Lately I'm spending some time coding some libraries in JS to achieve a solid & cross-browser framework. Not every browser out there supports the latest Javascript version, so a fallback solution is often needed. Some examples are the iterative methods in Array, as introduced in JS 1.7: every, filter, forEach, map, some, indexOf, lastIndexOf.
So, as extending a built-in is often a bad idea, the direction taken was to create a subclass of Array and extend this one. No wonder it's a hard path, but we can walk it on our own.