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Fix: no-constant-condition false positives with unary expressions #13927

merged 1 commit into from Dec 18, 2020


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@mdjermanovic mdjermanovic commented Dec 15, 2020

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: v7.15.0
  • Node Version: v12.18.4
  • npm Version: v6.14.6

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


Please show your full configuration:

module.exports = {
  parserOptions: {
    ecmaVersion: 2015

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

Online Demo

/* eslint no-constant-condition: error */

if (+(a || true));

What did you expect to happen?

No errors.

For example, with a = "0" the condition is falsy, with a = "1" the condition is truthy.

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

+(a || true) is reported as a constant condition:

  3:5  error  Unexpected constant condition  no-constant-condition

What changes did you make? (Give an overview)

Fixed the no-constant-condition rule to not treat the argument of a unary expression as a "boolean position", unless the operator is !.

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

  • valid tests were failing before this change.
  • invalid tests are regression tests, they were already passing as invalid tests before this change.
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@btmills btmills left a comment

LGTM. I appreciate the additional tests to guard against regression. The original bug, values that are simultaneously truthy and falsy depending on what route you use to coerce them, is a bit of a mind-bender.


@mdjermanovic mdjermanovic merged commit 301d0c0 into master Dec 18, 2020
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@mdjermanovic mdjermanovic deleted the noconstcond-unary branch Dec 18, 2020
@eslint-github-bot eslint-github-bot bot locked and limited conversation to collaborators Jun 17, 2021
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2 participants