Optional `new` operator

Question

Reading through the jQuery documentation about the event object and its constructor $.Event() I see:

The new operator is optional [when calling the constructor].

That's cool! How did the jQuery people pull such a trick?

Answer 1

John Resig explains this pretty well: http://ejohn.org/apps/learn/#36 and http://ejohn.org/apps/learn/#38

Basically, Event is a function and an object (functions are objects). The first line of Event checks if it is being called as a function or as an instance of the Event object (with the new operator).

If you are looking for specifically how jQuery does it, look at line 3134-3138 of the jQuery source:

jQuery.Event = function( src, props ) {
    // Allow instantiation without the 'new' keyword
    if ( !this.preventDefault ) {
        return new jQuery.Event( src, props );
    }

And explanation for this is on the jQuery forms.

Basically, on lines 3178-3194 the preventDefault event is added to the Event prototype. If the event is instantiated with new it will be given this preventDefault method. Otherwise, it won't be defined.