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…wn default symbol
`export default (function a() {})` and `export default (class a {})` bind
`a` in the scope of the expression. The parser also stored that symbol as
the symbol of the default export, which every later stage treats as a
module-scope binding.
The parser now makes the generated `<file>_default` symbol for these two
forms, as it does for every other expression. The name of the expression
is an ordinary nested symbol again. `S::ExportDefault.default_name` states
the rule, and the visit pass checks it in debug builds.
This corrects the output of `--no-bundle --minify-identifiers`, of minified
bundles, of bundles that import `export default (function a(a) {})`, of
`--splitting` and of a lowered top-level `using` for such a module.
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Status: ready for review. How I reproduced it, on a debug build of main:
PR: #46097 |
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Updated 9:05 AM PT - Oct 21st, 2026
❌ @robobun, your commit 5ccc59c has 1 failures in
🧪 To try this PR locally: bunx bun-pr 46097That installs a local version of the PR into your bun-46097 --bun |
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No actionable comments were generated in the recent review. 🎉 ℹ️ Recent review info
WalkthroughDefault-export expressions now reuse identifier references when applicable and use generated module-scope names otherwise. Lowered ChangesDefault export name binding
Suggested reviewers: Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to
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| Check name | Status | Explanation |
|---|---|---|
| Title check |  |
The title clearly and concisely describes the primary parser change for default-exported function and class expressions. |
| Linked Issues check | |
Issue #23810 requires a minified import to retain the value exported by its module. parse_stmt.rs now gives expression-form default exports a generated module-scope default symbol. This removes the … |
| Out of Scope Changes check | |
The parser, visitor assertion, cache version, lowered-using collision handling, and regression tests support the default-export symbol fix and its related minification failures. The tests cover bund… |
| Description check | |
The description clearly explains the problem, root cause, fix, scope, known limitations, and verification results. It does not use the exact template headings, but it provides the required information… |
Full details: Linked Issues check
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Explanation
Issue
#23810requires a minified import to retain the value exported by its module.parse_stmt.rsnow gives expression-form default exports a generated module-scope default symbol. This removes the nested-scope symbol that caused the imported value to use a function parameter. Newbundler_minifytests cover the reported minified bundle path and related import paths. The cache version increases to 36 for affected entries.
Full details: Out of Scope Changes check
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Explanation
The parser, visitor assertion, cache version, lowered-
usingcollision handling, and regression tests support the default-export symbol fix and its related minification failures. The tests cover bundling, no-bundle output, splitting, CommonJS, directeval, decorators, and transpilation. No unrelated product change is demonstrated.
Full details: Description check
-
Explanation
The description clearly explains the problem, root cause, fix, scope, known limitations, and verification results. It does not use the exact template headings, but it provides the required information through the Problem, Fix, and Suites sections.
✨ Finishing Touches: 💡 2
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- Resolve merge conflict in branch
robobun/8ff251fc/default-export-expr-own-symbol
- Resolve merge conflict in branch
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- Create a new PR
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Actionable comments posted: 1
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Inline comments:
Review comments at @test/bundler/bundler_minify.test.ts:
- Around line 2446-2448: Update the parameterized test for `transformSync` and
`transform` to use `test.concurrent.each`, keeping the existing cases and test
body unchanged.
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📒 Files selected for processing: 6
src/ast/s.rssrc/js_parser/p.rssrc/js_parser/parse/parse_stmt.rssrc/js_parser/visit/visit_stmt.rssrc/jsc/RuntimeTranspilerCache.rstest/bundler/bundler_minify.test.ts
💤 Files with no reviewable changes: 1
- src/js_parser/p.rs
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked two things in the deleted/added parser code and ruled them out: the removed EImportIdentifier + macro arm of default_name_for_expr was unreachable (the parse pass only constructs EIdentifier; EImportIdentifier is created in the visit pass, and the only parse-side mention in src/js_parser/parse/parse_jsx.rs is a match arm, not a constructor), and the new debug_assert! in src/js_parser/visit/visit_stmt.rs comparing against module_scope().generated is consistent with create_default_name pushing to the current scope, because export default is only parsed where allows_esm_import_export() holds (module scope, or a declare namespace whose body is not visited).
Extended reasoning...
The change removes default_name_for_expr from src/js_parser/p.rs, makes src/js_parser/parse/parse_stmt.rs generate a <file>_default symbol for every non-identifier default-export expression, adds a debug assertion in visit_stmt.rs, documents the invariant in src/ast/s.rs, and bumps the transpiler cache version to 36; it touches no security-sensitive surface. Two confirmed findings are posted inline, so this note only records the additional refactor-preservation and assertion-scope checks that were examined and found not to be problems.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟡
src/js_parser/visit/visit_stmt.rs— Users running an unbundled file that has a top-levelusingandexport default (function a(){})can now get a duplicatevar <file>_defaultSyntaxError or silent shadowing where the base ran correctly. src/js_parser/parse/parse_stmt.rs:1198 now generates<file>_defaultfor named function/class expressions, and src/js_parser/visit/visit_stmt.rs:520-556 emitsvar <file>_default = ...; export { <file>_default as default }with no renamer in --no-bundle mode, so any user binding with that name collides. Fix: in paths that skip the renamer, make create_default_name (p.rs:4535) pick a name not present in module_scope().members, so every expression form (object, arrow, call, and now function/class) is safe.Why this was flagged
Trigger: a file
lib.tswith top-levelusing x = ...(lowered),const lib_default = 1;(or any binding named<file>_default), andexport default (function a(){}), run withbun run lib.tsorbun build --no-bundle. parse_stmt.rs:1198 now produces the generated symbol for the function expression; visit_stmt.rs:520-556 then printsvar lib_default = function a(){}andexport { lib_default as default }by original name because the non-bundle printer does not rename. The user'sconst lib_defaultand the emittedvar lib_defaultcollide: SyntaxError (let/const) or a silently overwritten value (var). On the base the emitted binding for this input wasa, which cannot collide with<file>_default. The dismissal called it pre-existing because other expression forms already had this exposure; but the function/class expression population is new to this path, and the collision is name-derived from the file name, so it is deterministic for the affected file. Remedy: pick a collision-free generated name when no renamer runs.Verification: Trigger:
bun build --no-bundlewith a non-Bun target, a top-levelusing, a user binding named<file>_default, andexport default (function a(){}). src/js_parser/parse/parse_stmt.rs:1192-1199 sends named function/class expressions top.create_default_name, which makes a symbol named"{file}_default"with no hash suffix. On base, it printedvar a = function a(){}and did not collide.
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About the additional finding of the review: a lowered top-level
No change in this PR for it. |
…declares With a lowered top-level `using` and no bundle, `export default <expr>` prints as `var <file>_default = <expr>` and an export clause. A module that declares `<file>_default` itself printed that name twice. The `var` takes a temporary name in that case. Also shorten three comments.
…ult-export-expr-own-symbol # Conflicts: # src/jsc/RuntimeTranspilerCache.rs
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I re-reviewed this PR after the latest commit (a6a8f29) and found no bugs, but because it changes the symbol model for every default-exported named function/class expression in the parser, with user-visible output differences and the size trade-off listed in the description, a maintainer look is still worthwhile.
What was reviewed:
- The new lowered-
usingtaken-name guard in src/js_parser/visit/visit_stmt.rs:522-530: the lookup uses the module scope's member table (hoistedvar,function,const, and import bindings all land there), the generated<file>_defaultsymbol is not a member so it cannot match itself, and the bundle path is correctly left to the linker's renamer. - The
debug_assert!at visit_stmt.rs:464:export default <expr>is only visited at module scope (declare namespacebodies returnS::TypeScriptbefore visiting), and parse-passEIdentifierrefs are alwaysstore_name_in_refslices, so the identifier arm always reachescreate_default_namefirst. - The deleted
EImportIdentifierarm ofdefault_name_for_exprwas unreachable: nothing in src/js_parser/parse/ constructs anEImportIdentifier. - The
CopyExprDatamatch in parse_stmt.rs:1190 does not partially moveexpr; the cache version bump covers the changed transpiled output.
Extended reasoning...
The change touches the JS parser's export default handling (src/js_parser/parse/parse_stmt.rs, src/js_parser/visit/visit_stmt.rs), deletes P::default_name_for_expr from src/js_parser/p.rs, documents the invariant on S::ExportDefault in src/ast/s.rs, bumps the transpiler cache version, and adds ~490 lines of bundler and Bun.Transpiler tests. It touches no security-sensitive surface. The core logic is small and matches esbuild (createDefaultName for every expression form), the new guard and assertion check out, and the tests cover bundle/no-bundle, ESM/CJS, splitting, keep-names and lowered-using variants. Deferring rather than approving because the PR intentionally changes emitted output for a common input shape and the description itself lists size and naming trade-offs that a maintainer should weigh; no CODEOWNERS entry covers the changed paths.
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Update on the lowered
The anonymous declaration ( |
…6270) ### What does this PR do? Hardening follow-up to #46021. It fixes regressions from #46021 and #44618 that were never released, includes more of the open pull requests that fix a crash, and fixes a few crashes that had no pull request. #### Regressions fixed (none was released) - `spyOn(Bun, "$")`, and the same for `"sql"`, `"postgres"` and `"SQL"`, threw "does not support accessor properties yet". - Async `fs.read`, `fs.write` and `zlib.brotliCompress` on a view over a `WebAssembly.Memory` could lose data or copy far more than they needed. - `node:vm` with `vm.constants.DONT_CONTEXTIFY`: `this` in host accessors, prototype handling, `util.types.isProxy`, `Bun.deepEquals`, `assert.deepEqual` and `console.log` of a context. - `zlib` `stream.flush()` threw for arguments it used to accept, and streams had a new enumerable property. - `Bun.Transpiler` `exports.replace` with the name `"default"`. - `process.env.X = 3000` could store the number once the function was JIT-compiled. - `node:sqlite` and `bun:sqlite` could lose rows at exit. - Dev server: stylesheets shared between pages after a build error. - Shell `cp` of a file onto itself exited 1. - `error.stack`: the frame of `require()` reads `at anonymous (unknown)` again. - `import * as process from "process"` followed by `process.on(...)` threw at load. The EventEmitter methods that the module exports now act on `process`. - `delete res.statusCode; res.end()` in a `node:http` handler threw. It sends 200, as in Node. (from #46128) - `node:tls`, `node:https` and `node:http2`: connection teardown and write callbacks behave as in 1.4.3 and Node again. #### Crash fixes Bundler, parser and CSS: #42286, #46092, #46097, #46177, #46239, #36334, #43024, #40769. `bun test`, `--hot` and `--watch`: #30776, #38206, #38974. Shell: #43183, #37743, #46168, #46207, #34900. `node:http`, `node:http2`, `Bun.serve`, streams: #43449, #45380, #44457, #42197, #42485, #38082, #38124, #46074, #45999, #46045. `node:vm`, native addons, workers: #46129, #46198, #45615, #33412, #38031. Dev server: #42790, #42850, #39488. Other: #35991, #38030, #38586, #40526, #41830, #46029, #46078, #46114, and the bug of #42215. #### Behavior changes - `crypto.randomFill(view, cb)` reports `ERR_CRYPTO_OPERATION_FAILED` if it could not fill the view. It used to report success. - A function registered twice on `process` runs twice, as in Node. (#41830) - Under `--hot`, a reload after an uncaught exception cancels the timers that the failed run created. The new run's timers fire, and so do timers and `fs.watchFile` watchers that earlier runs kept on `globalThis`. The reload also resets the exit code to 0. (#38206) - The parent of a Worker no longer gets an `'error'` event for a failed `Bun.serve` request in that Worker. The error is printed, as on the main thread. - `hash.write()` after `digest()` drops the chunk, as in Node. (#35991) - `http2` over TLS with `server.emit("connection", socket)`: `session.socket` is a `TLSSocket` and a `'connection'` listener fires, as in Node. (#38124) - A `slice()` with no type of a body `Blob` has `type === ""`, as in Node. (#46114) - `util.types.isProxy(globalThis)` is false, as in Node. - A write on a `node:net` or `node:tls` socket that is still pending when the connection closes calls back with `ECANCELED`, as in Node. - `HTMLRewriter.on()` throws for a selector that is nested too deeply. - `Bun.Image`: `maxPixels` also bounds PNG metadata, `metadata()` of a GIF reports the size that gets decoded, and a path with a NUL byte throws. (#40526) - `bun build --no-bundle` with the `base64` or `dataurl` loader prints an error. (#36334) - Bundled output for a default-exported function or class expression changes to `var file_default = function name() {}`, as in esbuild. `.name` is unchanged. (#46097) - A Proxy in the prototype chain of a `node:vm` global is allowed. jsdom 30 with `runScripts` needs this. - `bun:sqlite` commits a write that `iterate()` left open when the main thread exits. An explicit `BEGIN` is still rolled back. - When a Worker stops while a native addon's async work is still running, the addon's finalizers run after that work is done, as in Node. - Shell: `mv -n` and `cp -n` don't overwrite, and `cp` reads `R`, `v` and `n` anywhere in a flag group. #### Performance - Ordinary `node:vm` contexts run at the same speed. With `DONT_CONTEXTIFY`, stores to globals and reads of inherited names in a hot loop cost 10 to 20 % more than on `main`. - A `process.env` write costs 20 to 35 % less than after #46021. - An async read into a `WebAssembly.Memory` view looks through the live memories: about 1.5 µs more with one memory. #### Not included - Taken in part, so these should stay open for the rest: #36334, #38974, #39488, #40526, #40769, #42485, #45615, #45999, #46029, #46092, #46114, #46168, #46207, #46239. - Needs a WebKit change: #34655 #39204 #40170 #40253 #41587 #41900 #42699 #42897 #42903 #43084 #43813 #44518 #44949 #45770 #45804 #45827. - #44723 (covered by #46097), #41675, #42753, #38762, #41475, #46016. ### How did you verify your code works? - Every item was reproduced first on release builds of `main`, of its parent and of 1.4.3, and compared with Node v26 where Node has the API. - Every fix has a test that fails on `main` and passes on a debug + ASAN build of this branch, except where that isn't practical (huge inputs, code that only runs on Windows or macOS). - The regressions were checked on grids of calls against the build before #46021 and against 1.4.3. - Bun's own test suites and Node's `test-vm-*`, `test-http2-*` and `test-zlib-*` files for the areas touched.
Fixes #23810
Problem
bun build --minifyprintsvar n=function(o,a,s){...};async function e(n,o){await o.register(n)}.--no-bundle --minify-identifiersgives a local the name of the function:TypeError: a is not a function.--splitting --minifyfails withpanic: index out of bounds: the len is 0 but the index is 0.default_name_for_expr(src/js_parser/p.rs:4559) makes the name of the expression the symbol of the default export. That name belongs to the scope of the expression. Each later stage expects a module-scope binding.Fix
<file>_defaultsymbol for these forms, as for every other expression and as esbuild does.export default function a() {}does not change.usingand no bundle, that symbol prints as avar. If the module declares<file>_defaultitself, thevartakes a temporary name.EXPECTED_VERSIONis 39. Verified: 23 new cases intest/bundler/bundler_minify.test.ts, 21 fail on main. Self-reviewed: 10 concerns raised, 4 addressed, 6 listed in Notes as not fixed here.Background
export default (function a() {})bindsaonly inside the function. The declaration form bindsain the module.print_ast. A bundle does not callprint_ast, so Error in minified single-file executable application #23810 stays.Downsides
var controller_default = function Controller() {}, and.nameiscontroller_defaultwhen--minify-syntaxremoves the name.--no-bundle --minify-identifiersminifies the name of the expression.const { inc } = require("./barrel")over a re-export of such a function grows from 247 to 834 bytes minified..text: -1,024 bytes for the parser change. Each cached source is transpiled again one time.Notes
The release numbers compare two release builds made on one machine (linux x64): main at 017b243, and the parser change of this PR on top of it. The doc comment, the
debug_assert!and theusingguard came later. The first two add no code to a release build. The guard is one hash lookup on the loweredusingpath without a bundle. Its size is not in the number above: I did not build the two release binaries again.What changes
--no-bundle --minify-identifiers:export default (function a(a) { var longname = 5; var othername = 7; return a + longname * othername; })var a = 5; var e = 7; return a + a * e,f(1)is 40export default (function a(n) { var longname = n - 1; return n <= 1 ? 1 : n * a(longname); })TypeError: a is not a functionexport default (class a { static make(n) { var count = n + 1; return new a(count); } ... })TypeError: 2 is not a constructorbun build --minifyof the #23810 shape, also with--compile --sourcemapbun buildwith no flags, importingexport default (function a(a) { return a + 1; })ReferenceError: a is not definedexport { a as default }--splitting --minify-identifiers, entry chunks without a nested scopepanic: index out of bounds: the len is 0 but the index is 0--no-bundle, lowered top-levelusing,let a = 1; export default (function a(n) {...})var atwice, the module'sais the functiontop 1--no-bundle, lowered top-levelusing,const lib_default = 9inlib.js,export default (n) => n + lib_defaultvar lib_defaulttwice, the call returns a stringrequire()orimport()of such a module,--minify-identifiersTypeErrorexport default (function a(n) { return eval(...) }), no flags101 1012 101export default @dec class Model {}imported elsewhere,--minify-identifiersTypeError: e.make is not a functionbun pm diffof two versions that differ only in local names+5 -5for the fileformatting onlyprint_astpin each export through its link chain. The name offunction a(a)links to its parameter, so the first row was correct before that change and is wrong since. Bun 1.4.3 has that change.export default (function foo(foo) {})as its "known sibling 4". Its testedgecase/ExportVarRedeclaredLaterKeepsNameWhenMinifiedis red on main because of the first row. With this change alone its input prints the expected2 36.History
createDefaultNameuntil e490577 (2021-08-08), which started to use the name of the expression "since it's a better name". esbuild uses a declared name only for the statement forms and callscreateDefaultName()for every expression.EImportIdentifierarm of the removed function could not run. The parse pass makesEIdentifieronly. The visit pass makes the import identifiers.export default <identifier>keeps its path: the visit pass makes its default name. A move of that to the parse pass would change the order of symbols, and so the minified names of other bindings.usingguard: the first commit of this PR printedvar lib_defaulttwice forconst lib_default = 9; export default (function work(n) { return n + lib_default; })inlib.jswith a loweredusing. Main printsvar workthere. The guard removes that, and the same collision that main has for the other expression forms.S::ExportDefault.default_namenow states the rule. The visit pass checks it with adebug_assert!. With the two old arms put back, that assert fires for the function and the class expression, and not for an object, an identifier or a declaration.Outputs that change in modes that work today
var controller_default = function Controller(a) {...}(main:var Controller = function Controller(a))..namestaysController. A top-levelclass Model {}in another module keeps its name now. Main printed it asModel2.--minify-syntaxand no--minify-identifiers: the unread name is removed, so.nameiscontroller_default(main:Controller). With--keep-namesit isControlleron both. esbuild 0.21.5 prints the samevar controller_default = function(a).--no-bundle --minify-identifiersandBun.Transpilerwithminify.identifiers:export default (function factorial(n) {...})printsexport default (function r(t) {...}), as for any other function expression and as in esbuild. Main keptfactorialbecause it was pinned.--keep-namesdoes not keep it: that flag has no effect on function expression names under--minify-identifiers, on main too.export default function factorialkeeps its name.usingwithout a bundle: the binding and the export clause are<file>_default.<file>_defaultprints asvar lib_default = function lib_default2(n), so its.nameislib_default2. esbuild does the same.require()pattern in Downsides: the linker binds arequire()pattern local only to a function symbol (binds_call_item,src/bundler/LinkerContext.rs). Main bound it because the export symbol was the name of the function expression. That binding is the Error in minified single-file executable application #23810 mechanism: with one more function that has a parameter, main printsfunction l(n){return n(n.n)}. An anonymous function, an arrow, a class and aconstalready read the namespace object.Measurements
gdbstepifrom the entry ofP::t_exportto its return, for onetransformSynccall of a one-statement module, on the unstripped release binaries. Six calls for each source, all six equal.export default foo;export default {a:1};export default (function a(n){return n});export default (class A {});export default function a(){}create_default_nameone time, asexport default {a:1}does. The class case also takes an allocator slow path.sizegives 106,766,588 bytes of text on main and 106,765,564 with the change, forbun-profile. The fourdefault_name_for_exprinstances (two distinct copies of 433 bytes) are gone (llvm-nm).test/node_modules. None has the shape.transformSyncwithout minify,transformSyncwithminify.identifiers, andBun.buildof each of the 3,110 files with a default export, with and withoutminify: 0 outputs differ (sha1).cmp -l). After the upgrade 2 of 2 entries are written again one time.perf,valgrindandstraceare not available on the build machine.Not fixed here
Main has each of these defects too. This change does not touch them.
_ssignature temp in the scope of the component and declares it at the top level (handle_react_refresh_hook_call,src/js_parser/p.rs, under aTODO: fix the renamer bug).bun build --react-fast-refresh --minify-identifierswith two hook components in two modules printsvar i = a();twice. A general assert ("a top-level declared symbol has no nested slot") cannot be added before that is fixed.export default function App() { useState() }under--react-fast-refreshprintsconst App = _s(function App() {...}); var App = App;.export default (function X() { useState() })under--react-fast-refreshprintconst default_exporttwice.const lib_default = 1; export default function () {}inlib.jsdoes not load:SyntaxError: Cannot declare a const variable twice: 'lib_default'. The anonymous declaration prints asfunction lib_default().EXPECTED_VERSIONwhile this PR was open. js_parser: parseexport default @dec classas a class declaration #44723 bumps it too, so the one that lands second takes the next number.Suites
bundler_minify(88, on the merge with main at 2d6d445),bundler_cjs,bundler_cjs2esm,bundler_barrel,bundler_regressions,bundler_minify_symbol_for,bundler_dynamic_import_dce,bundler_jsx,bundler_decorator_metadata,esbuild/dce,esbuild/importstar,esbuild/importstar_ts,esbuild/ts,esbuild/splitting,transpiler/transpiler,transpiler/runtime-transpiler,transpiler/es-decorators,bake/dev/esm,bake/dev/bundle,bake/dev/hot.bake/dev/productionandbake/dev/react-spapass with--timeout 120000. At the default limit, 10 and 2 of their tests time out in this debug build on the build machine.bundler_splittingandesbuild/defaulthave tests that stop at the 5 second limit in this debug build, such assplitting/BarrelCycleKeepsImportOrder*anddefault/TopLevelAwaitForbiddenRequireSourceMapCLI. Those two stop there on a debug build of main too. All other tests of those files pass.[human-review] gate passed · iteration 0 · 6 files touched
fails on main (without fix)
passes on PR (with fix)
diff hotspot
gate history · 1 passed · 0 rejected · iteration 0
evidence per changed file