Well that decision is pretty necessary when you realize that JS has no syntax to indicate a function is variadic (we use the arguments magic variable, but use of it does not necessarily indicate that a function is variadic) and that implementation supplied functions are not required to have their arity exposed via Function.prototype.length (http://es5.github.io/#x15.3.5.1).

There's no way to know, even at run-time, whether a function is being called with too few or too many arguments, since that's equivalent to the halting problem. So the sensible alternative is just to default everything to undefined, and silently ignore extraneous arguments.

But yes, if JS was strict with how it handled argument definition lists and had support for indicating infinite arity, I'd agree, this would be a WTF, or at least strange. But I think it makes a lot of sense, all things considered.