Override default behavior of comparison operators in JavaScript

Question

I have a custom Javascript class (created using John Resig's Simple Javascript Inheritance). I want to be able to compare two instances of this class, using the ==, <, >, >=, and <= symbols.

How do I override the comparators of my custom class?

Answer 1

Try overriding valueOf(). Then you can write stuff like this:

if (obj1.valueOf() === obj2.valueOf())
if (obj1.valueOf() < obj2.valueOf())
if (obj1.valueOf() > obj2.valueOf())

So whenever I need a special JavaScript object type to override the comparison I just add valueOf to the prototype. It works great for primitive types as well since valueOf just returns the value.

Just watch out for nulls.

Answer 2

Lee is correct, if you implement valueOf then when comparing objects (not with === or !===) this will be used but you'll have to use toString as well because it's used when sorting arrays for some reason.

function Test(value){
  this.value=value;
}
Test.prototype.toString=function(){
  console.log("tostring called");
  // could do something with case sensitiveness here
  return new String(this.valueOf());
}
Test.prototype.valueOf=function(){
  console.log("valueof called");
  return this.value;
}

var t1=new Test(11);
var t2=new Test(1.1);
var arr=[t1,t2];
console.log('sorted',arr.sort().map(o=>o.value));
console.log('larger',t1>=t2);