sindresorhus/eslint-plugin-unicorn

Rule proposal: `no-set-duplicates`

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#1,559 opened on Oct 25, 2021

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 (14 comments) (7 reactions) (0 assignees)JavaScript (5,022 stars) (468 forks)user submission
help wantednew rule

Description

Sometimes a hardcoded Set grows large enough for it to be hard to see if there is already a value in it. One could miss that a value one wants to add is already there and add a duplicate.

This rule should at minimum report hardcoded sets of literals and consts with duplicates.

This rule may also report consecutive .add calls with same arguments (this is probably much harder to implement). Or maybe this could be a separate rule like unicorn/no-array-push-push.

Fail

const allowed = new Set([
	'foo',
	'bar',
	'buz',
	'qux',
	'bar', // ← Remove duplicate value `'bar'` from the Set
]);

Pass

const allowed = new Set([
	'foo',
	'bar',
	'buz',
	'qux',
]);

Contributor guide

Tech stack
javascriptnodejs
Domain
toolingdeveloper experience
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Familiarity with ESLint rulesJavaScript AST (ESTree)
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60
Research direction
Study existing rules in the eslint plugin unicorn source (e.g., 'rules/' folder) to understand the pattern for creating a new rule. Examine the AST representation of `new Set([...])` and implement detection of duplicate literal elements. Write tests following the plugin's test structure.