feat: support TS syntax in `no-redeclare` #19563

snitin315 · 2025-03-22T21:23:26Z

Prerequisites checklist

I have read the contributing guidelines.

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

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

What changes did you make? (Give an overview)

Refs #19173

Adds TS support to no-redeclare. Added a new typescript only option similar to typescript-eslint/no-redeclare:

Examples of correct code with { ignoreDeclarationMerge: true }:

interface A {
  prop1: 1;
}
interface A {
  prop2: 2;
}

namespace Foo {
  export const a = 1;
}
namespace Foo {
  export const b = 2;
}

class Bar {}
namespace Bar {}

function Baz() {}
namespace Baz {}

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

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snitin315 · 2025-03-26T13:21:06Z

Tests are passing locally but failing on CI. I can reproduce the same behavior locally if I remove languageOptions.sourceType: "script option from ruleTesterTypeScript locally. I'm not sure why it's failing on CI.

tests/lib/rules/no-redeclare.js

JoshuaKGoldberg

I don't think I have enough context to weigh in on the sourceType points, but otherwise this looks very reasonable! Just one area of code I think can be deduplicated. 🚀

lib/rules/no-redeclare.js

tests/lib/rules/no-redeclare.js

snitin315 · 2025-04-09T05:50:15Z

@JoshuaKGoldberg This needs a re-review.

JoshuaKGoldberg

I have to admit my confidence in declaration merging logic has never been super high - but I think there are some points of simplification we could go for?

lib/rules/no-redeclare.js

JoshuaKGoldberg

🙌

nzakas · 2025-05-30T14:48:56Z

@snitin315 please double-check the lint failure.

@JoshuaKGoldberg @mdjermanovic @fasttime this PR has been auto-closed for inactivity multiple times. What do you want to do here?

snitin315 · 2025-05-30T15:05:06Z

I have some changes locally, let me push an update

fasttime · 2025-06-06T06:58:36Z

Sorry for losing track of this. I will review it soon.

fasttime

The typescript-eslint rule no-redeclare, upon which this PR is based, is not recommended for new projects, because it reports problems that are already reported by the TypeScript compiler, see https://typescript-eslint.io/rules/no-redeclare/. The built-in no-redeclare rule on the other hand is recommended, and it reports some problems that are not reported by tsc, like var redeclarations.

I'm not sure if we want to add the unrecommended TypeScript extensions to this rule without an option.

docs/src/rules/no-redeclare.md

tests/lib/rules/no-redeclare.js

nzakas · 2025-06-13T14:31:03Z

If the typescript-eslint rule is not recommended for us, maybe we just shouldn't update this rule?

fasttime · 2025-06-18T08:10:01Z

If the typescript-eslint rule is not recommended for us, maybe we just shouldn't update this rule?

I'm not sure. It seems that all new problems the rule would find will be also reported by TypeScript, provided that the file is tested.

nzakas · 2025-06-18T14:16:08Z

I'm not sure. It seems that all new problems the rule would find will be also reported by TypeScript, provided that the file is tested.

Right. If updating this rule just adds noise that is already caught by TypeScript, I'm not sure it's worth it.

fasttime · 2025-06-19T09:45:39Z

Then maybe we should update just the documentation and rule metadata, and keep only the new tests that ensure the rule works as intended with TypeScript files?

nzakas · 2025-06-19T15:00:14Z

Then maybe we should update just the documentation and rule metadata, and keep only the new tests that ensure the rule works as intended with TypeScript files?

Makes sense to me. 👍

fasttime

I've commented on some invalid cases which are actually considered valid TypeScript code and are also treated as valid by the typescript-eslint rule. Initially, I didn't realize our rule would report those usages as invalid without modification. To better align with user expectations, I think it would be good to ignore TS type-only declarations (the ones that are stripped out from code during compilation) like type, interface, namespace, declare function, etc. These are already handled by TypeScript and are safe to redeclare if the declarations are compatible.

tests/lib/rules/no-redeclare.js

github-actions · 2025-07-09T22:35:49Z

Hi everyone, it looks like we lost track of this pull request. Please review and see what the next steps are. This pull request will auto-close in 7 days without an update.

snitin315 · 2025-07-17T05:29:52Z

@fasttime I've addressed the feedback. PTAL

fasttime

Thanks for the update. There's a particular scenario I'm not sure about. What should happen when a value and a type share the same name? For example:

const a = 42; // value
type a = string; // type

or

let b: object; // value
interface b { foo: string } // type

or also

/* global c */
type c = number;

The TypeScript compiler considers these valid code because there is no collision between values and types even if they share the same name:

TypeScript Playground

tests/lib/rules/no-redeclare.js

github-actions · 2025-07-30T22:36:50Z

Hi everyone, it looks like we lost track of this pull request. Please review and see what the next steps are. This pull request will auto-close in 7 days without an update.

fasttime · 2025-07-31T09:32:15Z

@eslint/eslint-tsc I could use some feedback on #19563 (review).

nzakas · 2025-07-31T16:30:50Z

I think we should follow the typescript-eslint behavior, which is to flag this as a redeclaration:
https://typescript-eslint.io/play/#ts=5.8.2&fileType=.ts&code=LAKAxg9gdgzgLgAgIYILwICwCYDcCD0%2BCAbkgDYCuApqHAJ4AOVyaC8ATgJZQDmehCekyA&eslintrc=N4KABGBEBOCuA2BTAzpAXGUEKQHYHsBaaRAE0QGN4BDE9KfAM0cgBpxtIABAFwE8ADigrQAlgJ6EU8Ubh4B6AsTKUadDJETRo%2BaJA4BfEAaA&tsconfig=N4KABGBEDGD2C2AHAlgGwKYCcDyiAuysAdgM6QBcYoEEkJemy0eAcgK6qoDCAFutAGsylBm3TgwAXxCSgA&tokens=false

fasttime · 2025-08-04T13:30:36Z

tests/lib/rules/no-redeclare.js

These cases should be valid:

declare function a(): void; declare function a(): void; declare function b(): void; declare class b { } enum c { } namespace c { } declare enum d { } interface e { }

typescript-eslint doesn't report any violations: typescript-eslint Playground

fasttime · 2025-08-04T14:15:36Z

tests/lib/rules/no-redeclare.js

+	  function A() {}
+	  class A {}


There should be no need to report this case since it's a syntax error in JavaScript and it doesn't compile in TypeScript: typescript-eslint Playground.

fasttime · 2025-08-04T14:17:25Z

tests/lib/rules/no-redeclare.js

+	  function A() {}
+	  class A {}
+	  namespace A {}


As above, this is incorrect code, so the first error seems superfluous.

fasttime · 2025-08-04T14:24:55Z

lib/rules/no-redeclare.js

+			if (
+				interfaces > 1 &&
+				classes === 0 &&
+				namespaces === 0 &&
+				functions === 0
+			) {
+				return true;
+			}
+
+			// Multiple namespaces only
+			if (
+				namespaces > 1 &&
+				interfaces === 0 &&
+				classes === 0 &&
+				functions === 0
+			) {
+				return true;
+			}
+
+			// Interface + class (+ optional namespace)
+			if (interfaces === 1 && classes === 1 && functions === 0) {
+				return true;
+			}
+
+			// Class + namespace (no interface or function)
+			if (
+				classes === 1 &&
+				namespaces >= 1 &&
+				interfaces === 0 &&
+				functions === 0
+			) {
+				return true;
+			}
+
+			// Function + namespace (no interface or class)
+			if (
+				functions === 1 &&
+				namespaces >= 1 &&
+				interfaces === 0 &&
+				classes === 0
+			) {
+				return true;
+			}


With this logic, the rule is making explicit exceptions for certain combinations of declaration types that should not be reported.

I think it would be easier to do the opposite: start by allowing all combinations, and report an error only for specific cases, for example when counts.others > 0, counts.functions > 1, and probably not much more.

github-project-automation bot added this to Triage Mar 22, 2025

github-project-automation bot moved this to Needs Triage in Triage Mar 22, 2025

eslint-github-bot bot added the feature This change adds a new feature to ESLint label Mar 22, 2025

github-actions bot added the rule Relates to ESLint's core rules label Mar 22, 2025

snitin315 mentioned this pull request Mar 22, 2025

Change Request: Make rules TypeScript syntax-aware #19173

Open

19 tasks

snitin315 added 3 commits March 26, 2025 18:33

feat: support TS syntax in no-redeclare

6693b19

chore: fix test cases

de31139

docs: update examples no-redeclare

42b3fd8

snitin315 force-pushed the ts/no-redeclare branch from f38129c to 42b3fd8 Compare March 26, 2025 13:03

snitin315 added 3 commits March 26, 2025 18:37

ci: fix

86c3a4b

ci: fix

b7e67ea

ci: enable sourceType script

3ffcca8

snitin315 marked this pull request as ready for review March 26, 2025 13:19

snitin315 requested a review from a team as a code owner March 26, 2025 13:19

nzakas moved this from Needs Triage to Implementing in Triage Mar 27, 2025

fasttime reviewed Mar 28, 2025

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snitin315 added 2 commits March 31, 2025 13:37

ci: add parserOptions.sourcetype

491d403

ci: fix lint

e571236

This was referenced Mar 31, 2025

docs: update supported parser options typescript-eslint/typescript-eslint#11012

Closed

Docs: Mention sourceType in parser/parserOptions typescript-eslint/typescript-eslint#11016

Open

JoshuaKGoldberg requested changes Mar 31, 2025

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refactor: rm redundant code

9febe57

snitin315 added the accepted There is consensus among the team that this change meets the criteria for inclusion label Mar 31, 2025

snitin315 requested a review from JoshuaKGoldberg March 31, 2025 18:06

JoshuaKGoldberg requested changes Apr 9, 2025

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refactor: simplify code

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fix: improve ignoreDeclarationMerge implementation

c03442b

snitin315 moved this from Evaluating to Implementing in Triage May 31, 2025

fasttime reviewed Jun 11, 2025

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snitin315 added 2 commits June 28, 2025 22:50

refactor: update only meta & tests

53cec54

chore: fix formatting

486ba4b

fasttime reviewed Jun 29, 2025

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snitin315 added 3 commits July 10, 2025 11:58

fix: correct TS nodes behavior

362d68b

chore: refactor code

f007986

Merge branch 'main' into ts/no-redeclare

70fa0e6

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feat: support TS syntax in `no-redeclare` #19563

feat: support TS syntax in `no-redeclare` #19563

snitin315 commented Mar 22, 2025 •

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