jQuery.Deferred() - "new" operator is optional?

Question

How does jQuery implement its Deferred object so that new operator is optional as in var x = $.Deferred(); ?

Answer 1

Here is a pattern to achieve that...

$.Deferred = function() {
    if ( ! (this instanceof $.Deferred)) {
        return new $.Deferred;
    }
}

It works because this in a constructor is set to the new object. instanceof will tell you if the LHS operand has the RHS operand in its prototype chain. If this condition isn't true, the function will return an instantiated version of the object.