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Fix: allow fallthrough comment inside block (fixes #14701) #14702

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merged 2 commits into from Jun 18, 2021

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bakkot
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@bakkot bakkot commented Jun 14, 2021

Prerequisites checklist

What is the purpose of this pull request? (put an "X" next to an item)

[x] Bug fix
[x] Changes an existing rule

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

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?

warn

What will the rule do after it's changed?

not warn

What changes did you make? (Give an overview)

Now when the case consists of a single BlockStatement the code will look for the fallthrough comment to be the final comment in the case, as well where it looked previously.

Is there anything you'd like reviewers to focus on?

In the situation in question - i.e., when the case consists of exactly one statement, which is a BlockStatement - I've chosen to check for the fallthrough comment in both places. This means that it becomes possible to have a comment after the fallthrough comment, as long as it's after the block, as in

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

  // some other comment
  case 2: {
    doSomething();
  }
}

I think that's correct, personally. However, I'm happy to be more restrictive and say that the fallthrough comment must also be the final comment: that is, to only look in the new place when there are no comments after the block.

@eslint-github-bot eslint-github-bot bot added the triage An ESLint team member will look at this issue soon label Jun 14, 2021
@mdjermanovic mdjermanovic added accepted There is consensus among the team that this change meets the criteria for inclusion bug ESLint is working incorrectly rule Relates to ESLint's core rules and removed triage An ESLint team member will look at this issue soon labels Jun 14, 2021
@mdjermanovic
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mdjermanovic commented Jun 14, 2021

@bakkot thanks for the PR!

In the situation in question - i.e., when the case consists of exactly one statement, which is a BlockStatement - I've chosen to check for the fallthrough comment in both places. This means that it becomes possible to have a comment after the fallthrough comment, as long as it's after the block, as in

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

  // some other comment
  case 2: {
    doSomething();
  }
}

I agree, I think it's fine to allow any comments after } when the fallsthrough comment is last in { ... }.

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@mdjermanovic mdjermanovic left a comment

All changes look good, I only have some small requests about the documentation and a few additional tests.

docs/rules/no-fallthrough.md Show resolved Hide resolved
tests/lib/rules/no-fallthrough.js Show resolved Hide resolved
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@mdjermanovic mdjermanovic left a comment

LGTM, thanks!

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@snitin315 snitin315 left a comment

LGTM

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@btmills btmills left a comment

Thanks, and congrats on your first ESLint pull request, @bakkot! This will be released today.

@btmills btmills merged commit 6a1c7a0 into eslint:master Jun 18, 2021
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@bakkot bakkot deleted the fallthrough-comment-in-switch-case branch Jun 18, 2021
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evelynhathaway commented Jun 19, 2021

Oh yay! I submitted a PR for this a long time ago but it got closed. I think this also fixes #4652, #7889, #9080, and #9559

@eslint-github-bot eslint-github-bot bot locked and limited conversation to collaborators Dec 16, 2021
@eslint-github-bot eslint-github-bot bot added the archived due to age This issue has been archived; please open a new issue for any further discussion label Dec 16, 2021
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no-fallthrough should allow the comment inside of a block when the case is a single block
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