Question

Is it possible to override the equivalence comparison in Javascript?

The closest I have gotten to a solution is by defining the valueOf function and invoking valueOf with a plus in front of the object.

This works.

equal(+x == +y, true);

But this fails.

equal(x == y, true, "why does this fail.");

Here are my test cases.

var Obj = function (val) {
    this.value = val;
};
Obj.prototype.toString = function () {
    return this.value;
};
Obj.prototype.valueOf = function () {
    return this.value;
};
var x = new Obj(42);
var y = new Obj(42);
var z = new Obj(10);
test("Comparing custom objects", function () {
    equal(x >= y, true);
    equal(x <= y, true);
    equal(x >= z, true);
    equal(y >= z, true);
    equal(x.toString(), y.toString());
    equal(+x == +y, true);
    equal(x == y, true, "why does this fails.");
});

Demo here: http://jsfiddle.net/tWyHg/5/

Answer 1

That is because the == operator doesn’t compare only primitives, therefore doesn’t call the valueOf() function. Other operators you used do work with primitives only. I’m afraid you cannot achieve such thing in Javascript. See http://www.2ality.com/2011/12/fake-operator-overloading.html for some more details.