How can you define how objects are compared in JavaScript

I want to define a JavaScript class, Foo.

Foo = function(value){
    this.value = value;
};

I will create "instances" of my Foo:

foo1 = new Foo(1);
foo2 = new Foo(1);

and I want my instances of Foo to be comparable with each other using the standard == equality operator:

foo1 == foo2;      // this should be true

I can not find a way to do this. I thought I was on to something with the valueOf() function, but this is only useful when one side of the comparison is a primitive, not as above where both are of type object.

Have I missed something really simple akin to Ruby's

def ==(obj); end

From stackoverflow

• I don't think this is possible using javascript. You have to write your own function e.g. isEqual()

• JavaScript is in this regard like its namesake Java: No operator overloading. Use a custom method like equals() instead.

• JavaScript does not have operator overloading. See this discussion covering your question.

  Christoph : every minute an answer which says essentially the same thing - there sure are a lot of people with nothing better to do ;)

• Not in a clean way...

  Foo = function (value){
      this.value = value;
  };
  

  Override the toString function of your own object:

  Foo.prototype.toString = function( ){ return this.value.toString(); }
  

  Creating two test objects:

  foo1 = new Foo(1);
  foo2 = new Foo(1);
  

  If the values of your objects is strings or numbers, you can have the javascript engine to convert your objects to a string by adding them to an empty string:

  alert( ""+foo1 === ""+foo2 ); //Works for strings and numbers
  

  The same thing but cleaner:

  alert( foo1.toString() === foo2.toString() ); //Works for strings and numbers
  

  If the values of your objects is numeric only, you can use the unary + operator to convert the object to a number:

  alert( +foo1 === +foo2 ); //Works for numbers
  

  However, I recommend you to both define the toString as above and also an equals:

  Foo.prototype.equals=function(b){return this.toString() === b.toString();}
  

  And then call it like this:

  alert ( foo1.equals(foo2) );
  

  Since you now have defined toString you can:

  alert(foo1); // Alerts the value of foo1 instead of "[Object object]"
  alert("foo1: " + foo1); // Alerts "foo1: 1". Useful when debugging.
  

  some : You can ofcourse put whatever you want in the equals method to determinate if the two objects are equal enough without being the same object.