http: improve for-loop readability in _http_outgoing.js

[gengjiawen]

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

[BridgeAR]

Object.values() has not been improvement significantly since the last time I checked: https://bugs.chromium.org/p/v8/issues/detail?id=8071

If we change something, I suggest to use Object.keys() instead of in.

[gengjiawen]

@BridgeAR Can you review this again ? thanks.

[BridgeAR]

I would still check for if (headers != null) {. That way if there are no headers, we spare two extra checks.

[gengjiawen]

@BridgeAR Good point. I will add it.

[gengjiawen]

@BridgeAR Done.

[BridgeAR]

CI https://ci.nodejs.org/job/node-test-pull-request/21268/

[mcollina]

This is a very sensitive hot path. I’m -1 to any changes that do not improve our benchmarks.

[lpinca]

I think this has 0 impact on performance. Will run a benchmark CI to verify.

[mcollina]

@lpinca I'm pretty sure it has some impact. for-of  loops are slower than for(;;).
https://github.com/nodejs/node/pull/26408/files#diff-286202fdbdd74ede6f5f5334b6176b5cR306.

[lpinca]

https://jsperf.com/for-of-vs-for-loop they seem to be on par on Chrome 72.

[lpinca]

If this is a problem for 10.x 8.x and 6.x we can add the required "do not land" labels.

[mscdex]

https://jsperf.com/for-of-vs-for-loop they seem to be on par on Chrome 72.

The for-loop case in that benchmark consistently shows it being faster for me with Chrome 72

[gengjiawen]

On chrome 72, for-of is faster than for-loop.

Update:
sometimes, the perf is very near