doc: add explanation why keep var with for loop in async_hooks

[lrecknagel]

Refs: nceu19-async_hooks

This comment will help contributors to understand why keeping var in some for loop instead changing at to let, as discussed with @mcollina

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[Trott]

Welcome, @lrecknagel and thanks for the pull request. I'm guessing this is from a Code + Learn event. I'm not sure the "let inside a loop is much slower than var" thing is true anymore in current V8. /ping @nodejs/v8

[mcollina]

This is an extremely tight loop in a code path that is hit a lot.
This is still true on Node 10 and it would mean that we would not be able to backport this change.
Given that this is a cosmetic change that might cause issues, I prefer to not change them.

[mcollina]

LGTM

[Trott]

This is an extremely tight loop in a code path that is hit a lot.
This is still true on Node 10 and it would mean that we would not be able to backport this change.
Given that this is a cosmetic change that might cause issues, I prefer to not change them.

Cool. Let's see if we can get that information into the comment. As it reads now, it could encourage someone to go through and change all let in loops to var throughout core.

[Trott]

I left two optional suggestions. It will save someone a little bit of git blame archaeology should these lines get moved around, edited, etc. But totally optional. Thanks.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/26587/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/27032/

[gireeshpunathil]

the windows failure sequential/test-inspector-contexts is known as in #30519

the windows failure parallel/test-inspector-wait-for-connection is known as in #30619

the linux containerized failure parallel/test-readline-async-iterators-destroy is reported #30660

[gireeshpunathil]

landed in 4506991

[Aschen]

👋 Just passing by here and I was curious so I ran some benchmarks if anyone is curious about this.

At the time of Node.js 6 it was a real performance gain but nowadays it's almost the same

Source