ID mapping

gistfile1.js

function getId () {
    return getId.id++;
}
getId.id = 0;

function Foo() {
    this.__id__ = getId();
}

Foo.prototype.valueOf = function () {
    return this.__id__;
};



/* Example */
var foo = new Foo(),
    bar = {};

bar[foo] = 20;
bar[foo]; //20