bundler: apply the CommonJS unwrap list only to CommonJS files

[robobun]

Problem

• bun build (ESM output) of import React from "react" drops every export of react when node_modules/react holds an ES module ("react": "npm:@preact/compat", a local shim). It prints var require_react = __commonJS(function(exports) {});, and React.version is undefined for every importer. The same holds for the rest of the unwrap list. Found by fuzzing, no tracker issue.
• P::init (src/js_parser/p.rs) sets unwrap_all_requires from the package directory name alone, so an ES module, a JSON file and a text file get force_cjs_to_esm. On a default import, scan_imports_and_exports (scanImportsAndExports.rs:323) restores a __commonJS wrapper, which cannot hold export statements. import txt from "react/NOTICE.txt" gives an object.

Fix

• prepare_for_visit_pass clears unwrap_all_requires for an ES module (import / export syntax, "type": "module", .mjs), and to_lazy_export_ast for JSON and text. import / import() of such a file links like in any other package.
• Stacked on bundler: read destructured require() of an unwrapped package from its import namespace #39184 (the first two commits). Without it, const { version } = require("react") of an ES module prints a call to a wrapper that does not exist. Merge bundler: read destructured require() of an unwrapped package from its import namespace #39184 first, or merge this and close it.
• Behaviour change: a missing export of such a file is a build error (No matching export in "node_modules/react/index.js" for import "default"), as in other packages and bun run. It was a silent undefined.
• Verified: test/bundler/bundler_cjs2esm.test.ts. The released bun fails 12 of the 13 new or changed tests.

Background

• Unwrap list: for ESM output the bundler converts the CommonJS files of react, react-dom, scheduler and four more packages to ES modules (exports.x = ... becomes export { $x as x }). Each require() of them becomes import * as ns.
• FORCE_CJS_TO_ESM marks a converted file. A missing import of it reads undefined, and a default import that needs module.exports at run time restores the wrapper.
• bundler: read destructured require() of an unwrapped package from its import namespace #39184: a destructured require("react") reached the printer as a marker, and the printer used FORCE_CJS_TO_ESM to pick what to print. bundler: read destructured require() of an unwrapped package from its import namespace #39184 gives the destructuring the namespace identifier and deletes that path.

Notes

Commits:

1. bundler: destructured require() of an unwrapped package reads the import namespace (bundler: read destructured require() of an unwrapped package from its import namespace #39184, unchanged).
2. js_printer: remove the was_unwrapped_require require() printing path (bundler: read destructured require() of an unwrapped package from its import namespace #39184, unchanged).
3. bundler: remove the last readers of the unwrapped require() marker. Main gained two readers after bundler: read destructured require() of an unwrapped package from its import namespace #39184 was written (require_namespace_ref, value_is_import_namespace). Commit 1 makes both dead. It also adds a test for a destructured require() of an external package.
4. bundler: apply the CommonJS unwrap list only to CommonJS files. This is the fix for the report: three source lines, the tests, and the removal of a !has_es_module_syntax check in parse_entry.rs that the field now implies.
5. bundler: an ES module by type is not in the CommonJS unwrap list either (from review). A "type": "module" or .mjs file with no import / export statement kept the flag, so its require("./impl.cjs") of a function became a namespace import and the call threw.
6. bun_core: remove GenericIndexOptional::is_some. The printer path that commit 2 deletes was its only caller.
7. js_parser: drop two checks for an unwrapped require() marker that never reach one (from review). visit_decls consumes the marker of an identifier binding before its split_require block runs, and the branches of a conditional initializer never get one.

Repro from the report:

mkdir -p node_modules/react
printf '{"name":"react","version":"19.0.0","type":"module","main":"./index.js"}' > node_modules/react/package.json
printf 'export const version = "19";\nexport default { version };\n' > node_modules/react/index.js
printf 'import React from "react";\nconsole.log(typeof React, React && React.version);\n' > b.js
bun build --target=bun b.js --outfile=out.js && bun out.js   # before: object undefined, after: object 19

An ES module under node_modules/react (or react-dom), bun build output compared with bun run:

entry	before	after	bun run
import React from "react"	object undefined	object 19	object 19
import R, { version } from "react"	both undefined	19 19	19 19
the same with --target=browser --minify	undefined	19	19
default import, plus a .cjs file that calls require("react")	both undefined	both 19	both 19
import React from "react", no default export	builds, undefined	No matching export ... for import "default"	SyntaxError: Missing 'default' export
import { nope } from "react"	builds, undefined	No matching export ... for import "nope"	SyntaxError: Export named 'nope' not found
await import("react") with --splitting, no default export	typeof m.default is object	undefined	undefined
the ES module calls require("./impl.cjs"), which exports a function	TypeError: impl is not a function	works	works
import next to exports.foo = x in react/index.js, default import	React.foo is undefined	5, as under any other name	throws
import txt from "react/NOTICE.txt"	typeof txt is object	string	string
import pkg from "react/package.json" with --splitting	Object.keys(pkg) has default	name,version	name,version
import *, named import, require(), await import()	correct	same output

The four readers of unwrap_all_requires: transpose_require (every require() in the file becomes an import), the checkDCE(); module.exports = require() rewrite and the EsmWithDynamicFallbackFromCjs exclusion in parse_entry.rs, and force_cjs_to_esm in to_ast. #41162 and #41188 each added !has_es_module_syntax next to one reader. The field is now cleared once.

What stays special for these package names: require("react") from any file is still turned into import * as ns by the specifier. For an ES module target that reads the namespace object itself, without the __toCommonJS copy (and its __esModule key) that require() of an ES module in another package gets.

Not changed: the CommonJS files of the unwrap list (the real react, react-dom, scheduler). bundler_npm.test.ts checks the exact size of a real react-dom/server bundle.

Suites run with the debug build: bundler_cjs2esm, bundler_cjs, bundler_jsx, bundler_edgecase, bundler_npm, bundler_splitting, bundler_dynamic_import_dce, bundler_minify, bundler_barrel, bundler_regressions, bundler_browser, bundler_bun, bundler_plugin, bundler_loader, bundler_string, bundler_html, esbuild/default, esbuild/importstar, esbuild/importstar_ts, esbuild/dce, esbuild/tsconfig, esbuild/splitting, esbuild/loader, esbuild/packagejson, transpiler/react-compiler, regression/issue/03844.

[robobun]

^{Updated 12:25 PM PT - Sep 12th, 2026}

✅ @robobun, your commit 5a3a0b88958a5241aa4c2a0bd184dd13c99ede72 passed in Build #114803! 🎉

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🧪   To try this PR locally:

bunx bun-pr 42500

That installs a local version of the PR into your bun-42500 executable, so you can run:

bun-42500 --bun

[robobun]

Status: ready for review.

Reproduced on bun 1.4.3-canary.1+6a92015fc with the script in the PR notes: an ES module at node_modules/react/index.js, import React from "react", bun build --target=bun b.js --outfile=out.js && bun out.js prints object undefined (bun b.js prints object 19). With this branch it prints object 19.

test/bundler/bundler_cjs2esm.test.ts holds the repro and its variants. The released bun fails 12 of the 13 new or changed tests, the debug build of this branch passes all 113.

[coderabbitai]

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Walkthrough

The change updates CommonJS-to-ESM require unwrapping, removes unwrapped-require metadata from printer and linker APIs, and expands bundler tests for destructuring, ES modules, lazy resources, dynamic imports, and interop.

Changes

CommonJS-to-ESM unwrapping

Layer / File(s)	Summary
Parser unwrapping rules src/ast/e.rs, src/js_parser/p.rs, src/js_parser/parse/parse_entry.rs, src/js_parser/parser.rs, src/js_parser/visit/mod.rs, src/bun_core/util.rs	Parser state limits require unwrapping to eligible CommonJS files. Identifier declarations retain the immediate-assignment marker. Bundled require namespace tracking covers records with an unwrapped ID. Lazy exports disable require unwrapping.
Require/import metadata and printing src/bundler/LinkerContext.rs, src/js_printer/lib.rs	The require/import metadata pipeline no longer carries unwrapped-require state. Bundled imports use wrapper and export references, while dev-server imports retain their dedicated path.
Bundler behavior coverage test/bundler/bundler_cjs2esm.test.ts	Tests cover destructured requires, ES modules, lazy JSON and text resources, dynamic imports, CommonJS interop, missing exports, splitting, and mixed module syntax.

Suggested reviewers: jarred-sumner

Priority: ⬆️ High

Merge Risk: ⚪ Minimal · up to 5a3a0

The CommonJS unwrapping change is ready to merge with no unresolved material risk identified.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The description clearly explains the problem, fix, behavior changes, reproduction, and verification. It does not use the template headings exactly, but it provides the required information in equivale…
Title check	✅ Passed	The title is concise, specific, and accurately describes the primary change: limiting the CommonJS unwrap list to CommonJS files.

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[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/js_parser/p.rs`:
- Line 3333: The unwrap_all_requires assignment must also disable require
unwrapping when options::ModuleType::Esm configures the file as ESM, even if
has_es_module_syntax is false. Update the logic near transpose_require to
account for the configured module type, preserving unwrapping only for non-ESM
files.

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• src/js_parser/parse/parse_entry.rs
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• src/js_parser/visit/mod.rs
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[claude]

Code review found no issues

No high-confidence issues detected in this change.

[claude]

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

Additional findings (outside the current diff — GitHub can't attach inline comments there):

• 🟡 src/js_parser/visit/mod.rs — nit: req.unwrapped_id.get().is_some() is now dead — a BIdentifier decl whose require() value carries an unwrapped_id always hits continue 'outer at line 406 first, so this branch of 'split_require is only reached with unwrapped_id == NONE. Fix: drop the || req.unwrapped_id.get().is_some() term, matching commit 3's "remove the last readers of the unwrapped require() marker" and REVIEW.md's delete-dead-code rule.

  Extended reasoning...

  unwrapped_id is set only in transpose_require (p.rs:1729) when state.is_require_immediately_assigned_to_decl is true, which after this PR requires matches!(decl.binding.data, BData::BIdentifier(_)) (visit/mod.rs:359) and a direct EString argument. When set, transpose_require also pushes to imports_to_convert_from_require (p.rs:1706), so back in visit_decls should_unwrap_common_js_to_esm() is true (same feature flag), prev_require_to_convert_count < len is true, decl.binding.data is BIdentifier, decl.value is the ERequireString written at line 368, and req.unwrapped_id.get() is Some — every guard at 394–405 passes and line 406 does continue 'outer. 'split_require at 534 therefore never sees an ERequireString with unwrapped_id set for a BIdentifier binding, and non-identifier bindings now receive E::Identifier instead of ERequireString. No user-visible failure; the term is a leftover reader of the marker this PR set out to remove.

  Verification: nit — the term is unreachable, and it was already unreachable on the base commit; commit 482258f in this PR ("remove the last readers of the unwrapped require() marker") swept the sibling reader in require_namespace_ref but skipped this one. Reachability trace for visit/mod.rs:562 with req.unwrapped_id.get().is_some(): - Line 558 requires decl.binding.data == BIdentifier; line 535…

[robobun]

Applied the nit in 5a3a0b8. The || req.unwrapped_id.get().is_some() term in the split_require block of visit_decls is gone. The same commit drops the req.unwrapped_id.get().is_none() guard in conditional_namespace_records, which is also never false: e_if ignores its ExprIn, so the branches of a conditional initializer never get a marker. The marker now has one reader, the identifier branch of visit_decls, plus the debug_assert! in the printer. bundler_cjs2esm (113) and bundler_dynamic_import_dce (274) pass with the debug build.

[claude]

Code review found no issues

No high-confidence issues detected in this change.