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Show an anonymous default export as default, not starDefault - #47502

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Show an anonymous default export as default, not starDefault#47502
robobun wants to merge 3 commits into
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robobun/2cdb9aa5/default-export-display-name

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@robobun robobun commented Oct 30, 2026 •
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Problem

  • Stack frames, Bun.inspect, snapshots and the debugger show an anonymous default export as starDefault. Examples: at new starDefault, [class starDefault]. The name property and Node say default.
  • The text is JavaScriptCore's private name for the binding. The engine stored it as the name of the function.
  • No user reported this (Notes). Since bun test: Vitest compatibility #44833, main gives it as a mock name too: jest.fn(fn).name.

Fix

  • Use the WebKit build of [JSC] Store the name default for an anonymous default export WebKit#950, which stores the name default. Bun changes no name itself.
  • JSC__JSValue__getNameProperty loses a dead fallback (src/jsc/bindings/bindings.cpp:5406). The ahead-of-time compiler keeps HAS_EXPLICIT_NAME for export default class {}, so compiled code names the class as plain Bun does.
  • Verified: the 14 new tests fail with main's engine and pass with this one (13 files, Notes).
  • Self-reviewed: 8 concerns raised, 8 addressed. Left out: a test of the new toStrictEqual result, which Jest does not give.

Background

Downsides

  • Recorded text changes for export default class {}: snapshot values, describe(Class) titles and jest.fn(Class).name. Notes has the list.
  • toStrictEqual compares class names. An instance of an anonymous default class now equals a like instance of another class named default.
  • Not reached: export default function () {} from a file is still <file>_default (js_parser: attach the generated default name to an anonymous default function only in a bundle #34932). An async arrow with an await has an unnamed frame.
Notes

This is a draft for CI. WEBKIT_VERSION is the preview build of oven-sh/WebKit#950 (autobuild-preview-pr-950-c5dabb29: Bun's pin 4d45755fed plus that one commit). It must not merge like this. Before a merge the pin has to be a merged WebKit commit with builds for every platform. Fork main is ahead of Bun's pin (six commits on 1 November), so the commit reaches Bun main with the first pin bump after it merges, or with a bump to a merged commit of this one alone. The tests and the two source changes here land with that bump or right after it.

Where the rule is. oven-sh/WebKit#950 has the cause, the design, the measurements and four questions for a maintainer: is the change of text wanted, is JavaScriptCore the place, patch or minor release, and one compare per new function. #38517 and #38530 are the other place for the rule, a filter in Bun's bindings. They stay open until that is answered. If WebKit#950 merges, this pull request takes their place. #34932 lands after this one.

Text that changes

  • Snapshot values, re-recorded with bun test -u: [class starDefault] to [class default], starDefault {} to default {}, <starDefault /> to <default />, Any<starDefault> to Any<default>. GitHub code search finds no public Bun Snapshot v1 file with starDefault in it, of about 5,400.
  • Titles from describe(Class), test(fn) and .each: starDefault > title to default > title. A snapshot key, a JUnit name and a -t pattern with starDefault in it change with the title. Such a -t pattern matches nothing any more.
  • jest.fn(fn).name and mock(fn).getMockName(): "default" for every form in the table below. Bun 1.4.3 gives "". Main gives "starDefault" since bun test: Vitest compatibility #44833, which reads the raw name of the function (copyNameAndLength in src/jsc/bindings/JSMockFunction.cpp). jest.fn(fn).getMockName() is "jest.fn()" on main, whatever fn is.
  • Stack frames: at new default, was at new starDefault. A tool that groups errors by frame text sees a new group once.
  • The two errors that name a class declaration: Cannot call a class constructor default without |new|, Cannot return a non-object type in the constructor of a derived class default.
  • In the debugger, the frame of the constructor that JavaScriptCore makes for export default class {} has a scope with a variable default. A class with a written name shows its name there in the same way.

What gets the name, by form

form after this change
export default class {} default in frames, Bun.inspect, snapshots, both class errors, mock names, heap snapshots, the debugger, --cpu-prof
export default () => {}, (function () {}), (class {}) default in frames, Bun.inspect and mock names. The name is inferred, so a snapshot prints [Function] or [class], and the class errors have no name
export default function () {}, also function* and async function from a file the transpiler names them <file>_default first, as before (#34932). They get default where the text reaches JavaScriptCore as written: node:vm SourceTextModule, and a file that starts with // @bun
export default async () => { await x } an unnamed frame, before and after. With oven-sh/WebKit#820 the body gets the name

Not changed

  • A frame of a getter or setter, and a frame of a function with a displayName, still change text once .name is read (functionName in src/jsc/bindings/ErrorStackTrace.cpp). That is not about default exports.
  • The variable edge of the module binding in a heap snapshot is still starDefault, and so are three SyntaxErrors for a second export default declaration in one module. They show the binding, not the function.
  • Error.captureStackTrace(err, fn) matches frames by name as before. A check of the source range emptied the stack for return super.method(), so it is not here (see Error.captureStackTrace: keep frames when the callee frame was elided by a tail call #32136).
  • A bundle names the function <file>_default, as before.

Source changes

  • JSC__JSValue__getNameProperty: the deleted arm returned the raw name of the executable when JSFunction::name() was empty. With the new engine the two are the same string. bun:test: print a function's own name instead of its AsyncFunction / GeneratorFunction tag #38959 changes the same function, and the second of the two to land rebases.
  • finish_constructor_header in the ahead-of-time compiler: a class declaration with no name is export default class {}. It keeps HAS_EXPLICIT_NAME when the name is its own, as src/aot/lower/function/header.rs does for export default function () {}. Compiled code then gives the same mock names, class errors and snapshot text as plain Bun. Before, plain Bun said starDefault there and compiled code said nothing.
  • Neither adds work at run time. The cost of the engine change is in [JSC] Store the name default for an anonymous default export WebKit#950: +2 instructions for each function executable that is made, and name reads get cheaper.
  • Measured on release builds of main (8971677) and of this branch, linux x64, same toolchain. .text of bun: 108,951,913 to 108,951,337 bytes (-576). .rodata, .data and .bss do not change. require("typescript"): +56,613 user instructions of 814,854,823 (+0.007%, minimum of 9 interleaved rounds, control +7,940). After that load, no count in heapStats().objectTypeCounts differs (70 object types).
  • CI's size test (test/bundler/compile-aot-size.test.ts, red on main for its budget) reports the bytes that a trimmed --aot hello world maps: 25,953,252 here and 25,961,444 on main at the same commit (builds 136363 and 136333), 8,192 fewer.

Tests

  • test/js/bun/test/stack.test.ts: error.stack, CallSite#getFunctionName, the two class errors and Bun's error printer, for nine forms, before and after .name is read.
  • test/js/bun/util/inspect.test.js, test/js/bun/test/snapshot-tests/snapshots/moremore.test.ts: Bun.inspect and snapshot text.
  • test/js/bun/test/mock-fn.test.js, test/js/bun/test/describe.test.ts: mock names and describe(Class) titles.
  • test/js/node/v8/capture-stack-trace.test.js: Error.captureStackTrace with a resumed anonymous default function.
  • test/js/bun/util/v8-heap-snapshot.test.ts, test/cli/inspect/inspect.test.ts, test/cli/run/cpu-prof.test.ts, test/v8/v8.test.ts: both heap snapshot formats, the debugger, an inlined frame of the profiler, v8::Function::GetName.
  • test/cli/test/test-aot.test.ts, test/aot/ir/lower/classes/anonymous-class-names.ts, test/aot/parallel/errors/names-of-functions-behind-export-default: compiled code against plain Bun.
  • On a debug ASAN build of this branch (main cee9dd9) with the preview engine: the 14 new tests pass. In ten of the eleven bun test files 1,178 tests pass and 1 fails. In test/v8/v8.test.ts I ran the new test alone, and it passes.
  • The same objects linked against the engine that main pins (4d45755fed): the 14 new tests fail, and no other result changes.
  • The 1 failure is a matcher after .resolves or .rejects waits for a pending promise in test/cli/test/test-aot.test.ts, a test that main has. On a debug build it fails with both engines: plain bun test prints one more <anonymous> frame than the compiled run.
  • cargo test -p bun_aot --test ir_tests: the changed file passes. The programs under test/aot/parallel need a canary build, so CI runs them.
  • Two public traces with the old text: treetrum/amazon-kindle-bulk-downloader issue 148 and extractus/article-extractor issue 412 have at starDefault frames. Neither asks for another name.

no test proof · iteration 3 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/v8/v8.test.ts, test/js/bun/util/inspect.test.js, test/js/bun/test/describe.test.ts, test/cli/run/cpu-prof.test.ts

@robobun

robobun commented Oct 30, 2026 •
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Updated 3:18 AM PT - Nov 1st, 2026

❌ @robobun, your commit 9926993 has 41 failures in Build #136363 (All Failures):

🧪   To try this PR locally:

bunx bun-pr 47502

That installs a local version of the PR into your bun-47502 executable, so you can run:

bun-47502 --bun

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robobun commented Oct 30, 2026 •
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Status

This was referenced Oct 30, 2026
Open
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@robobun
robobun force-pushed the robobun/2cdb9aa5/default-export-display-name branch from 5870c1d to f18aaf0 Compare October 31, 2026 01:15
@robobun
Add tests for the name of an anonymous default export
713fec4 
JavaScriptCore binds an anonymous default export to the private name
`*default*`, whose text is `starDefault`. The tests hold each reader of a
function name to `default`: stack frames and call sites, `Bun.inspect`, mock
names, `describe(Class)` titles, the two heap snapshot formats, the debugger,
`--cpu-prof` and `v8::Function::GetName`.

The tests pass with the JavaScriptCore change that stores `default` in the
executable. `JSC__JSValue__getNameProperty` no longer reads the raw name of
the executable when `JSFunction::name()` is empty: the two are the same
string now.
@robobun
Give export default class {} its declared name in compiled code
4196393 
JavaScriptCore now has the name `default` as the declared name of an
anonymous default class. The ahead-of-time compiler kept that class as one
with an inferred name, so compiled code left the name out of the two class
errors, of mock names and of snapshot text. `finish_constructor_header`
keeps `HAS_EXPLICIT_NAME` for the class, as `header.rs` does for the
function of `export default function () {}`.

More tests: snapshot text, the scope that holds the built-in constructor
under its name in the debugger, a source that is already transpiled, and
`bun test --aot` against `bun test`. The `--cpu-prof` test runs again
when the profile has no sample in the function it looks for.
@robobun
Use the preview build of oven-sh/WebKit#950
9926993
@robobun
robobun force-pushed the robobun/2cdb9aa5/default-export-display-name branch from f18aaf0 to 9926993 Compare November 1, 2026 05:52

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