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.

A slight addition to this as people may encounter this in the context of service workers, in which case it means something slightly different.

You might see this in a service worker module:

self.addEventListener('install', function(e) {
  console.log('[ServiceWorker] Install');
});

Here self refers to the WorkerGlobalScope, and this is the standard method for setting event listeners.

From Mozilla docs:

By using self, you can refer to the global scope in a way that will work not only in a window context (self will resolve to window.self) but also in a worker context (self will then resolve to WorkerGlobalScope.self).