String.prototype.foo = function() {
   if (!this.bar) {
        this.bar = [];
   }
   // this.bar stays always undefined

String.prototype.bar = [];

String.prototype.foo = function() {
   this.bar.push({
     o: this,
     p: "baz"
   });
   return this;

x.foo();
y.foo();

alert("".bar[0].o === "".bar[1].o);

yields false in "all" browsers. Chrome returns true.

Any comments on that?

Gregor

It's a lousy standard at that point - requireing the creation of a new
object every time you access a property of a basic string value
... like the *length*! - but still, it is the spec.

I.e., some optimization that prevents creating new objects, that
probably works well for built-in functions, is also applied to
user-added methods on String.prototype.
Same problem exists for Number and Boolean.