Deprecate `spec` and `loose` plugin options by liuxingbaoyu · Pull Request #17972 · babel/babel

liuxingbaoyu · 2026-05-02T08:27:10Z

Q	A
Fixed Issues?	`Fixes #1, Fixes #2`
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link	babel/website#3201
Any Dependency Changes?
License	MIT

babel-bot · 2026-05-02T08:29:08Z

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/61532

pkg-pr-new · 2026-05-02T08:36:48Z

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commit: 6e367c2

nicolo-ribaudo · 2026-05-04T16:18:29Z

+  if ("loose" in options) {
+    console.warn(
+      "@babel/plugin-transform-template-literals: The 'loose' option has been deprecated, " +
+        "use the `ignoreToPrimitiveHint` and `mutableTemplateObject` assumptions instead.",


Suggested change

"use the `ignoreToPrimitiveHint` and `mutableTemplateObject` assumptions instead.",

"use the `ignoreToPrimitiveHint` and `mutableTemplateObject` assumptions instead (https://babeljs.io/assumptions).",

(and in all the others too)

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chatgpt-codex-connector · 2026-05-06T15:00:16Z

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babel/packages/babel-plugin-transform-private-methods/src/index.ts

Line 20 in dc18506

    
           "use the `privateFieldsAsProperties`, `privateFieldsAsSymbols` and `setPublicClassFields` assumptions instead. (https://babeljs.io/assumptions).",

Avoid recommending mutually exclusive assumptions

When users migrate loose: true by enabling every assumption listed in this warning, the plugin forwards them into createClassFeaturePlugin, which rejects privateFieldsAsProperties and privateFieldsAsSymbols when both are true (packages/babel-helper-create-class-features-plugin/src/index.ts:68). This makes the suggested replacement configuration throw before any transform runs instead of preserving the deprecated option's behavior.

babel/packages/babel-plugin-transform-private-property-in-object/src/index.ts

Line 23 in dc18506

    
           "use the `privateFieldsAsProperties`, `privateFieldsAsSymbols` and `setPublicClassFields` assumptions instead. (https://babeljs.io/assumptions).",

Avoid recommending mutually exclusive assumptions

When users migrate loose: true by enabling every assumption listed in this warning, the class-features helper rejects privateFieldsAsProperties and privateFieldsAsSymbols when both are true (packages/babel-helper-create-class-features-plugin/src/index.ts:68). Following this message for #x in obj therefore produces a configuration error rather than a working replacement for the deprecated option.

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chatgpt-codex-connector · 2026-05-06T15:36:32Z

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babel/packages/babel-plugin-transform-for-of/src/index.ts

Line 9 in 0aba8be

/** @deprecated Use the `iterableIsArray` assumption instead. */

Mention iterator closing in the deprecated type hint

For TypeScript users migrating from loose: true, this generated JSDoc only points them at iterableIsArray, but the plugin still maps the old option to skipForOfIteratorClosing below (api.assumption("skipForOfIteratorClosing") ?? options.loose). Following this hint alone leaves iterator closing enabled, so non-array iterators with return() behave differently after the migration; the fresh evidence is that the runtime warning was updated to mention both assumptions, but the exported deprecation hint was not.

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chatgpt-codex-connector · 2026-05-06T15:43:41Z

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babel/packages/babel-plugin-transform-arrow-functions/src/index.ts

Lines 13 to 14 in d75542c

    
           "@babel/plugin-transform-arrow-functions: The 'spec' option has been deprecated, " + 
        
             "use the 'noNewArrows' assumption instead. (https://babeljs.io/assumptions).",

Clarify the inverted spec migration

When this warning is shown for spec: true, the behavior users need to preserve is noNewArrows = false because the code below computes it as api.assumption("noNewArrows") ?? !options.spec. Following the warning literally and setting assumptions.noNewArrows: true changes the generated arrow-function semantics, so the deprecation message should spell out the inverted mapping (spec: true -> noNewArrows: false, and vice versa).

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nicolo-ribaudo

I'd like to merge this and release soon the next RC.

I think the above suggestion from codex is an improvement, the rest looks good.

JLHwung

Thank you.

A few wording fixes to indicate that privateFieldsAsProperties and privateFieldsAsSymbols are exclusive, and generally privateFieldsAsProperties should be preferred unless user run into specific shadowing issue.

Co-authored-by: Huáng Jùnliàng <[email protected]>

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Co-authored-by: Huáng Jùnliàng <[email protected]>

deprecate

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nicolo-ribaudo reviewed May 4, 2026

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nicolo-ribaudo added PR: Deprecation 📢 A type of pull request used for our changelog categories labels May 5, 2026

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Comment thread packages/babel-plugin-transform-for-of/src/index.ts Outdated

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nicolo-ribaudo added the PR: Needs Docs label May 6, 2026

nicolo-ribaudo approved these changes May 9, 2026

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Wording fixes

a1c7bfd

nicolo-ribaudo mentioned this pull request May 9, 2026

Add docs for loose/spec deprecation babel/website#3201

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nicolo-ribaudo removed the PR: Needs Docs label May 9, 2026

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Co-authored-by: Huáng Jùnliàng <[email protected]>

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Comment thread packages/babel-plugin-transform-arrow-functions/src/index.ts Outdated

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Comment thread packages/babel-plugin-transform-arrow-functions/src/index.ts Outdated

Update packages/babel-plugin-transform-arrow-functions/src/index.ts

6e367c2

Co-authored-by: Huáng Jùnliàng <[email protected]>

nicolo-ribaudo merged commit 7a3142c into babel:main May 12, 2026
101 of 102 checks passed

	"use the `ignoreToPrimitiveHint` and `mutableTemplateObject` assumptions instead.",
	"use the `ignoreToPrimitiveHint` and `mutableTemplateObject` assumptions instead (https://babeljs.io/assumptions).",

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liuxingbaoyu commented May 2, 2026 •

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