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Add parse support for the "structs" proposal (only non-shared)#17624

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JLHwung wants to merge 4 commits intobabel:mainfrom
JLHwung:parse-structs
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Add parse support for the "structs" proposal (only non-shared)#17624
JLHwung wants to merge 4 commits intobabel:mainfrom
JLHwung:parse-structs

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@JLHwung JLHwung commented Nov 28, 2025

Q                       A
Fixed Issues? Parse & print the non-shared struct proposed in https://github.com/tc39/proposal-structs
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature? Yes
Tests Added + Pass? Yes
Documentation PR Link
Any Dependency Changes?
License MIT

This PR adds support of the non-shared structs with the following AST design:

interface StructDeclaration <: Declaration {
  type: "StructDeclaration";
  id: Identifier | null;
  superClass: Expression | null;
  body: StructBody;
}
interface StructBody <: Node {
  type: "StructBody";
  body: [ ClassMethod | ClassPrivateMethod | ClassProperty | ClassPrivateProperty | StaticBlock ];
}

The member within the struct body is still class members. I have not yet decided whether we should leave it as-is or rename them from Class* to Struct*.

@JLHwung JLHwung added the PR: New Feature 🚀 A type of pull request used for our changelog categories label Nov 28, 2025
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babel-bot commented Nov 28, 2025 •
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Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/60773

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commit: 49299be

@JLHwung JLHwung force-pushed the parse-structs branch 2 times, most recently from 7ecda07 to 7f6e64d Compare December 1, 2025 21:46
@JLHwung JLHwung marked this pull request as ready for review December 2, 2025 13:51
@nicolo-ribaudo nicolo-ribaudo self-requested a review December 5, 2025 16:34
nicolo-ribaudo
nicolo-ribaudo reviewed Jan 23, 2026
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packages/babel-parser/test/fixtures/experimental/structs/invalid-struct-decorators/input.js
@@ -0,0 +1,4 @@
@f
struct S {}
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Agree with disallowing this but allowing accessors

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@JLHwung JLHwung Jan 24, 2026 •
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Right, and the member decorators are not allowed as well.

IMO accessors are essentially syntax sugar serving for the purpose of decorating otherwise accessor pairs once. They can be decoupled from decorators so I see no reason to ban them in non-shared structs.

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nicolo-ribaudo approved these changes Jan 23, 2026
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Nice :)

@nicolo-ribaudo nicolo-ribaudo changed the title Parse structs Add parse support for the "structs" proposal (only non-shared) Jan 23, 2026
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nicolo-ribaudo commented Jan 29, 2026

@JLHwung could you rebase?

@nicolo-ribaudo nicolo-ribaudo added PR: New Feature (next major) 🚀 A type of pull request used for our changelog categories for next major release and removed PR: New Feature 🚀 A type of pull request used for our changelog categories labels Jan 29, 2026
@nicolo-ribaudo nicolo-ribaudo added this to the v7.29.0 milestone Jan 29, 2026
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JLHwung commented Jan 29, 2026

@JLHwung could you rebase?

Rebased.

Given that this PR only implements the non-shared part of the proposal, I think it'd be better shipped as Babel 8 only, otherwise Babel 7 will come with an incomplete proposal, plus we have not yet implemented the transform part.

WDYT about the AST shape? Currently the member within the struct body is still class members. Do you think it is better stay with class members or rename all Class* members to Struct* members?

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liuxingbaoyu commented Jan 29, 2026

I agree that this only applies to releases in Babel 8.
I'm slightly inclined to rename them.
How does estree handle this?

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JLHwung commented Jan 29, 2026

I agree that this only applies to releases in Babel 8. I'm slightly inclined to rename them. How does estree handle this?

Here is the ESTree proposal: https://github.com/estree/estree/blob/master/experimental/structs.md

ESTree reuses class members because their names do not contain the Class prefix. For Babel an alternative approach is to stick with ESTree's MethodDefinition and PropertyDefinition, but we will have deal with two slightly different member AST.

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nicolo-ribaudo commented Jan 30, 2026

Maybe let's just wait for 8.1, targeting end of march then? So we don't need to rush this.

WDYT about the AST shape? Currently the member within the struct body is still class members. Do you think it is better stay with class members or rename all Class* members to Struct* members?

Do we have transforms that just look for the Class* member nodes, rather than for a whole class, and that would have problem with structs?

@nicolo-ribaudo nicolo-ribaudo removed this from the v7.29.0 & v8.0.0-rc milestone Jan 31, 2026
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PR: Needs Docs PR: New Feature (next major) 🚀 A type of pull request used for our changelog categories for next major release Spec: JavaScript Structs

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