Override the Equivalence Comparison in Javascript

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.

Answer 2

Piggybacking on @Corkscreewe:

This is because you are dealing with Objects and the equivalency operators are only going to compare whether two variables reference the same Object, not whether the two Objects are somehow equal.

One solution is to use "+" in front of the variables and define a valueOf method for the Objects. This calls the valueOf method on each object to "cast" its value to a Number. You have already found this, but understandably do not seem very satisfied with it.

A more expressive solution might be to define an equals function for your Objects. Using your examples above:

Obj.prototype.equals = function (o) {     return this.valueOf() === o.valueOf(); };  var x = new Obj(42); var y = new Obj(42); var z = new Obj(10);  x.equals(y); // true x.equals(z); // false 

I know this doesn't do exactly what you want (redefine the equivalency operators themselves), but hopefully it will get you a little closer.