Question

Possible Duplicate:
JS: var self = this?

When looking at arbitrary code written in JavaScript (e.g. on GitHub), many developers use var self = this and then use self instead of this to reference the current object.

What is the rationale behind this approach?

Answer 1

The value of this is contextual. Writing var self = this is a way to save the value of this in one context so that it can be used in another.

Example:

function Test() {
    var self = this;
    console.log(this);
    setTimeout(function() {
        console.log(this);
        console.log(self);
    }, 1000);
}

Prints:

Test {}
Window 11918143#comment15869603_11918143
Test {}

Notice that the value of this has changed, but we can still refer to the original value using self.

This works because functions in JavaScript “close over” the variables in their lexical scope (which is just a more technical way of saying that the inner function can see variables declared in the outer function). This is why we write var self = this; the variable self is available to all inner functions, even if those functions don’t execute until long after the outer function has returned.