as it is here), this will be the element that is the target of the event.
• If it is called as a constructor with new, this will be a newly-created object whose prototype is set to the prototype property of the constructor function.
• If the function is the result of a bind operation, the function will always and forever have this set to the first argument of the bind call that produced it. (This is the single exception to the "functions don't have a fixed this" rule -- functions produced by bind actually do have an immutable this.)

Using var that = this; is a way to store the this value at function definition time (rather than function execution time, when this could be anything, depending on how the function was invoked). The solution here is to store the outer value of this in a variable (traditionally called that or self) which is included in the scope of the newly-defined function, because newly-defined functions have access to variables defined in their outer scope.

Answer 2

Because this changes its value based on the context it's run in.

Inside your selectButtonClick function the this will refer to that function's context, not the outer context. So you need to give this a different name in the outer context which it can be referred to by inside the selectButtonClick function.

Answer 3

There's lexical scope: variables declared in functions and arguments passed to functions are visible only inside the function (as well as in its inner functions).

var x = 1; // `1` is now known as `x`
var that = this; // the current meaning of `this` is captured in `that`

The rules of lexical scope are quite intuitive. You assign variables explicitly.

Then there's dynamic scope: this. It's a magical thing that changes it's meaning depending on how you call a function. It's also called context. There are several ways to assign a meaning to it.

Consider a function:

function print() { console.log(this); }

Firstly, the default context is undefined in strict mode and the global object in normal mode:

print(); // Window

Secondly, you can make it a method and call it with a reference to an object followed by a dot followed by a reference to the function:

var obj = {};
obj.printMethod = print;
obj.printMethod(); // Object

Note, that if you call the method without the dot, the context will fall back to the default one:

var printMethod = obj.printMethod;
printMethod(); // Window

Lastly, there is a way to assign a context is by using either call/apply or bind:

print.call(obj, 1, 2); // Object
print.apply(obj, [ 1, 2 ]); // Object

var boundPrint = print.bind(obj);
boundPrint(); // Object

To better understand context, you might want to experiment with such simple examples. John Resig has very nice interactive slides on context in JavaScript, where you can learn and test yourself.

Answer 4

Storing it in a variable lets you access it in other scopes where this may refer to something else.

See https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Operators/this, http://www.quirksmode.org/js/this.html and What is the scope of variables in JavaScript? for more information about the this keyword.

Answer 5

The this reference doesn't work when a method of a class is called from a DOM event. When a method of an object is used as an event handler for onclick, for example, the this pointer points to the DOM node where the event happened. So you have to create a private backup of this in the object.

Answer 6

this is a keyword in javascript, not a default variable defined within every function, hence, as Gareth said, this will refer to the context in which the function is invoked, or the global object if there's no context.