If you're not in a JIT, for-of performance is going to be terrible too. Instead of creating a single function (which may not happen _anyway_ in the non-jit cas

If you're not in a JIT, for-of performance is going to be terrible too. Instead of creating a single function (which may not happen _anyway_ in the non-jit case, depending on how it's implemented) you now have to create an iterator result object for every single thing you get out of the iterator. In a JIT, forEach is actually _easier_ to optimize well (just need to inline the callback function, though of course that can fail for various reasons) than for-of (need to inline the calls to next() on the iterator, need to do escape analysis on the return value of next(), need to do scalar replacement on the return value of next; note that this is a strict superset of the work needed to optimize forEach). for-of can be nicer to read, and can work on arbitrary iterables. Those are its key strengths. Performance just isn't, unfortunately.