no-fallthrough should allow the comment inside of a block when the case is a single block

[bakkot]

What rule do you want to change?

no-fallthrough

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

fewer

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

new default behavior

Please provide some example code that this change will affect:

switch (foo) {
  case 1: {
    let x = value;
    console.log(value);
    // falls through
  }
  case 2: {
    doSomething();
  }
}

What does the rule currently do for this code?

Error

What will the rule do after it's changed?

Not error

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

Yes

---

This has been brought up before, in #9559 and #9080, and with a PR in #11016. On each of those occasions it's been closed automatically without any interest. But those just asked for the new behavior without really explaining why.

I want to make a case the proposed change is the right behavior. The fallthrough comment is intended to replace a break, which means that a reader of the code will be looking for it where they would expect to find the break. And the convention when using a braced case is that the break occurs within the braces, not outside of it. So that is where a reader should expect to find the comment, not (as eslint currently requires) after the braces. The placement required by the current rule actually makes the comment easier to miss.

[aladdin-add]

as a workaround, you can write the comment out the block:

switch (foo) {
  case 1: {
    let x = value;
    console.log(value);
  }
  // falls through
  case 2: {
    doSomething();
  }
}

IMHO, it seems a bug, no new option needed. but let's see what others thoughts. :)

[mdjermanovic]

IMHO, it seems a bug, no new option needed. but let's see what others thoughts. :)

Agreed. If consequent consists of only one statement, and that statement is BlockStatement, then the rule should also look for the comment before its closing }. The rule will still allow comment after }.

[bakkot]

I'll prepare a PR for this unless someone is already working on it.

[aladdin-add]

@bakkot please feel free to open a PR, thanks!

[bakkot]

Done: #14702.