What is this javascript idiom?

Question

I was going through broccoli plugins and I see this line a lot. What is it used for?

function MyCompiler (arg1, arg2, ...) {
  if (!(this instanceof MyCompiler)) return new MyCompiler(arg1, arg2, ...);
  ...
};

Answer 1

That is so that you can use it with or without the new keyword.

E.g.:

var comp = new MyCompiler();

or:

var comp = MyCompiler();

If you call it as a function, it will call itself with the new keyword and return the instance.