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Update: fix let logic in for-in and for-of loops in no-extra-parens #14011

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merged 1 commit into from Jan 30, 2021

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@mdjermanovic mdjermanovic commented Jan 18, 2021

Prerequisites checklist

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

[ ] Documentation update
[x] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

Tell us about your environment

  • ESLint Version: v7.18.0
  • Node Version: v12.18.4
  • npm Version: 6.14.6

What parser (default, @babel/eslint-parser, @typescript-eslint/parser, etc.) are you using?

default

Please show your full configuration:

Configuration
module.exports = {
  parserOptions: {
    ecmaVersion: 2015
  }
};

What did you do? Please include the actual source code causing the issue.

Online Demo

/* eslint no-extra-parens: error */

for ((let) in foo);

for ((let.a) of foo);

What did you expect to happen?

Per the ForInOfStatement specification, ForInStatement#left expression cannot start with let[, and ForOfStatement#left expression cannot start with let:

for ( [lookahead ≠ let [] LeftHandSideExpression[?Yield, ?Await] in Expression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

for ( [lookahead ≠ let] LeftHandSideExpression[?Yield, ?Await] of AssignmentExpression[+In, ?Yield, ?Await] ) Statement[?Yield, ?Await, ?Return]

So, I would expect:

  • 1 error for for ((let) in foo);
  • no errors for for ((let.a) of foo);

What actually happened? Please include the actual, raw output from ESLint.

  • no errors for for ((let) in foo);
  • 1 error for for ((let.a) of foo);
  5:6  error  Unnecessary parentheses around expression  no-extra-parens

✖ 1 problem (1 error, 0 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

After the autofix:

/* eslint no-extra-parens: error */

for ((let) in foo);

for (let.a of foo);

for (let.a of foo); is indeed a syntax error in engines:

Uncaught SyntaxError: The left-hand side of a for-of loop may not start with 'let'.

Espree currently allows that, but it shouldn't. Reported acornjs/acorn#1009

What changes did you make? (Give an overview)

Split "ForInStatement, ForOfStatement"(node) into two functions, as they're already different enough, and fixed the logic about expressions starting with let identifier.

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

This fix produces more warnings for for-in loops and fewer warnings for for-of loops.

This is similar to the previous fix for for loops #13981

@mdjermanovic mdjermanovic added bug ESLint is working incorrectly rule Relates to ESLint's core rules accepted There is consensus among the team that this change meets the criteria for inclusion labels Jan 18, 2021
nzakas
nzakas approved these changes Jan 28, 2021
@btmills btmills merged commit c60e23f into master Jan 30, 2021
3 checks passed
@btmills btmills deleted the noextraparens-forinforof-let branch Jan 30, 2021
@medianick
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This looks to have broken typescript-eslint; see typescript-eslint/typescript-eslint#2986

@mdjermanovic
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It seems that @typescript-eslint/no-extra-parens rule should be updated, and that will fix the problem.

@mdjermanovic
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@medianick

This looks to have broken typescript-eslint; see typescript-eslint/typescript-eslint#2986

Fixed in @typescript-eslint/eslint-plugin v4.15.0

This was referenced Mar 17, 2021
@eslint-github-bot eslint-github-bot bot locked and limited conversation to collaborators Jul 30, 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 Jul 30, 2021
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4 participants