What values can a constructor return to avoid returning this?

Question

What are the exact circumstances for which a return statement in Javascript can return a value other than this when a constructor is invoked using

Answer 1

As a side note, the return value or this is just part of the equation.

For example, consider this:

function Two() { return new Number(2); }
var two = new Two;
two + 2; // 4
two.valueOf = function() { return 3; }
two + 2; // 5
two.valueOf = function() { return '2'; }
two + 2; // '22'

As you can see, .valueOf() is internally used and can be exploited for fun and profit. You can even create side effects, for example:

function AutoIncrementingNumber(start) {
    var n = new Number, val = start || 0;
    n.valueOf = function() { return val++; };
    return n;
}
var auto = new AutoIncrementingNumber(42);
auto + 1; // 43
auto + 1; // 44
auto + 1; // 45

I can imagine this must have some sort of practical application. And it doesn't have to be explicitly a Number either, if you add .valueOf to any object it can behave as a number:

({valueOf: function() { return Math.random(); }}) + 1; // 1.6451723610516638

You can exploit this to make an object that always returns a new GUID, for instance.