Daddy, what does the "new" keyword do?

Sit down, and I'll tell you. Ok, it creates a new context object for you, then you'll have a new this to work with. This object will be the return value of the expression starting with new. For example,

> function Dog() { this.name = "poo" }
undefined
> new Dog().name
'poo'

But daddy, what happens if you accidentally forget to use new?

Um, er, lemme see.

> Dog().name
TypeError: Cannot read property 'name' of undefined

So, your constructor gets called, but nothing is returned and your Dog is undefined.

But what happened to the name?

Well, it's the name of the Window or what ever your "global object" is, depending on your execution environment. But its bed time now, good night.

< one hour passes >

Dad! I can't get no sleep! What if I return something from my Dog constructor?

< starts node.js >

Well, if you call it with new, the return value is discarded. If you forget the new, the expression evaluates to the return value.

But dad, that makes no sense and can cause unexpected application errors at runtime!!

That's right, son. That's why daddy avoids the new and this keywords. Now good night.