What does “return this” do within a javascript function?

It refers to the object instance on which the method is currently being called. It’s used for chaining. For example, you could do something like this:

myObject.foo().bar();

Since foo returns this (a reference to myObject), bar will be called on the object too. This is the same thing as doing

myObject.foo();
myObject.bar();

But requires less typing.

Here is a more complete example:

function AnimalSounds() {}

AnimalSounds.prototype.cow = function() {
    alert("moo");
    return this;
}

AnimalSounds.prototype.pig = function() {
    alert("oink");
    return this;
}

AnimalSounds.prototype.dog = function() {
    alert("woof");
    return this;
}

var sounds = new AnimalSounds();

sounds.cow();
sounds.pig();
sounds.dog();

sounds.cow().pig().dog();

http://jsfiddle.net/jUfdr/