Question

Typically, in a constructor function, the object that is bound to this within the function is returned by it when it is called with the new prefix. But I would imagine that it’s also possible (I think I even read this in crockford’s book) to manually return your own value. Are there any places where such a practice is useful?

Answer 1

If you return a value type from the constructor, you’ll get different behavior depending on if new was used. That’s how String works. Look at this object in your JavaScript console:

{
    s: String("abc"),
    S: new String("abc")
}

Little s contains a string value, but big S contains a string Object. A subtle difference, perhaps.

You could go way beyond that and use the same function for different purposes:

function Foo() {
    return "abc";
}
Foo.prototype.DoSomething = function () {
    // something completely unrelated to "abc"
};
var a = Foo();      // The string "abc".  Does not have a DoSomething() method.
var b = new Foo();  // An Object with a DoSomething() method.  Doesn't know about "abc".

Depending on whether new was used, you’d get back something completely different.