Update: add options to (dis)allow comma expression in for-loops in no-sequences

[danielrentz]

What rule do you want to change?

no-sequences

This is a follow-up of #14197 (Add option to disable the "in-parentheses" exception).
Consequently, the rule should contain an option to decide whether commas are allowed in for-loop heads.

Does this change cause the rule to produce more or fewer warnings?

More.

How will the change be implemented? (New option, new default behavior, etc.)?

Currently, comma expressions are always allowed in for-loop init and update part, but not in test part where it does not make any sense.

I have two alternatives in mind:

1. One option for init and update part of the for-loop, e.g. "allowInForLoop". But this is confusing as it obfuscates the fact that comma will still not be allowed in test part.
2. Distinct options for init and update part:
   • allowForLoopInit (following "ignoreForLoopInit" from init-declarations)
   • allowForLoopAfterthoughts (as in no-plusplus) (or allowForLoopUpdate which is shorter and better remarkable?)

All options will be true by default for compatibility.

Please provide some example code that this change will affect:

let i, j;
for (i = 0, j = 0; i < 100; i += 1, j += 2) {
  
}

What does the rule currently do for this code?

Passes.

What will the rule do after it's changed?

Pass or warn according to options.

Are you willing to submit a pull request to implement this change?

Yes.

[mdjermanovic]

I'm not much in favor of this option, as it's mostly stylistic. Comma in a for-loop init/update doesn't look like a possible error or a bad practice. It's also easy to disallow this with no-restricted-syntax, the selector would be:

"ForStatement > SequenceExpression.init, ForStatement > SequenceExpression.update".

I'm in favor of #14197 as it can catch possible errors, and it doesn't seem easy to write a selector that would disallow sequence expressions everywhere except in for-loop init/update.

[danielrentz]

Sure it does not hide errors like #14197 but is mostly for completeness :)

[nzakas]

We aren’t moving forward with this, so closing.