The inheritance of javascript

Maybe this question is easy,but I can't understand now.

String.prototype.self=function()
{
    return this;
}
var s="s";

alert开发者_开发技巧("s".self()=="s".self()) //false;
alert(s.self()==s.self()) //false;

If you know the reason, please tell me why the result is "false".

---

That's because when a property is accessed from a primitive value, such as "s", the property accesors coerce it internally ToObject, and the comparison fails because it checks two different object references.

For example:

String.prototype.test = function() {
  return typeof this;
}

"s".test(); // "object"

It's like comparing:

new String("s") == new String("s"); // false