js_parser: rewrite every assignment target of a lowered `#private` member by robobun · Pull Request #42651 · oven-sh/bun

robobun · 2026-09-13T19:47:49Z

Problem

With a standard decorator on #x, each write except plain = fails at load: this.#x++ (Postfix ++ operator applied to value that is not a reference.), this.#x += 1 (Left side of assignment is not a reference.), [this.#x] = a (Invalid destructuring assignment target.), for (this.#x of a) (Cannot reference undeclared private names: "#x").
rewrite_private_accesses_in_expr (src/js_parser/lower/lower_decorators.rs:274) knows a read, a plain assignment and #x in o. Every other target gets the read form, __privateGet(this, _x)++. The loop head (:571) is skipped.

Fix

Add esbuild's __privateWrapper helper. __privateWrapper(o, _x)._ is a property that reads and writes the member. It replaces the operand of ++ and --, each destructuring target and each for-in or for-of head.
A compound or logical assignment through this or an identifier becomes __privateSet(o, _x, __privateGet(o, _x) + v) or __privateGet(o, _x) ?? __privateSet(o, _x, v), as in esbuild. Any other receiver uses the wrapper, which evaluates it once.
Verified: test/bundler/transpiler/es-decorators.test.ts. The new privateTargets fixture prints what node prints for the same classes without decorators. Main fails 335 of 423 tests. Other decorator and transpiler suites pass.

Background

Standard decorator lowering (js_parser: lower standard decorators without moving class members #40833) keeps undecorated #private members native. A name with a decorated member becomes a WeakMap, so the decorator helpers can reach it.
__privateGet and __privateSet are bun:wrap helpers that read and write such a member after a brand check. src/ast/runtime.rs lists them.
An assignment target must be a reference: an identifier, a property access, or a pattern. A call is a value.

Notes

Found while working on #31910. There is no user report.

The parser output changed, so EXPECTED_VERSION of the runtime transpiler cache goes to 33.

Two bundler snapshots (bundler_promiseall_deadcode.test.ts, cyclic-imports-async-bundler.test.js) get a new debugId. The debugId prints the content hash of the chunk. That hash mixes in the part index of each statement the chunk takes from a file (generate_isolated_hash, src/bundler/LinkerContext.rs:1872). __esm and __promiseAll are statements of runtime.js below the new helper, so their part index moves by one. The bundles and their source maps are byte for byte the same.

Output for @dec #x, with f() as a receiver that cannot be repeated:

source	output
`this.#x++`	`__privateWrapper(this, _x)._++`
`this.#x += 2`	`__privateSet(this, _x, __privateGet(this, _x) + 2)`
`o.#x ??= 5`	`__privateGet(o, _x) ?? __privateSet(o, _x, 5)`
`f().#x += 2`	`__privateWrapper(f(), _x)._ += 2`
`[this.#x, o.#x = 1, ...f().#x] = a`	`[__privateWrapper(this, _x)._, __privateWrapper(o, _x)._ = 1, ...__privateWrapper(f(), _x)._] = a`
`for (this.#x of a)`	`for (__privateWrapper(this, _x)._ of a)`
`this.#acc++` (decorated `accessor`)	`__privateWrapper(this, _acc, _acc_acc.set, _acc_acc.get)._++`
`this.#g++` (getter and setter)	`__privateWrapper(this, _g, _g_set, _g_get)._++`

Differences from esbuild 0.21.5 (--target=es2021, which lowers every private name):

esbuild captures a receiver like f() in a temporary: __privateSet(_a = f(), _x, __privateGet(_a, _x) + 2). Here a temporary made in a field initializer or a parameter default is shared between evaluations (js_parser: declare decorator lowering temporaries per evaluation in parameter defaults and field initializers #38904). The wrapper needs no temporary, so this lowering uses it for every receiver that cannot be repeated.
esbuild prints [o._x = 1] = a and for (this._x of a) for a defaulted target and a loop head. Both lose the private member.
++ and -- use the wrapper in both. __privateGet(o, _x) + 1 is wrong for a BigInt and for a string.

Cost of the wrapper: 5e6 iterations of this.#x += 1 take about 65 ms through __privateSet/__privateGet and about 300 ms through __privateWrapper (release build, x64).

Not writable members keep their TypeError: a write to a method or to a getter without a setter reaches member.set on a WeakSet, as this.#m = v did before. For a method the wrapper gets () => _m_fn as its getter, so a read gives the method: f().#m ??= v gives the method and f().#m++ throws on the write, as the native member does.

The new fixture section covers each kind (field, BigInt field, accessor, getter and setter, static field, method, getter alone, setter alone), each operator, the value of each expression, the number of receiver and right side evaluations, nested functions and classes, a field initializer, a static block, a parameter default, and the calls a decorated accessor receives. A bundle for --target=browser of the same fixture gives the same output under node.

An alternative that this PR does not take: keep a decorated #x native and give __decorateElement { has, get, set } closures made inside the class, as tsc does. That removes the WeakMap, the walker and the production code of this PR. #40833 kept the WeakMap for decorated names on purpose, so this PR extends the walker. The privateTargets fixture does not depend on the lowering and can be the acceptance test of such a change.

Other forms the walker does not know, not fixed here: import(this.#x) is left as written, and a tagged template this.#tag`x` loses its receiver. Optional chains are #31910.

[human-review] gate passed · iteration 0 · 8 files touched

fails on main (without fix)

ASAN without fix: 338 FAILED
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/bundler_promiseall_deadcode.test.ts test/bundler/transpiler/es-decorators.test.ts test/regression/issue/cyclic-imports-async-bundler.test.js
bun test v1.4.3 (b99371011)

test/bundler/bundler_promiseall_deadcode.test.ts:
72 |       },
73 |     },
74 |     onAfterBundle(api) {
75 |       const bundled = api.readFile("out.js");
76 | 
77 |       expect(bundled).toMatchInlineSnapshot(`
                           ^
error: expect(received).toMatchInlineSnapshot(expected)

  
  "var __esm = (fn, res, err) => () => {
    if (fn)
      try {
        res = fn(fn = 0);
      } catch (e) {
        err = [e];
      }
    if (err)
      throw err[0];
    return res;
  };
  var __promiseAll = (args) => Promise.all(args);
  
  // StoreDependencyAsync.ts
  var somePromise;
  var init_StoreDependencyAsync = __esm(async () => {
    somePromise = await Promise.resolve("Hello World");
  });
  
  // StoreDependency.ts
  function StoreDependency() {
    return "A string from StoreFunc" + somePromise;
  }
  var init_StoreDependency = __esm(async ()
... (truncated)

release without fix: 360 FAILED
bun test v1.4.3-canary.1 (b99371011)

test/bundler/bundler_promiseall_deadcode.test.ts:
72 |       },
73 |     },
74 |     onAfterBundle(api) {
75 |       const bundled = api.readFile("out.js");
76 | 
77 |       expect(bundled).toMatchInlineSnapshot(`
                           ^
error: expect(received).toMatchInlineSnapshot(expected)

  
  "var __esm = (fn, res, err) => () => {
    if (fn)
      try {
        res = fn(fn = 0);
      } catch (e) {
        err = [e];
      }
    if (err)
      throw err[0];
    return res;
  };
  var __promiseAll = (args) => Promise.all(args);
  
  // StoreDependencyAsync.ts
  var somePromise;
  var init_StoreDependencyAsync = __esm(async () => {
    somePromise = await Promise.resolve("Hello World");
  });
  
  // StoreDependency.ts
  function StoreDependency() {
    return "A string from StoreFunc" + somePromise;
  }
  var init_StoreDependency = __esm(async () => {
    await init_StoreDependencyAsync();
  });
  
  // SecondElementImport.ts
  function SecondElementImport() {
    console.log("SecondElementImport called", formValue.key);
    return formValue.key;
  }
  var init_SecondElementImport = __esm(async () => {
    await init_
... (truncated)

passes on PR (with fix)

ASAN with fix: all passed
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test "--reporter=junit" "--reporter-outfile=/tmp/pr_gate.xml" test/bundler/bundler_promiseall_deadcode.test.ts test/bundler/transpiler/es-decorators.test.ts test/regression/issue/cyclic-imports-async-bundler.test.js
bun test v1.4.3 (b99371011)

test/bundler/bundler_promiseall_deadcode.test.ts:
(pass) bundler > bundler/__promiseAll is tree-shaken when only one async import exists but __esm remains [1525.88ms]
(pass) bundler > bundler/__promiseAll is included when multiple async imports exist with __esm [912.04ms]
(pass) bundler > bundler/__promiseAll is tree-shaken when no async imports despite circular deps with __esm [670.70ms]

test/regression/issue/cyclic-imports-async-bundler.test.js:
(pass) cyclic imports with async dependencies should generate async wrappers [1148.21ms]

test/bundler/transpiler/es-decorators.test.ts:
(pass) ES Decorators > class decorators > basic class decorator [338.11ms]
(pass) ES Decorators > class decorators > class decorator receives correct context [342.42ms]
(pass) ES Decorators > class decorators > class decorator can replace class [429.84ms]
(pass) ES Decorators > 
... (truncated)

release with fix: all passed
$ bun scripts/build.ts --profile=release
[configured] bun-profile → bun (stripped)
  target       linux-x64-gnu
  build type   Release
  build dir    ./build/release
  revision     6ab7b84e2a
  features     baseline

23 deps, 131 codegen, 1176 objects in 895ms

ninja: Entering directory `/workspace/bun/build/release'
[1/1248] install /workspace/bun
bun install v1.4.3-canary.1 (b99371011)

Checked 22 installs across 61 packages (no changes) [26.00ms]
[2/1248] gen ErrorCode+*.h
[3/1248] install /workspace/bun/packages/bun-error
bun install v1.4.3-canary.1 (b99371011)

Checked 1 install across 2 packages (no changes) [1.00ms]
[4/1248] install /workspace/bun/src/node-fallbacks
bun install v1.4.3-canary.1 (b99371011)

Checked 111 installs across 104 packages (no changes) [14.00ms]
[5/1248] fetch zlib
[zlib] up to date
[6/1248] gen bindgenv2
[7/1248] gen node-fallbacks/react-refresh.js
Bundled 1 module in 10ms

  react-refresh.js  4.81 KB  (entry point)

[8/1248] fetch tinycc
[tinycc] up to date
[9/1247] gen bake.{client,server,error}.js
-> bake.client.js, bake.server.js, bake.error.js
[10/1247] fetch libjpeg-turbo
[libjpeg-turbo] up to date
[11/1247] gen .bind.ts → G
... (truncated)

diff hotspot

src/ast/op.rs                                      |  22 ++
 src/ast/runtime.rs                                 |  24 +-
 src/js_parser/lower/lower_decorators.rs            | 252 +++++++++++++++++----
 src/jsc/RuntimeTranspilerCache.rs                  |   3 +-
 src/runtime.js                                     |   8 +
 test/bundler/bundler_promiseall_deadcode.test.ts   |   4 +-
 test/bundler/transpiler/es-decorators.test.ts      | 236 +++++++++++++++++++
 .../issue/cyclic-imports-async-bundler.test.js     |   2 +-
 8 files changed, 499 insertions(+), 52 deletions(-)

gate history · 1 passed · 0 rejected · iteration 0

evidence per changed file

file                                                      reads  edits  tests
src/ast/op.rs                                                 1      1     24
src/ast/runtime.rs                                            1      0     24
src/js_parser/lower/lower_decorators.rs                       5      2     24
src/jsc/RuntimeTranspilerCache.rs                             1      1     24
src/runtime.js                                                1      1     24
test/bundler/bundler_promiseall_deadcode.test.ts              0      0      4
test/bundler/transpiler/es-decorators.test.ts                 4      1     20
…t/regression/issue/cyclic-imports-async-bundler.test.js      0      0      4

Standard decorator lowering turns a #private name with a decorated member into a WeakMap and rewrites each access. The walker knew a read, a plain assignment and `#x in o`. Every other position that writes to the member got the read form, a call, which is not a valid assignment target: `this.#x++`, `this.#x += 1`, `this.#x ??= 1`, `[this.#x] = a`, `({ a: this.#x } = o)`. The head of `for (this.#x of a)` was left as written, a private name that no longer exists. Add esbuild's __privateWrapper helper. `__privateWrapper(o, _x)._` is a property that reads and writes the member, so it is valid wherever a reference is: the operand of ++ and --, a destructuring target (nested, with a default, or a rest element), and the head of for-in and for-of. A compound or logical assignment through `this` or an identifier reads and writes with __privateGet and __privateSet, as esbuild does; any other receiver goes through the wrapper, which evaluates it once.

robobun · 2026-09-13T19:48:31Z

Status: ready for review. The diff is green. CI build 115295 has one red test, test/js/bun/spawn/spawn-pipe-leak.test.ts (an RSS test of Bun.spawn pipes, x64 lanes). This PR does not touch that code, its base is the head of main, and the same test was red in build 115261. Each other failure in that build passed on retry.

Self-reviewed: 4 points raised, 3 addressed. A lowered method now reads as itself through the wrapper (6ab7b84). The Notes now say that there is no user report, and they give the correct reason for the two debugId snapshot updates. Not addressed here: import(this.#x) and the tagged template receiver. They are reads, not assignment targets, and the tagged template needs the receiver capture that the work on #31910 adds. The Notes list both.

How I reproduced it: on main (09bb546, canary 1.4.3-canary.1+09bb54630) each form in the table below fails to load. On this branch each prints what node prints for the same class without the decorator.

function dec() { return (v, ctx) => {}; }
class Foo {
  @dec() #x = 1;
  run() { this.#x++; return this.#x; }   // SyntaxError: Postfix ++ operator applied to value that is not a reference.
}
console.log(new Foo().run());            // expected 2

body of `run()`	main	this branch and node
`this.#x++`, `++this.#x`, `this.#x--`	`Postfix/Prefix ++/-- operator applied to value that is not a reference.`	2, 2, 0
`this.#x += 2`, `**= 3`, `??= 5`, `\|\|= 5`, `&&= 5`	`Left side of assignment is not a reference.`	3, 1, 1, 1, 5
`[this.#x] = [9]`, `({ a: this.#x } = { a: 7 })`	`Invalid destructuring assignment target.`	9, 7
`for (this.#x of [4]) {}`, `for (this.#x in { k: 1 }) {}`	`Cannot reference undeclared private names: "#x"`	4, "k"

bun bd test test/bundler/transpiler/es-decorators.test.ts passes 423 of 423 on this branch. The same file run by canary 09bb546 fails 335.

coderabbitai · 2026-09-13T19:56:38Z

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src/js_parser/lower/lower_decorators.rs
test/bundler/transpiler/es-decorators.test.ts

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Walkthrough

The change adds runtime wrapper support and lowering for decorated private members used in assignment-target positions. It covers compound, logical, unary, destructuring, spread, and loop forms, with tests for evaluation behavior and errors.

Changes

Decorated private assignment targets

Layer / File(s)	Summary
Runtime contracts and operator mapping `src/ast/op.rs`, `src/ast/runtime.rs`, `src/runtime.js`	Adds compound-assignment operator mapping and registers the exported `__privateWrapper` runtime helper.
Assignment-target lowering and cache update `src/js_parser/lower/lower_decorators.rs`, `src/jsc/RuntimeTranspilerCache.rs`	Rewrites decorated private members in compound, logical, unary, destructuring, spread, and loop assignment targets. Updates the runtime transpiler cache version.
Assignment-target coverage `test/bundler/transpiler/es-decorators.test.ts`, `test/bundler/bundler_promiseall_deadcode.test.ts`, `test/regression/issue/cyclic-imports-async-bundler.test.js`	Adds tests for assignment forms, accessors, receiver evaluation, nested cases, bundling modes, expected errors, and updated bundle snapshot identifiers.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to 6ab7b

The decorated private assignment-target lowering is covered across the changed forms, and no actionable merge risk remains.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the main change: rewriting assignment targets for lowered `#private` members.
Description check	✅ Passed	The description explains the problem, implementation, scope, and verification results. It does not use the template headings exactly, but it provides the required information and is sufficiently compl…

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Inline comments:
In `@src/js_parser/lower/lower_decorators.rs`:
- Line 300: Add __privateWrapper to Bake’s synthetic bun:wrap export object and
its bake.private.d.ts declaration, matching the existing normal runtime
registration so decorated private assignment targets resolve during Bake/HMR.

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📒 Files selected for processing (6)

src/ast/op.rs
src/ast/runtime.rs
src/js_parser/lower/lower_decorators.rs
src/jsc/RuntimeTranspilerCache.rs
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test/bundler/transpiler/es-decorators.test.ts

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robobun · 2026-09-13T19:59:48Z

^{Updated 2:16 PM PT - Sep 13th, 2026}

❌ @robobun, your commit 6ab7b84 has 1 failures in Build #115295 (All Failures):

test/js/bun/spawn/spawn-pipe-leak.test.ts - code 1 on 🐧 25.04 x64
test/js/bun/spawn/spawn-pipe-leak.test.ts - code 1 on 🐧 3.23 x64

🧪 To try this PR locally:

bunx bun-pr 42651

That installs a local version of the PR into your bun-42651 executable, so you can run:

bun-42651 --bun

The source map of a bundle maps `__esm` and `__promiseAll` to their position in runtime.js, and the debugId is a hash of the map. The new `__privateWrapper` helper sits above both.

claude

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

…rapper `f().#m ??= v` gives the method and `this.#m++` throws on the write, as the native member does. The wrapper had no getter for a method, so the read threw.

claude

Code review found no issues

No high-confidence issues detected in this change.

github-actions Bot added the claude label Sep 13, 2026

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Comment thread src/js_parser/lower/lower_decorators.rs

test: update the debugId of two bundler snapshots

07f6dec

The source map of a bundle maps `__esm` and `__promiseAll` to their position in runtime.js, and the debugId is a hash of the map. The new `__privateWrapper` helper sits above both.

github-actions Bot reviewed Sep 13, 2026

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