Difference between this and self in JavaScript

Unless set elsewhere, the value of self is window because JavaScript lets you access any property x of window as simply x, instead of window.x. Therefore, self is really window.self, which is different to this.

window.self === window; // true

If you’re using a function that is executed in the global scope and is not in strict mode, this defaults to window, and therefore

function foo() {
    console.log(
        window.self === window, // is self window?
        window.self === this,   // is self this?
        this === window         // is this window?
    );
}
foo(); // true true true

If you’re using a function in a different context, this will refer to that context, but self will still be window.

// invoke foo with context {}
foo.call({}); // true false false

You can find window.self defined in the W3C 2006 working draft for the Window Object here.