Map.prototype.forEach fires the callback for an empty Map

This bug in the polyfill impacts React 16 users, and we are shipping React 16 very soon.

Specifically, Map.prototype.forEach attempts to skip deleted items but then always calls the callback. The problem is that it may be the case that all items were deleted, in which case it should have skipped the callback. In other words, this check should be done just before calling the callback as well.

Specific reproducing case:

var x = new Map();
x.set(42, 'hi');
x.delete(42);
x.forEach(function() {
  throw new Error('Should never get here!');
});

This breaks with the provided Map polyfill but works with the native implementation.

[JSteunou]

@green-arrow 3.22 was shipped with the fix

[gaearon]

Yeah, I didn’t meant to be pushing 😃. Just letting you know.