js_parser: fix TypeScript experimental decorator lowering by robobun · Pull Request #40830 · oven-sh/bun

robobun · 2026-08-28T17:14:14Z

Problem

With experimentalDecorators, lower_class (src/js_parser/p.rs) moved every decorated field out of the class body. That reordered initializers (a c b), gave decorated fields [[Set]] semantics under useDefineForClassFields: true, dropped @dec w; ("w" in new A() was false), and hoisted this/super out of static initializers (A.t = this.a threw a TypeError, A.s = super.x was a SyntaxError).
The same lowering evaluated a computed key twice, parsed parameter decorators in the method's scope and await context (@pd(arg) arg = 2 bound to the parameter, await was rejected), and emitted decorators that read Foo.#priv outside the class (SyntaxError: Cannot reference undeclared private names).
The parser sent export default @dec class Foo {} through parse_expr, so Foo was never bound and the decorator was dropped. Decorators on class expressions were dropped silently, and accessor was a syntax error.

Fix

Decorated fields stay in the body. The lowering only emits the __legacyDecorateClassTS calls after the class. With useDefineForClassFields: false, visit_class already moves instance fields into the constructor and the decorated field without an initializer is omitted, as tsc does.
A decorated computed key is captured once: [_key = expr] in the body, _key in the decorator call (var _key; before the class). Parameter decorators are parsed with the enclosing await context and scope and visited in the class body scope. When a decorator reads a #private name, the decorator calls go into a static block at the end of the body, the shape tsc emits.
export default @dec class is a declaration. Experimental decorators on a class expression, its members, or its parameters report an error (tsc: TS1206, esbuild: the same messages). accessor x = init becomes #x_accessor_storage = init plus a getter/setter pair. A decorated accessor receives the descriptor like a method.
Verified: test/bundler/transpiler/decorators-legacy-lowering.test.ts (each repro runs with bun, bundled and minified, and as tsc's emit under node, all three must agree), decorators.test.ts, es-decorators.test.ts, ts-use-define-for-class-fields.test.ts, test/bundler/esbuild/ts.test.ts.

Background

lower_class is the post-visit pass that turns one TypeScript class statement into the class plus its __legacyDecorateClassTS and __legacyDecorateParamTS calls. It runs after visit_class, which resolves names and applies the useDefineForClassFields: false field lowering.
A static {} block in a class body runs during class definition, in order with static field initializers, with the class's private names in scope. tsc uses one when a decorator expression mentions a private name. Class decorators still run after the class.
The visit pass replays the scopes the parse pass recorded, in order. Parameter decorators are therefore still visited inside visit_args, with current_scope swapped to the class body scope, the same way esbuild does it.

Notes

Behavior changes to be aware of:

Property decorators that install an accessor on the prototype (TypeORM, MobX, Angular style) now see the same thing as with tsc and esbuild: under the default useDefineForClassFields: true the field's own property shadows the accessor. Projects that rely on the accessor need useDefineForClassFields: false in tsconfig, as with tsc. The existing "decorators random" test asserted the old behavior; it now checks [[Define]] semantics, and its [[Set]] assertions moved to decorators-set-semantics/set-semantics-fixture.ts, which runs with its own tsconfig.
With useDefineForClassFields: false, a class that has an instance field with a computed key keeps every instance field native (pre-existing behavior of visit_class, it does not hoist keys). Decorated fields now follow that rule too instead of getting their own this[key] = init with a second key evaluation.
"Decorators are not valid here" replaces Expected "class" but found "@" for @x export @y class.
RuntimeTranspilerCache EXPECTED_VERSION is bumped because the output changes for the same input.

Later review rounds moved the accessor rewrite into visit_class (so the useDefineForClassFields: false pass moves its storage initializer in source order), gave storage names a _N suffix when the class already declares the name, kept decorated abstract accessor members, parsed parameter decorators with the enclosing yield and super context too, tracked private-name use per class body scope, and made visit_stmts set its statement list before the TypeScript enum pre-pass so class expressions inside enum initializers can declare temporaries. The tsc comparison in the test runs the typescript package in node because loading it in a debug build of bun takes longer than a test may.

Overlapping open PRs that this supersedes: #35537 ([[Define]] semantics), #38953 (static initializers), #38142 (computed key once), #38125 (accessor), and the class-expression part of #38095.

Not changed here: static fields with useDefineForClassFields: false keep [[Define]] semantics (tsc emits static { this.x = init }), and the inner class binding is the same symbol as the outer one, so a class decorator that returns a new class is not visible from inside the body. Both are pre-existing.

While testing, two unrelated bugs were found and handed off: the runtime applies the cwd's tsconfig experimentalDecorators/useDefineForClassFields to every module instead of the nearest tsconfig.json, and the standard decorator lowering shares one _init temporary between two decorated classes in one scope.

Repro set, all under {"compilerOptions":{"experimentalDecorators":true}}, before and after:

r1 (field order, this/super)      SyntaxError: super is not valid    -> a b c x y z x y z / 2 1 5 true
r2 (computed keys)                decorating k2, k3; n = 5           -> decorating k1, k2; n = 2
r3a (@pd(arg) arg = 2, minified)  ReferenceError                     -> dec arg = 1
r3b (await in param decorator)    "await" can only be used inside    -> dec arg = 42
r3c (@pd(Q.#p))                   Cannot reference undeclared        -> dec arg = 1 (static block)
r4 (export default @cd class Bar) Bar is not defined                 -> decorated Bar / 1
r5 (const X = @cd class Y {})     silently dropped                   -> error: cannot be used in expression position
r6 (accessor y = 2)               Expected ";" but found "y"         -> 2 3

@dec

With `experimentalDecorators`, `lower_class` moved every decorated field out of the class body (`Class.key = init` after the class, `this.key = init` in the constructor). That changed the initialization order, gave decorated fields [[Set]] semantics under `useDefineForClassFields: true`, dropped decorated fields that have no initializer, and left `this` and `super` in hoisted static initializers pointing at the wrong thing. Decorated fields now stay where TypeScript's own field lowering puts them and only the `__legacyDecorateClassTS` calls follow the class. Other fixes in the same lowering: - A computed key of a decorated member is captured in a temporary (`[_key = expr]`) so it is evaluated once and the decorator call uses the same key. - Parameter decorators are parsed with the await context and scope that enclose the class, and visited in the class body scope, so a name in them is not bound to a parameter and `await` works inside an async function. - When a member or parameter decorator reads a `#private` name, the decorator calls run in a static block at the end of the class body, where the name is in scope (the shape tsc emits). - `export default @dec class Foo {}` is parsed as a class declaration, so the `Foo` binding exists and the decorator is applied. With standard decorators an anonymous class on that path keeps the name "default". - Experimental decorators on a class expression, or on its members and parameters, report an error instead of being dropped. - `accessor` fields are parsed in experimentalDecorators projects and lowered to a `#x_accessor_storage` field with a getter/setter pair; decorated accessors receive the descriptor like a method. - Decorators on both sides of `export` report "Decorators are not valid here".

coderabbitai · 2026-08-28T17:14:22Z

Walkthrough

The parser now tracks decorator scopes and private-name usage. TypeScript decorator lowering preserves static blocks, reuses computed keys, transforms auto-accessors, and handles decorated declarations and exports. Tests cover runtime behavior, diagnostics, metadata, and generated output.

Changes

TypeScript decorator lowering

Layer / File(s)	Summary
Decorator metadata contracts `src/ast/*`, `src/js_parser/parser.rs`, `src/js_parser/p.rs`, `src/js_parser/lower/lower_decorators.rs`	AST nodes and parser state now preserve decorator scopes, private-name usage, class-expression status, and auto-accessor getter information.
Decorator parsing and validation `src/js_parser/parse/*`	Parsing propagates decorator scopes, recognizes auto-accessors, validates expression-position decorators, and handles decorated exports.
Decorator visitation and class lowering `src/js_parser/visit/*`, `src/js_parser/p.rs`, `src/js_parser/lower/lower_decorators.rs`	Lowering tracks private-name references, preserves static blocks, captures computed keys, transforms auto-accessors, and emits decorator metadata.
Decorator regression coverage `test/bundler/transpiler/*`, `src/jsc/RuntimeTranspilerCache.rs`	Tests compare Bun and TypeScript output, cover field and accessor semantics, validate diagnostics and metadata, snapshot transformed output, and update the cache format version.

Suggested reviewers: jarred-sumner

Merge Risk: 🟡 Moderate · up to 0508d

The decorator-lowering changes can still omit decorators and metadata from abstract auto-accessors, causing incorrect output for affected TypeScript code; this should be fixed or explicitly accepted before merge.

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robobun · 2026-08-28T17:14:34Z

^{Updated 3:16 PM PT - Aug 28th, 2026}

❌ @robobun, your commit 2fe2ee3 has 1 failures in Build #108002 (All Failures):

test/js/web/url/url.test.ts - code 1: TypeError: Invalid URL on 🍎 any x64

🧪 To try this PR locally:

bunx bun-pr 40830

That installs a local version of the PR into your bun-40830 executable, so you can run:

bun-40830 --bun

robobun · 2026-08-28T17:14:41Z

Status: the diff is ready for review. CI is green on every lane except test/js/web/url/url.test.ts on macOS x64, which fails on main too (the Unicode 16 IDNA test) and is reported for main-break triage.

Reproduced every case from the report with the released build (v1.4.1-canary) under {"compilerOptions":{"experimentalDecorators":true}}: the super is not valid SyntaxError and a c b order, computed keys evaluated twice, the parameter decorator bound to its parameter and await rejected, Cannot reference undeclared private names, Bar is not defined for export default @cd class Bar, the silently dropped class-expression decorator, and Expected ";" but found "y" for accessor.

Each repro is a fixture in test/bundler/transpiler/decorators-legacy-lowering.test.ts. A fixture runs with bun, bundled and minified with Bun.build, and as tsc's emit under node, and all three outputs have to match. The new file fails on every fixture with the released build and passes with this branch.

A decorated "declare" field with a computed key, or a decorated field that "useDefineForClassFields: false" omits, is removed from the class body. Its key expression still runs once, in a static block, because the decorator call reads the temporary. The omission of fields without an initializer only applies when visit_class actually lowered the instance fields, which it does not do when one of them has a computed key.

robobun · 2026-08-28T18:59:37Z

The body lists #38125 as superseded. I built this branch and ran the tests of #38125 against it. The parse change, the lowering of undecorated and decorated accessors, the design:type metadata, and the #29197 repro all pass here. Three cases do not:

useDefineForClassFields: false. The backing field keeps its initializer in the class body, so it runs before the constructor assignments of the other fields. For a = 1; accessor x = this.a + 1; b = 2; this branch prints x: null and the order x, a, b. tsc moves the initializer into the constructor in source order (a, x, b, x: 2).
Class::can_be_moved in src/ast/g.rs does not count a static accessor initializer as a side effect. const names = ["p"]; class S { static accessor s = names[0] } is hoisted above const names and throws ReferenceError: Cannot access 'names' before initialization. This also happens in .js files on main.
@dec abstract accessor g: number is dropped together with its decorators. tsc emits __decorate([dec, __metadata("design:type", Number)], Entity.prototype, "g", void 0).

#38125 fixes all three (its accessor lowering runs in visit_class, before the useDefineForClassFields block). It stays open and will rebase onto this PR once it lands.

…ators A class expression inside a decorator that uses its own #name no longer sends the outer class's decorator calls into a static block. The check now compares the resolved private name against the members of the class body scope whose decorators are being visited.

claude

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

A class expression nested in a decorator could discard the flag the outer class needs. The parser now keeps the list of class body scopes whose decorators read one of their private names, and each class looks itself up when its visit ends.

coderabbitai

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src/js_parser/parse/parse_property.rs (1)
467-475: 🎯 Functional Correctness | 🟠 Major | 🏗️ Heavy lift

Preserve decorated abstract auto-accessors.

abstract accessor value: T; now parses as PropertyKind::AutoAccessor. The abstract-property path only retains PropertyKind::Normal, so a decorated abstract accessor is discarded with its decorators and metadata. Preserve and lower this form, or report an explicit diagnostic.
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In `@src/js_parser/parse/parse_property.rs` around lines 467 - 475, Update the
abstract-property handling in the property parsing flow around
PropertyKind::AutoAccessor so decorated abstract auto-accessors such as
“abstract accessor value: T;” are preserved and lowered with their decorators
and metadata, rather than being discarded because only PropertyKind::Normal is
retained; if preservation is unsupported, emit an explicit diagnostic instead.

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Inline comments:
In `@src/js_parser/p.rs`:
- Line 6780: Update the membership check in the storage-name loop to use
storage_names.contains with a reference to storage_name instead of iter().any(),
preserving the existing loop behavior while resolving the Clippy warning.
- Around line 6706-6710: Update the logic around lower_class_expr_auto_accessors
to obtain the nearest mutable statement list before calling
drain_capture_temp_decls, and only drain capture temporaries when that list
exists. Preserve the temporary declarations in temp_refs_to_declare when
nearest_stmt_list_mut() returns None so rewritten computed keys remain declared.

In `@src/js_parser/parse/parse_fn.rs`:
- Around line 231-247: Update the parameter-decorator handling in parse_fn to
save the current allow_super_call and allow_super_property values, restore the
enclosing method-option values while parse_type_script_decorators runs, then
restore the inner values afterward. Ensure restoration occurs even when
decorator parsing returns an error, preserving parser state for all subsequent
processing.

In `@test/bundler/transpiler/decorators-legacy-lowering.test.ts`:
- Around line 225-259: Replace the manual Object.entries(fixtures) for-loop with
describe.each(Object.entries(fixtures)), passing each fixture name and value
into a per-fixture describe block. Keep the three existing test lanes—bun,
bundled and minified, and tsc emit—unchanged in behavior and grouped under each
fixture.

In `@test/bundler/transpiler/decorators-set-semantics/set-semantics-fixture.ts`:
- Around line 43-45: Update the assertions around iceCream.flavor to pass the
actual flavor value to expect and compare it with the expected string using the
matcher, preserving the existing vanilla and chocolate expectations while
improving failure diagnostics.

---

Outside diff comments:
In `@src/js_parser/parse/parse_property.rs`:
- Around line 467-475: Update the abstract-property handling in the property
parsing flow around PropertyKind::AutoAccessor so decorated abstract
auto-accessors such as “abstract accessor value: T;” are preserved and lowered
with their decorators and metadata, rather than being discarded because only
PropertyKind::Normal is retained; if preservation is unsupported, emit an
explicit diagnostic instead.

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src/ast/expr.rs
src/ast/g.rs
src/js_parser/lower/lower_decorators.rs
src/js_parser/p.rs
src/js_parser/parse/mod.rs
src/js_parser/parse/parse_fn.rs
src/js_parser/parse/parse_prefix.rs
src/js_parser/parse/parse_property.rs
src/js_parser/parse/parse_stmt.rs
src/js_parser/parser.rs
src/js_parser/visit/mod.rs
src/js_parser/visit/visit_binary.rs
src/js_parser/visit/visit_expr.rs
src/jsc/RuntimeTranspilerCache.rs
test/bundler/transpiler/decorators-legacy-lowering.test.ts
test/bundler/transpiler/decorators-set-semantics/set-semantics-fixture.ts
test/bundler/transpiler/decorators-set-semantics/tsconfig.json
test/bundler/transpiler/decorators.test.ts
test/bundler/transpiler/es-decorators.test.ts

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… parameter decorators A decorated abstract accessor keeps its decorator call like an abstract field. Parameter decorators also parse with the enclosing super flags. A class expression only captures a computed accessor key when there is a statement list to declare the temporary in.

claude

This pull request has now been reviewed several times and this review found new issues. Before patching these one by one, step back: would one root-cause fix close several of them? Is the pull request's scope growing with each push? Prefer root-cause fixes, keep scope fixed, and note out-of-scope improvements as follow-ups.

The accessor rewrite now runs in visit_class before the useDefineForClassFields: false pass, so the storage field's initializer moves into the constructor in source order like any other field. The getter keeps the member's decorators and type metadata, marked with a property flag, so lower_class treats it like a decorated method and emits design:type only. One rewrite covers class statements and class expressions.

coderabbitai

Actionable comments posted: 1

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Inline comments:
In `@src/js_parser/p.rs`:
- Around line 6699-6706: Update the computed-key handling in
lower_ts_auto_accessor so the nearest_stmt_list == None path evaluates the
auto-accessor key once and reuses its stored value for both generated members.
Replace the raw prop.key fallback with an expression-local temporary or
equivalent single-evaluation form, while preserving the existing
capture_computed_key behavior when can_declare_temps is true.

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src/ast/lib.rs
src/js_parser/p.rs
src/js_parser/parse/parse_fn.rs
src/js_parser/parse/parse_property.rs
src/js_parser/visit/mod.rs
src/js_parser/visit/visit_expr.rs
test/bundler/transpiler/decorators-legacy-lowering.test.ts
test/bundler/transpiler/decorators-set-semantics/set-semantics-fixture.ts

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src/js_parser/visit/visit_expr.rs

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…ass too visit_stmts now points nearest_stmt_list at its prefix list before it pre-visits TypeScript enums, so a class expression in an enum initializer can declare its computed accessor key temporary. The accessor lowering always captures a computed key once.

claude

This pull request has now been reviewed several times and this review found new issues. Before patching these one by one, step back: would one root-cause fix close several of them? Is the pull request's scope growing with each push? Prefer root-cause fixes, keep scope fixed, and note out-of-scope improvements as follow-ups.

A user-written `[k = expr]` computed key on a decorated member gets its own temporary again. The linear-lowering test compares M=25000 with M=50000 after a warm-up instead of M=100, so fixed costs do not decide the ratio.

coderabbitai

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test/bundler/transpiler/decorators.test.ts (1)
932-980: 🎯 Functional Correctness | 🟡 Minor | ⚡ Quick win

Make the benchmark distinguish the claimed regression.

Line 932 holds N fixed and doubles only M. An O(M*N) implementation then has a 2x ratio, not 4x. The tLarge < tMid * 3 assertion accepts that regression.

Scale N with M, or use a test matrix that makes the quadratic case exceed the threshold.
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In `@test/bundler/transpiler/decorators.test.ts` around lines 932 - 980, Update
the benchmark around gen and time so the compared runs distinguish O(M*N)
behavior: scale the generated constructor padding N with M, or use multiple M/N
cases where quadratic work exceeds the assertion threshold. Keep the warm-up and
initializer-presence checks, but tighten the final ratio assertion so the
claimed regression cannot pass.

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In `@test/bundler/transpiler/decorators.test.ts`:
- Around line 932-980: Update the benchmark around gen and time so the compared
runs distinguish O(M*N) behavior: scale the generated constructor padding N with
M, or use multiple M/N cases where quadratic work exceeds the assertion
threshold. Keep the warm-up and initializer-presence checks, but tighten the
final ratio assertion so the claimed regression cannot pass.

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Doubling M with N fixed cannot tell O(M*N) from O(M+N). A small M against a large one can: parsing the body dominates the linear case, while a quadratic splice copies the body once per field.

claude

Code review found no issues

No high-confidence issues detected in this change.

robobun · 2026-08-28T22:39:19Z

On the CodeRabbit risk summary: the "decorated abstract auto-accessors may be lost" item refers to an earlier head. Since 054ee93 abstract accessor x: T keeps the Abstract kind when decorated, so its decorator call and design:type metadata are emitted like for an abstract field. The "accessor fields are lowered and can be decorated" fixture has @dec abstract accessor q: number and checks the call against tsc's output.

Current state: every review thread is resolved and CI on 2fe2ee3 is green except for test/js/web/url/url.test.ts on macOS x64, which fails on main as well.

robobun · 2026-08-29T07:01:24Z

Also fixes #20664. The issue's repro, three decorated fields without initializers under experimentalDecorators, prints ["id","name","date"] with this branch and [] on 1.4.1.

Dedupe notes. I ran the tests of #35537, #38953 and #38142 against a build of this branch and closed them in favor of this PR. The details are in a comment on each. Two things remain:

js_parser: keep [[Define]] semantics for decorated class fields #35537 also changed the standard decorator lowering (__publicField for decorated public fields, three tests in es-decorators.test.ts). This PR does not cover that part. js_parser: lower standard decorators without moving class members #40833 does, and those three tests pass on a build of it.
js_parser: fix panic on decorated declare/abstract fields in class expressions with experimentalDecorators #38095 stays open. It drops decorated declare and abstract members of a class expression silently. This PR rejects them with an error. A maintainer needs to pick one.

The mordant lint job is red on this branch: PropertyOpts.is_class (src/js_parser/parser.rs:1321) is only ever written next to the new decorator_scope field, true with Some and false with None. The lint asks to drop the bool and call decorator_scope.is_some() where it is read.

robobun · 2026-10-07T21:17:29Z

#44723 holds the T::TAt line of the parse_stmt.rs hunk here, and it names an anonymous default class default in the statement lowering. If #44723 merges first, a rebase of this PR does not need the token line, or the conversion of an anonymous class with should_lower_standard_decorators to an expression in the export default arm.

robobun assigned dylan-conway Aug 28, 2026

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js_parser: keep [[Define]] semantics for decorated class fields #35537

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js_parser: keep legacy-decorated static field initializers inside the class body #38953

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js_parser: evaluate the computed key of a legacy-decorated member once #38142

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Accept and lower the accessor keyword in TypeScript files using experimentalDecorators #38125

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js_parser: parameter decorators use the enclosing yield context too

23465fc

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Comment thread src/js_parser/visit/mod.rs Outdated

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Comment thread src/js_parser/p.rs

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js_parser: shorten accessor lowering comments

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js_parser: shorten the nearest_stmt_list comment

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js_parser: only reuse a captured key on an accessor getter

0508d41

A user-written `[k = expr]` computed key on a decorated member gets its own temporary again. The linear-lowering test compares M=25000 with M=50000 after a warm-up instead of M=100, so fixed costs do not decide the ratio.

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test: compare M=100 with M=20000 in the linear lowering check

2fe2ee3

Doubling M with N fixed cannot tell O(M*N) from O(M+N). A small M against a large one can: parsing the body dominates the linear case, while a quadratic splice copies the body once per field.

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robobun mentioned this pull request Aug 29, 2026

js_parser: fix panic on decorated declare/abstract fields in class expressions with experimentalDecorators #38095

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robobun mentioned this pull request Sep 18, 2026

js_parser: reject parameter decorators when experimental decorators are off #43321

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robobun mentioned this pull request Oct 7, 2026

js_parser: parse export default @dec class as a class declaration #44723

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robobun mentioned this pull request Oct 9, 2026

js_parser: reject a decorator on a class static block, keep decorators out of the body of declare global #44883

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