Return false for `#x in` on static private fields before initialization by hoobnn · Pull Request #18265 · babel/babel

hoobnn · 2026-10-02T04:24:55Z

Q	A
Fixed Issues?	Fixes #18262
Patch: Bug Fix?	Yes
Major: Breaking Change?	No
Minor: New Feature?	No
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?	No
License	MIT

For a static private field, #x in obj compiles to checkInRHS(obj) === Class. But the field's value lives in var _x = { _: value }, which isn't assigned until the class body has finished evaluating. So any code that runs before that point (a static block, an earlier static field initializer, or anything after the field's initializer has thrown) gets true, even though natively the class doesn't have the field yet.

This PR adds && _x !== void 0 to the check for static private fields. checkInRHS still runs first, so a non-object right-hand side throws as before, and it's only evaluated once. Static private methods and accessors are left as they are, because the class already has them before any field initializer runs. Under noUninitializedPrivateFieldAccess the storage holds the raw value, so undefined doesn't mean "not initialized yet"; in that case the check is skipped and the output doesn't change. privateFieldsAsProperties / privateFieldsAsSymbols go through the hasOwnProperty path and aren't affected.

Tests:

private/static-field-initialization (exec): brand checks on a static field, a field with no initializer, a static method and a static getter, before and after each initializer. It also checks that the RHS is evaluated once and that the getter is never called.
private/half-constructed-static (exec): returns false after the field's initializer throws.
assumption-noUninitializedPrivateFieldAccess/static-field (output): no extra check under the assumption.
Six existing output fixtures now include && _x !== void 0. private-loose/static-shadow and to-native-fields/static-shadow override their plugins to transform-class-properties, so their change is the spec-mode one; actual loose-mode output is unchanged.

With only the new fixtures, the private-in suite has 8 failures. With the change it passes (67 tests), and so does babel-helper-create-class-features-plugin (17). I checked this on Linux and macOS.

One known gap: when the same class declaration is evaluated more than once, for example in a loop, var _x is shared between evaluations, so only the first one is fixed. Natively you get [false, true, false, true]; with this PR it's [false, true, true, true], and before it was [true, true, true, true]. This comes from how static private field storage is hoisted, which affects reads and writes as well, so I've kept it out of this PR.

This also extends noUninitializedPrivateFieldAccess to cover in checks. If you'd like, I can open a PR on the website to update the docs for that assumption.

github-actions · 2026-10-02T04:25:06Z

Thanks for opening this pull request! 🎉

We really appreciate you taking the time to contribute, @hoobnn.

A maintainer will take a look as soon as they can. In the meantime, please make sure that:

the description explains what changed and why
any related issues are linked
existing tests still pass

If anything needs adjusting we'll leave comments here. Thanks again!

babel-bot · 2026-10-02T04:26:51Z

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/62220

pkg-pr-new · 2026-10-02T04:34:40Z

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commit: 219bf51

nicolo-ribaudo

Thank you!

hoobnn · 2026-10-02T09:18:30Z

Thank you!

Done, thanks!

nicolo-ribaudo

Very minor, but could you also add a test for the loop case that you mentioned, with a comment saying that that output is actually wrong?

Btw, your Claude is adding to the commit messages links to sessions that are not actually accessible.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nicolo-ribaudo · 2026-10-02T19:31:20Z

@JLHwung I was also confused, but that assumption already exists :) https://babeljs.io/docs/assumptions#nouninitializedprivatefieldaccess

(if you merge this PR, please squash-merge replacing Co-authored-by with Assisted-by)

JLHwung · 2026-10-02T19:41:26Z

@JLHwung I was also confused, but that assumption already exists :) https://babeljs.io/docs/assumptions#nouninitializedprivatefieldaccess

(if you merge this PR, please squash-merge replacing Co-authored-by with Assisted-by)

Oh my bad. I didn't realize we already have this assumption. In that case this PR is indeed a bug fix.

## pnpm-workspace.yaml (default) ## Dependency Updates | Package | From | To | Type | | --- | --- | --- | --- | | `@babel/core` | 8.0.6 | 8.0.7 | patch | | `@react-email/ui` | 6.11.0 | 6.11.1 | patch | | `ai` | 7.0.128 | 7.0.130 | patch | | `react-email` | 6.11.0 | 6.11.1 | patch | | `shadcn` | 4.21.3 | 4.21.4 | patch | ## Release Notes <details> <summary>@babel/core (8.0.6 → 8.0.7)</summary> ## v8.0.7 (2026-10-07) Thanks @brunoborta, @CINC0S, @hoobnn, and @jpradelle for your first PRs! #### 🐛 Bug Fix * `babel-helper-create-regexp-features-plugin`, `babel-plugin-transform-dotall-regex`, `babel-plugin-transform-unicode-sets-regex`, `babel-preset-env` * [#18295](babel/babel#18295) Bump regexpu-core to 6.5.3 ([@JLHwung](https://github.com/JLHwung)) * `babel-plugin-transform-for-of` * [#18292](babel/babel#18292) Skip `for await` in `transform-for-of` ([@brunoborta](https://github.com/brunoborta)) * `babel-helpers`, `babel-plugin-transform-async-generator-functions`, `babel-runtime-corejs3` * [#18283](babel/babel#18283) Fix `asyncIterator` helper forwarding `.throw()` to `.return()` ([@brunoborta](https://github.com/brunoborta)) * `babel-parser` * [#18288](babel/babel#18288) fix(parser): allow await as a function expression name inside async arrow ([@JLHwung](https://github.com/JLHwung)) * `babel-helpers`, `babel-plugin-transform-class-properties`, `babel-plugin-transform-modules-systemjs`, `babel-runtime-corejs3` * [#18284](babel/babel#18284) Guard `@@toPrimitive` usage in toPrimitive helper ([@nicolo-ribaudo](https://github.com/nicolo-ribaudo)) * `babel-plugin-transform-block-scoping` * [#18195](babel/babel#18195) fix: closures in for-loop init capture wrong binding ([@joelle-a-dev](https://github.com/joelle-a-dev)) * `babel-helper-create-class-features-plugin`, `babel-plugin-transform-private-property-in-object` * [#18265](babel/babel#18265) Return false for `#x in` on static private fields before initialization ([@hoobnn](https://github.com/hoobnn)) * `babel-traverse` * [#18151](babel/babel#18151) Evaluate TypeScript expression wrappers in `NodePath#evaluate` ([@CINC0S](https://github.com/CINC0S)) * [#18250](https://github …[full notes](https://github.com/babel/babel/releases/tag/v8.0.7) </details> <details> <summary>react-email (6.11.0 → 6.11.1)</summary> ### Patch Changes * 54769fc: Prevent `email export` from deleting the project, the home directory, the email templates or the static assets when `--outDir` points at them. * 6926bac: `<Markdown>` now applies `markdownCustomStyles.th` to table header cells. Header cells were previously styled with `td`, so any `th` styles were ignored. </details> <details> <summary>shadcn (4.21.3 → 4.21.4)</summary> ### Patch Changes - [#12189](shadcn-ui/ui#12189) [`76fd499595ba7cc01f49d159bd7ac82d7aca8e63`](shadcn-ui/ui@76fd499) Thanks [@shadcn](https://github.com/shadcn)! - Drop the ts-morph dependency from @shadcn/registry. - [#12184](shadcn-ui/ui#12184) [`995c2cfff44c373bd210d451089726ec097ab917`](shadcn-ui/ui@995c2cf) Thanks [@shadcn](https://github.com/shadcn)! - Run the icons and asChild transformers without ts-morph. - [#12160](shadcn-ui/ui#12160) [`6efecd8fe9aa167886fe2cc0c05c5623a5bb5670`](shadcn-ui/ui@6efecd8) Thanks [@shadcn](https://github.com/shadcn)! - Edit the Next.js layout for font items without ts-morph, and skip it with a warning instead of writing a broken layout. - [#12188](shadcn-ui/ui#12188) [`efa11781f756c86debb0392fbea4fe468250b41b`](shadcn-ui/ui@efa1178) Thanks [@shadcn](https://github.com/shadcn)! - Rewrite imports and crawl file imports without ts-morph. - [#12177](shadcn-ui/ui#12177) [`e8c3143b1cd191280befcd6c9538284bb43399a8`](shadcn-ui/ui@e8c3143) Thanks [@shadcn](https://github.com/shadcn)! - Edit tailwind.config without ts-morph, and skip it with a warning instead of writing a broken config. - [#12183](shadcn-ui/ui#12183) [`f56bbd7282f0116a925a601cb6d7e0c3fede448b`](shadcn-ui/ui@f56bbd7) Thanks [@shadcn](https://github.com/shadcn)! - Run the rsc, import, CSS variable, cleanup, font and menu transformers without ts-morph. - [#12185](shadcn-ui/ui#12185) [`4a90344c42dc9ae47494179065219377ff5f2a8f`](https://github.com/shadcn-ui/u …[full notes](https://github.com/shadcn-ui/ui/releases/tag/shadcn%404.21.4) </details> --- *This PR was auto-generated by [catalog-update-action](https://github.com/brandhaug/catalog-update-action).* Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

nicolo-ribaudo reviewed Oct 2, 2026

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Comment thread ...in-transform-private-property-in-object/test/fixtures/private-loose/native-classes/output.js Outdated

Comment thread packages/babel-helper-create-class-features-plugin/src/fields.ts Outdated

nicolo-ribaudo approved these changes Oct 2, 2026

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nicolo-ribaudo added the PR: Bug Fix 🐛 A type of pull request used for our changelog categories label Oct 2, 2026

hoobnn and others added 3 commits October 2, 2026 17:33

Return false for #x in on static private fields before initialization

9047b4a

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Use !!_x for the static private field check

978632c

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Add a test for a static private field redeclared in a loop

219bf51

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

hoobnn force-pushed the fix/static-private-in-before-init branch from 0b4ddd0 to 219bf51 Compare October 2, 2026 09:33

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nicolo-ribaudo merged commit 5a9b954 into babel:main Oct 2, 2026
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hoobnn deleted the fix/static-private-in-before-init branch October 3, 2026 02:05

brandhaug mentioned this pull request Oct 10, 2026

chore(deps): bump all-patch-updates dependencies brandhaug/b2b-saas-starter#655

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Return false for `#x in` on static private fields before initialization - #18265

Return false for `#x in` on static private fields before initialization#18265
nicolo-ribaudo merged 3 commits into
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hoobnn:fix/static-private-in-before-init

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