[discussion] for loops: benchmarks for io.js wanted

[hellboy81]

The only benchmarks I found are not up-do-date

V8 (Chrome, Node)
Array: for loop      :: 64 +/- 1%
Array: reverse while :: 60 +/- 0%
Array: forEach()     :: 235 +/- 0%

Should in io.js for Array-s also be used for loop instead of forEach?

[Fishrock123]

@hellboy81 these apply to all javascript runtimes.

forEach() is always usually (see below) slower than inline? loops. (for & reverse while)

[petkaantonov]

forEach() is always slower than inline? loops. (for & reverse while)

Under very specific conditions it could theoretically be almost equally fast 😃

[mscdex]

This jsperf (and its various revisions) has some interesting results regarding loop performance when visiting array elements (both destructive and not).

[chrisdickinson]

The problem with including this sort of benchmark is that it depends on a variety of factors:

1. What you're looping over (is it a sparse array? a dense array? a uint8array? a smiarray?)
2. What happens after the loop (does it have to soft de-opt due to instructions after OSR?)
3. What's in the body of the loop (can the operations inside the loop be OSR'd? can functions in the body be inlined?)
4. How is the body of the loop defined? If it's a forEach function, is the function in-situ, or is it hoisted to an outer scope to preclude allocations / share optimizations?

There's no one good answer aside from to benchmark your specific use case. If it comes down to choosing one form over another, in nearly all cases I would suggest avoiding basing the decision on the apparent performance differences, since they're subject to change due to optimizations in new versions of V8. That is to say, if you need to close over iterated values, use .forEach or for-of, if you don't, use for or while.

It's unlikely that we'll include a benchmark for various for-loop forms in this repository, since the data from such a benchmark could be misleading. If you're interested in these sorts of things, though, you might watch @trevnorris' esbench project, since I believe he's attempting collect that sort of perf information.