JavaScript: Augmenting types - Purpose of "return this" [duplicate]

Question

I'm reading JavaScript - The Good Parts currently. So I've dealing with augmentation of types. I understand the motivation and the implementation. But if I look at the code ...

Function.prototype.method = function(ident, funct) {
    this.prototype[ident] = funct;
    return this; // No idea. For what?
};

... then I don't understand the purpose of the return. I have put the return in comments. That doesn't have an effect. It functioned anyway.

My complete code:

Function.prototype.method = function(ident, funct) {
    this.prototype[ident] = funct;
    return this;
};

Date.method('sayHello', function() {
    alert(new Date().toString());
});

var myDate = new Date();

myDate.sayHello();

So what it is for?

Answer 1

Typically this is done so you can chain method calls, so called "fluent interfaces":

obj.method().anotherMethod().yetAnotherMethod()

E.g.:

'string'.toUpperCase().substr(2).repeat(3)

In case of the string, another new string is being returned instead if this, but you get the idea why it's useful.