Question

I need to do some experiment, and I need to use some kind of unique identifier for objects in JavaScript so I can see if they are the same.

I don’t want to use equality operators. Instead, I need something like the id() function in Python.

Does something like this exist?

Answer 1

Update My original answer below was written 6 years ago in a style befitting the times and my understanding. In response to some conversation in the comments, a more modern approach to this is as follows:

(function() {
    if ( typeof Object.id != "undefined" ) return;

    var id = 0;

    Object.id = function(o) {
        if ( typeof o.__uniqueid != "undefined" ) {
            return o.__uniqueid;
        }

        Object.defineProperty(o, "__uniqueid", {
            value: ++id,
            enumerable: false,
            // This could go either way, depending on your 
            // interpretation of what an "id" is
            writable: false
        });

        return o.__uniqueid;
    };
})();

var obj = { a: 1, b: 1 };

console.log(Object.id(obj));
console.log(Object.id([]));
console.log(Object.id({}));
console.log(Object.id(/./));
console.log(Object.id(function() {}));

for (var k in obj) {
    if (obj.hasOwnProperty(k)) {
        console.log(k);
    }
}
// Logged keys are `a` and `b`

If you have archaic browser requirements, check here for browser compatibility for Object.defineProperty.

The original answer is kept below (instead of just in the change history) because I think the comparison is valuable.

---

You can give the following a spin. This also gives you the option to explicitly set an object’s ID in its constructor or elsewhere.

(function() {
    if ( typeof Object.prototype.uniqueId == "undefined" ) {
        var id = 0;
        Object.prototype.uniqueId = function() {
            if ( typeof this.__uniqueid == "undefined" ) {
                this.__uniqueid = ++id;
            }
            return this.__uniqueid;
        };
    }
})();

var obj1 = {};
var obj2 = new Object();

console.log(obj1.uniqueId());
console.log(obj2.uniqueId());
console.log([].uniqueId());
console.log({}.uniqueId());
console.log(/./.uniqueId());
console.log((function() {}).uniqueId());

Take care to make sure that whatever member you use to internally store the unique ID doesn’t collide with another automatically created member name.