javascript - a String.prototype's "this" doesn't return a string?

Question

What is going on here? Just when I thought I knew JS inside and out, this gem comes up.

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

alert(typeof 'foo'.doNothing()) // object
alert(typeof 'foo')             // string

http://jsfiddle.net/dJBmf/

This is breaking some things that expect a string, such as jQuery's .text(str) method.

Answer 1

To make sure you're always getting a string, trying using this code:

String.prototype.doNothing = function() {
    return this.toString();
};

alert(typeof 'foo'.doNothing())
alert(typeof 'foo')

In your original code, this is being returned as the string object and not the actual string.