Understanding context of self and this

By default, self refers to window object. Actually self is window.self which is different to this.

If we are using a function that is executed in the global scope and is not in strict mode, this defaults to window, hence

    window.self === window; // true

 function checkContext() {
    console.log(window.self === window); // true
    console.log(window.self === this) // true
    console.log(this === window) // true
}
checkContext(); 
//Output
// true
// true
// true

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

checkContext.call({}); 
//Output
// true
// false
// false

In the context of service workers, self refers to the WorkerGlobalScope

As per MDN, 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).