JavaScript identity operator on strings

Question

I'm trying to write a prototype for determining if a string is empty. It's really just playing with JS and prototype, nothing important. Here's my code:

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

Notice I used the === identity comparison instead of == equality. When I run the function with the above definition:

"".IsEmpty(); // false

If I chagne the definition to use == as:

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

The new def'n will do:

"".IsEmpty(); // true

I don't understand why === doesn't work since "" is identical to ""