Recursive Array.flat

There was much talk about Array.prototype.flat during its early stages, starting with the name alone. Many developers preferred the name flatten but the spec differed from MooTools' implementation. MooTools would recursively flatten an array but the new, official flat implementation defaults one level of flattening,.

The current implementation of Array.prototype.flat is:

[1, 2, [3], [[4]]].flat(/* depth */);
// [1,2,3,[4]]

.flat only flattens arrays to one level by default, but what if you want a truly flattened array? You can use Infinity and flat's depth argument to make that happen:

[1, 2, [3], [[4]], [[[[[[6]]]]]]].flat(Infinity);
// [1,2,3,4,6]

I find the method name a bit misleading but I understand why they went to a single level. The method name smush was thrown around, which would've been the worst method name since stringify!