This

and

Prototype

This attached

At its fundamental, its the calling context. 

var foo = {
   bar: 123,
   bas: function () {
       console.log('inside this.bar is:', this.bar);
   }
}

console.log('foo.bar is: ', foo.bar); // foo.bar is: 123
foo.bas(); // inside this.bar is: 123

IMHO better than self  in python.

This Global

In the absence of a context ... its window

function foo() {
    console.log('is this called from global? : ', this === global); // true
}
foo();

First Class functions

Just attach a function to an object and you get the *right* this.

var foo = {
    bar: 123 
};

function bas() {
    if (this === global)
        console.log('called from global');
    if (this === foo)
        console.log('called from foo');
}

// global context
bas(); // called from global

// from foo 
foo.bas = bas;
foo.bas(); // called from foo

Advantage of calling context

Forcing this Inside a function

Ways of this