Depends on the team. I would not approve anything other than using a runtime library or importing 'core-js/fn/array/is-array' from the wonderfully modular core-

Depends on the team. I would not approve anything other than using a runtime library or importing 'core-js/fn/array/is-array' from the wonderfully modular core-js: https://github.com/zloirock/core-js https://github.com/zloirock/core-js Although for this particular strawman I think `foo.constructor === Array` is probably enough.