fix(eslint-plugin): [prefer-optional-chain] skip optional chaining when it could change the result #11702

mdm317 · 2025-10-16T16:03:42Z

PR Checklist

Addresses an existing open issue: fixes Bug: @typescript-eslint/prefer-optional-chain flags situations where an optional chain would change behavior #11700
That issue was marked as accepting prs
Steps in Contributing were taken

When applying optional chaining, if the transformation has the potential to alter the evaluation result,
the previous operand should not be included, even when it appears useless.

Conversely, if the transformation is guaranteed to preserve the result,
the previous operand should be included in the chain, even when it’s useless.

Not convert to optional-chain

declare const a: { b?: string };

a && a.b != "4"
a == null || a.b === "foo"
...more cases

Conver to optional-chain

declare const a: { b?: string };

a && a.b != null
a && a.b != undefined
a && a.b  !== undefined
...more cases

see Algorithm

typescript-eslint · 2025-10-16T16:03:46Z

Thanks for the PR, @mdm317!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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codecov · 2025-10-16T16:15:10Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.66%. Comparing base (3f5fbf6) to head (fd98e65).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11702      +/-   ##
==========================================
- Coverage   90.67%   90.66%   -0.01%     
==========================================
  Files         518      518              
  Lines       52466    52415      -51     
  Branches     8694     8679      -15     
==========================================
- Hits        47571    47520      -51     
  Misses       4881     4881              
  Partials       14       14

Flag	Coverage Δ
unittest	`90.66% <100.00%> (-0.01%)`	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../rules/prefer-optional-chain-utils/analyzeChain.ts	`100.00% <100.00%> (ø)`

🚀 New features to boost your workflow:

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kirkwaiblinger · 2025-10-17T06:32:28Z

packages/eslint-plugin/tests/rules/prefer-optional-chain/prefer-optional-chain.test.ts

+      `,
+      `
+        declare const foo: { bar: number };
+        foo != null && foo.bar !== x;


Looks like x is undeclared here, so this is an error type. Might be good to make it explicit?

kirkwaiblinger · 2025-10-17T06:59:38Z

packages/eslint-plugin/tests/rules/prefer-optional-chain/prefer-optional-chain.test.ts

      'foo != null && foo.bar != x;',
+      `
+        declare const foo: { bar: number };
+        foo && foo.bar == x;


Let's declare x with a type here? Unless we're intentionally testing undeclared variables?

I didn’t declare it because I wanted to test that it doesn’t convert to an optional chain when we don’t know x’s type.
But thinking about what you said, testing with an error type feels odd and doesn’t clearly show the test’s purpose.

How about declaring x with the type any instead?

I think both are potentially valid tests! But I would just make the intention explicit like:

declare const foo: { bar: number }; declare const anee: any; foo && foo.bar == anee;

// intentionally tests that an undeclared variable doesn't trigger an autofix declare const foo: { bar: number }; foo && foo.bar == undeclaredVar;

kirkwaiblinger · 2025-10-17T07:09:13Z

packages/eslint-plugin/src/rules/prefer-optional-chain-utils/analyzeChain.ts

Looks like there's a minor bug somewhere with:

declare const foo: { bar: number | null } | undefined; if (foo === undefined || foo.bar === null) { // suggests to foo?.bar === null, which is not equivalent. } else { foo.bar.toExponential(); // after accepting fix, this crashes if `foo` is `undefined`. }

I guess this should either not report or it could maybe try to use foo?.bar == null? But this seems kind of bad since it is impossible to work with for an === nullishValue-prefering user

typescript-eslint/packages/eslint-plugin/src/rules/prefer-optional-chain-utils/analyzeChain.ts

Lines 277 to 284 in 3f5fbf6

if (

includesType(parserServices, operand.comparedName, ts.TypeFlags.Null)

) {

// we know the next operand is not a `null` check and that this

// operand includes `null` - which means that making this an

// optional chain would change the runtime behavior of the expression

return null;

}

I believe this bug didn’t originate from this PR — it seems to have existed beforehand.
Since I wasn’t the original author, it might take some time to fix it properly.

If possible, we could apply a quick hotfix for the current issue first and then open a new issue to track it, but it’s just a suggestion — feel free to decide as you think best.

In any case, I’ll start digging into it now.

That's fine if we defer it!

Just out of curiosity, do you mean to say it didn't originate in this PR, ie #11702, or in #11533? (either way we can defer it just good to know)

kirkwaiblinger

Looks like it's on the right track, but minor touchups and a question about one bug that I saw.

Thanks for looking at this so quickly! ❤️

| datasource | package | from | to | | ---------- | -------------------------------- | ------ | ------ | | npm | @typescript-eslint/eslint-plugin | 8.46.1 | 8.46.2 | | npm | @typescript-eslint/parser | 8.46.1 | 8.46.2 | ## [v8.46.2](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8462-2025-10-20) ##### 🩹 Fixes - **eslint-plugin:** \[prefer-optional-chain] skip optional chaining when it could change the result ([#11702](typescript-eslint/typescript-eslint#11702)) ##### ❤️ Thank You - mdm317 You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website.

@undsoft

| datasource | package | from | to | | ---------- | -------------------------------- | ------ | ------ | | npm | @typescript-eslint/eslint-plugin | 8.46.1 | 8.46.3 | | npm | @typescript-eslint/parser | 8.46.1 | 8.46.3 | ## [v8.46.3](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8463-2025-11-03) ##### 🩹 Fixes - **eslint-plugin:** \[no-duplicate-enum-values] support signed numbers ([#11722](typescript-eslint/typescript-eslint#11722), [#11723](typescript-eslint/typescript-eslint#11723)) - **eslint-plugin:** \[no-misused-promises] expand union type to retrieve target property ([#11706](typescript-eslint/typescript-eslint#11706)) ##### ❤️ Thank You - Evgeny Stepanovych [@undsoft](https://github.com/undsoft) - tao You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website. ## [v8.46.2](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8462-2025-10-20) ##### 🩹 Fixes - **eslint-plugin:** \[prefer-optional-chain] skip optional chaining when it could change the result ([#11702](typescript-eslint/typescript-eslint#11702)) ##### ❤️ Thank You - mdm317 You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website.

@undsoft

| datasource | package | from | to | | ---------- | -------------------------------- | ------ | ------ | | npm | @typescript-eslint/eslint-plugin | 8.46.1 | 8.46.3 | | npm | @typescript-eslint/parser | 8.46.1 | 8.46.3 | ## [v8.46.3](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8463-2025-11-03) ##### 🩹 Fixes - **eslint-plugin:** \[no-duplicate-enum-values] support signed numbers ([#11722](typescript-eslint/typescript-eslint#11722), [#11723](typescript-eslint/typescript-eslint#11723)) - **eslint-plugin:** \[no-misused-promises] expand union type to retrieve target property ([#11706](typescript-eslint/typescript-eslint#11706)) ##### ❤️ Thank You - Evgeny Stepanovych [@undsoft](https://github.com/undsoft) - tao You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website. ## [v8.46.2](https://github.com/typescript-eslint/typescript-eslint/blob/HEAD/packages/eslint-plugin/CHANGELOG.md#8462-2025-10-20) ##### 🩹 Fixes - **eslint-plugin:** \[prefer-optional-chain] skip optional chaining when it could change the result ([#11702](typescript-eslint/typescript-eslint#11702)) ##### ❤️ Thank You - mdm317 You can read about our [versioning strategy](https://typescript-eslint.io/users/versioning) and [releases](https://typescript-eslint.io/users/releases) on our website.

mdm317 added 3 commits October 17, 2025 00:23

fix : remove isAlwaysTruthyOperand

66b1c1a

test : fix base case

a382696

test : add some case for array/index access

ca323ed

mdm317 changed the title ~~fix(eslint-plugin): [prefer-optional-chain] remove unnecessary operator check~~ fix(eslint-plugin): [prefer-optional-chain] skip optional chaining when it could change the result Oct 16, 2025

fix : lint err

87605b0

mdm317 marked this pull request as ready for review October 16, 2025 16:39

kirkwaiblinger reviewed Oct 17, 2025

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kirkwaiblinger requested changes Oct 17, 2025

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mdm317 mentioned this pull request Oct 17, 2025

Bug: @typescript-eslint/prefer-optional-chain flags situations where an optional chain would change behavior #11700

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mdm317 added 2 commits October 18, 2025 02:24

refactor : undeclaredVar

837db2a

fix : weird test csae

fd98e65

kirkwaiblinger approved these changes Oct 17, 2025

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kirkwaiblinger added the 1 approval >=1 team member has approved this PR; we're now leaving it open for more reviews before we merge label Oct 17, 2025

bradzacher approved these changes Oct 19, 2025

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bradzacher merged commit 698e7a8 into typescript-eslint:main Oct 19, 2025
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chrisbbreuer mentioned this pull request Oct 20, 2025

chore(deps): update all non-major dependencies stacksjs/eslint-plugin#67

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1 task

This was referenced Oct 21, 2025

Bump @typescript-eslint/parser from 8.46.0 to 8.46.2 fossas/fossa-action#229

Merged

Update dependency @typescript-eslint/parser to v8.46.2 ericcornelissen/js-regex-security-scanner#1281

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	if (
	includesType(parserServices, operand.comparedName, ts.TypeFlags.Null)
	) {
	// we know the next operand is not a `null` check and that this
	// operand includes `null` - which means that making this an
	// optional chain would change the runtime behavior of the expression
	return null;
	}

Uh oh!

fix(eslint-plugin): [prefer-optional-chain] skip optional chaining when it could change the result #11702

fix(eslint-plugin): [prefer-optional-chain] skip optional chaining when it could change the result #11702

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mdm317 commented Oct 16, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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PR Checklist

Not convert to optional-chain

Conver to optional-chain

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