This is part of the language not just a an interface (though I'm not sure it is an interface, I think it is an object primitive). You wouldn't be able to write

This is part of the language not just a an interface (though I'm not sure it is an interface, I think it is an object primitive). You wouldn't be able to write this using plain Javascript. Which is why there are no polyfills for client-side code. Incidentally, PHP does it a similar way to Python with so-called "magic methods" but I can see the benefit to doing it this way.