What underlies this JavaScript idiom: var self = this?

What underlies this JavaScript idiom: var self = this?

I saw the following in the source for WebKit HTML 5 SQL Storage Notes Demo:
function Note() {
var self = this;

var note = document.createElement(‘div’);
note.className = ‘note’;
note.addEventListener(‘mousedown’, function(e) { return self.onMouseDown(e) }, false);
note.addEventListener(‘click’, function() { return self.onNoteClick() }, false);
this.note = note;
// …
}

The author uses self in some places (the function body) and this in other places (the bodies of functions defined in the argument list of methods). What’s going on? Now that I’ve noticed it once, will I start seeing it everywhere?

Solutions/Answers:

Solution 1:

See this article on alistapart.com

self is being used to maintain a reference to the original this even as the context is changing. It’s a technique often used in event handlers (especially in closures).

Solution 2:

I think the variable name ‘self’ should not be used this way anymore, since modern browsers provide a global variable self pointing to the global object of either a normal window or a WebWorker.

To avoid confusion and potential conflicts, you can write var thiz = this or var that = this instead.

Solution 3:

Yes, you’ll see it everywhere. It’s often that = this;.

See how self is used inside functions called by events? Those would have their own context, so self is used to hold the this that came into Note().

The reason self is still available to the functions, even though they can only execute after the Note() function has finished executing, is that inner functions get the context of the outer function due to closure.

Solution 4:

It should also be noted there is an alternative Proxy pattern for maintaining a reference to the original this in a callback if you dislike the var self = this idiom.

As a function can be called with a given context by using function.apply or function.call, you can write a wrapper that returns a function that calls your function with apply or call using the given context. See jQuery’s proxy function for an implementation of this pattern. Here is an example of using it:

var wrappedFunc = $.proxy(this.myFunc, this);

wrappedFunc can then be called and will have your version of this as the context.

Solution 5:

The variable is captured by the inline functions defined in the method. this in the function will refer to another object. This way, you can make the function hold a reference to the this in the outer scope.

Solution 6:

It’s a JavaScript quirk. When a function is a property of an object, more aptly called a method, this refers to the object. In the example of an event handler, the containing object is the element that triggered the event. When a standard function is invoked, this will refer to the global object. When you have nested functions as in your example, this does not relate to the context of the outer function at all. Inner functions do share scope with the containing function, so developers will use variations of var that = this in order to preserve the this they need in the inner function.