Question

I understand why :

 output = new Array();

and

 output = [];

but why does this work?

 output = Array();

Answer 1

The Array() constructor is simply implemented such that calling it with new is unnecessary. It’s part of its semantic definition.

Built-in constructors like Array() are (probably) not written in JavaScript, but you can get the same effect in your own code:

function MyConstructor() {
  "use strict";
  var newObj = this || {};

  // ...

  return newObj;
}

When you invoke with new, the constructor will see that’s it’s got something bound to this. If you don’t, then this will be undefined (because of “use strict”; you could alternatively check to see whether this is the global object, which you’d have to do for old IE).

The return value from a constructor is not the value of a new expression – that’s always the newly-created object. When you call it without new, however, the return value will be used.

edit — RobG points out in a comment that for this to really work properly, the “synthetic” newObj that the function creates needs to be explicitly set up so that it’s got the proper prototype etc. That’s kind-of tricky; it might be simplest for the code to simply do this:

function MyConstructor() {
  "use strict";
  if (!this) return new MyConstructor();
  // ... or possibly using "apply" if you need parameters too
}

T.J. Crowder has written some awesome answers here on the subject of object/inheritance wrangling.