bundler: pick the __toESM interop from the importer's module type - #41150
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The bundler passed `isNodeMode = 1` to `__toESM` for every importer that uses ESM syntax, because `input_module_type` came from `exports_kind`. A default import of a CommonJS module that sets `__esModule` then got the whole `module.exports` instead of `exports.default`, unlike `bun run` and esbuild. TypeScript and Babel output sets that marker, so this broke the default import of many packages (styled-components, react-bootstrap). Carry the resolver's module type on `BundledAst` and hand that to the printer. Only an ESM-by-type importer (`.mjs`, `.mts`, or package.json `"type": "module"`) gets Node's interop. The external `require()` path now prints `, 1` under the same rule, as esbuild does. The resolver no longer turns a matched `"import"` or `"require"` export condition into a module type, and the extension now wins over any package.json `"type"`. Node decides the format of a file from its extension and the nearest package.json only. A task that bypasses the resolver (a plugin result, an in-memory source) falls back to the extension, and a server component re-parse keeps the original's type.
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Status Reproduced with the released bun (1.4.1) on the snippet in the PR body: Fix is in this PR. Locally: Supersedes #35656 (closed). |
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WalkthroughChangesThe bundler now preserves resolved module type separately from Module type resolution
Suggested reviewers: Merge Risk: 🔵 Low · up to The PR corrects CommonJS default-import interop based on the importing file’s module type. Merge is otherwise reasonable, but one bundler test assertion should be updated to match the emitted spacing before or with merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
Full details: Linked Issues checkExplanation The changes address both linked issues [ Comment |
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In `@test/bundler/bundler_cjs.test.ts`:
- Line 776: Update the expectation in the relevant bundler test to match the
printer’s spacing by changing the __toESM assertion to include a space after the
comma, while preserving the existing outputPaths configuration and /out/entry.js
target.
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src/ast/nodes.rssrc/bundler/AstBuilder.rssrc/bundler/LinkerContext.rssrc/bundler/ParseTask.rssrc/bundler/bundle_v2.rssrc/bundler/bundled_ast.rssrc/js_printer/lib.rssrc/options_types/bundle_enums.rssrc/resolver/lib.rssrc/resolver/package_json.rssrc/resolver/resolver.rstest/bundler/bundler_cjs.test.tstest/bundler/bundler_edgecase.test.tstest/bundler/bundler_npm.test.ts
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I reviewed this PR and didn't find any bugs. Because it changes the bundler's CJS/ESM default-import interop semantics — user-visible output behavior — and rewires how the resolver assigns module_type (dropping the export-condition contribution and moving the extension check to run unconditionally over any package.json "type"), a human look would still be worthwhile.
What was reviewed:
- The
module_typeplumbing throughBundledAst,ParseTask::run,LinkerContext, andbundle_v2::enqueue_file— the new column is initialized on every construction path (empty_in,init,AstBuilder, server-components re-parse). - The resolver change in
finalize_result: the extension check now keys offresult.path_pair.primary.name().extafter all path rewriting, so.mjs/.cjswin over"type"regardless of how the file was reached; the removedext.len() == 4guard was redundant with the string map. - The printer's new
, 1on the external-require()__toESMwrapper mirrors the existing bundled-require_*()path and matches the esbuild output cited in the description. - The six existing
bundler_cjsassertions that flipped from "whole exports object" toexports.default— each is a.jsimporter with no"type", consistent with the new rule.
Extended reasoning...
Overview
This PR changes how bun build picks the isNodeMode argument to the __toESM interop helper. Previously it was derived from the parsed ExportsKind (any file with import/export syntax got isNodeMode=1); now it is derived from a new module_type: ModuleType column on BundledAst, populated from the resolver's determination — the file extension (.mjs/.mts/.cjs/.cts) or the nearest package.json "type". The resolver stops treating a matched "import"/"require" export condition as a module-type signal, and the extension check moves to the end of finalize_result to run unconditionally on the resolved primary path. ParseTask fills in the extension-derived type for tasks that bypass the resolver (plugin results, in-memory sources), and bundle_v2::enqueue_file threads the original file's module type into server-components SSR/browser re-parses. The printer also gains the , 1 suffix on the external-require() __toESM path. ~50 lines of source change, ~360 lines of new itBundled tests, plus three test/snapshot updates for the shifted output columns.
Security risks
None identified. This is bundler output-shaping logic; no untrusted input parsing, no auth/crypto/permissions surface. The resolver changes remove code rather than adding new lookup paths, and module_type_from_ext is a fixed comptime string map — making it pub doesn't expose anything sensitive.
Level of scrutiny
High. This is a user-visible behavioral change to CJS/ESM interop — a notoriously subtle area where "matches esbuild and Node" is the correctness bar and getting it wrong silently breaks default imports of TypeScript/Babel-compiled packages. Six pre-existing test assertions were flipped to the new behavior, and the resolver's module_type output is consumed elsewhere (the parser's ambiguous-file hint, per the PR notes), so the removal of the export-condition contribution has effects beyond the __toESM printer path. The PR description is unusually thorough (cites esbuild 0.21.5 output per importer kind, names the superseded PR, documents a known gap around nameless package.json), which raises confidence, but the semantic surface area warrants a maintainer confirming this is the interop model Bun wants to commit to.
Other factors
Test coverage is strong — 17 new itBundled cases covering .ts/.mjs/.mts importers, "type": "module"/"commonjs", per-file interop within one bundle, dynamic import with and without splitting, external CJS in CJS output, and four "exports"-map scenarios — and the PR states 17 of the 45 tests fail on released Bun (satisfying the "test fails without the fix" bar). The From<ExportsKind> for ModuleType impl and its comment stub are deleted with their last consumer, per the dead-code rule. The bundled_ast.rs field-count comment was updated (26 → 29), though I did not independently count. No CODEOWNERS check was performed; the bundler/resolver is core enough that maintainer sign-off is appropriate regardless.
…M argument The happy-dom and libraries.js outputs no longer pass `, 1` to __toESM, so their JS and bytecode hashes moved. Every CI platform (Linux glibc and musl on x64 and aarch64, ASAN, Windows x64, macOS aarch64) produced the same new values.
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Updated 7:05 PM PT - Sep 1st, 2026
✅ @robobun, your commit 0e6f964e6baef8bb9e1f007ef118dab40f2c23fa passed in 🧪 To try this PR locally: bunx bun-pr 41150That installs a local version of the PR into your bun-41150 --bun |
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…amespace (#41162) ### Problem - `import React from "react"` undoes the CommonJS to ESM lifting of `react`, `react-dom`, `scheduler` and any `exports.x = ...` file. `scanImportsAndExports.rs` put a `FORCE_CJS_TO_ESM` module back into a `__commonJS` wrapper once one import record had `CONTAINS_DEFAULT_ALIAS`. Every importer then read each hook through a `__toESM` getter. - A same-chunk `import()` of a lifted module had no `default` (#14061). ### Fix - The linker binds the default import of a lifted module to its `exports_ref`, like `import *`. The member binding from #41009 then resolves `React.useState` to the lifted `$useState`. The namespace object exists only when `React` escapes. - A module that sets `exports.__esModule` keeps its wrapper when the importer is not an ES module by type, because the default import then depends on that flag at run time. The new AST flag `COMMONJS_LIFTED_TO_ESM` excludes real ESM files of the react family. - A same-chunk `import()` of a lifted module prints `__toESM((init_x(), exports_x))`, so `default` is `module.exports`. - Verified: `test/bundler/bundler_cjs2esm.test.ts` (14 new tests, all fail on the released bun), `bundler_cjs.test.ts`, and the suites in the notes. ### Background - Lifting turns `exports.foo = x` into `var $foo = x; export { $foo as foo }`. The default import of a CommonJS module is `module.exports`. For a lifted module that is the module namespace, so `React.useState` reads a static export. - `exports_ref` is the symbol of a module's namespace object. `bind_import_property_accesses` (#41009) rewrites `X.name` on an import bound to it into the export and drops the use of `X`, so the namespace part tree-shakes. - `__toESM(mod, isNodeMode)` builds the ESM view of a CommonJS object: `default` is `mod` unless `mod.__esModule` is truthy and `isNodeMode` is 0 (#41150). <details><summary>Notes</summary> Suites run: `esbuild/importstar`, `esbuild/importstar_ts`, `esbuild/default`, `esbuild/dce`, `bundler_splitting`, `bundler_edgecase`, `bundler_jsx`, `bundler_npm`, `bundler_barrel`, `bundler_minify`, `bundler_browser`, `bundler_bun`, `bundler_loader`, `bundler_plugin`, `bundler_compile`, `bundler_string`, `bundler_html`, `bun-build-api`, `transpiler/react-compiler`, `regression/issue/03844`. `this` in a bound call: `React.createElement(...)` compiles to `$createElement(...)`, so `this` is `undefined` instead of `module.exports`. That is what a named import of the same lifted module already did, and what the `import *` member binding (#41009) and esbuild do for namespace members. The react family never reads `this` in an exported function (the files are compiled from ES modules, where module-level `this` is `undefined`). A module whose exported function reads `this` keeps it only when the call goes through the namespace object. Measured on the repro from the request, with `react@19.2.8` and `NODE_ENV=production --minify`: `import React from "react"; export const el = React.createElement("div")` went from 8206 to 1392 bytes, the same as the `import *` form. A small counter app with `react` and `react-dom/client`, `--production --target=browser`: 192699 to 185777 bytes. Semantics, all identical to the existing `import *` path documented in `docs/bundler/index.mdx`: `React === R` for `import * as R` (the namespace object doubles as `module.exports`), the namespace object has no separate `default` key, and `React.fn()` passes `undefined` as `this`. Node gives `module.exports` as `this` there. Key order: `Object.keys(React)` on an escaped default import used to be assignment order (through `__toESM`) and the `import *` namespace was sorted. Both are assignment order now (`doStep5.rs` skips the sort for a lifted module). Two tests in `bundler_cjs.test.ts` changed: `__toESM_import_syntax_namespace` (order) and `__toESM_mixed_import_styles` (the `import *` namespace no longer has a `default` key once the module stays lifted; a lone `import *` never had one). Decision table for `import X from "./lifted"`, by the lifted module's exports and the importer's module type: - no `__esModule`: `X` is the namespace, for any importer. `X.default` is the lifted `default` export if there is one, and otherwise the namespace itself, the same as `ns.default` on `import * as ns` (so `X === ns` and `X.default === ns.default`). - `__esModule` and an ESM-by-type importer (`.mjs`, `.mts`, `"type": "module"`): `X` is the namespace (Node ignores `__esModule`). - `__esModule` and any other importer: the module keeps its `__commonJS` wrapper and `__toESM` decides at run time, as before. `.default` on a namespace of such a module (`import * as ns`, `export * as Lib`) stays a property read, so it is `exports.default` (`undefined` when absent), as `__toESM` would give. `ns.default` on `import * as ns from "./lifted"`: when the module exports `default`, it stays bound to that export (the namespace object's own `default` key, as before). Otherwise it takes the same table, so `ns.default === ns` without `__esModule`, where it was `undefined` before. #34935 reports the missing `default` and proposes to wrap the module instead. `import()`: a same-chunk dynamic import of a lifted `Esm` module used to print `Promise.resolve().then(() => (init_x(), exports_x))`, with no `default`. It now prints `__toESM((init_x(), exports_x))`, with `, 1` for an ESM-by-type importer. The printer change moves the comma-operator parentheses inside the `__toESM(` call. The three scenarios from #35696 (`await import()`, `yield import()`, mixed static named import) pass without a wrapper. A split `import()` keeps the synthetic default export it had. Found on the way and handed off: `sideEffect(); module.exports = require("./x")` in a react-family package (`react-dom/index.js`, `react-dom/client.js`) never takes the `export *` conversion in `parse_entry.rs`, so those two files keep their wrapper in production builds. </details>
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… no default (#41213) ### Problem - Since #41150, `import D from "pkg"` in a `.js` or `.ts` file gives `D === undefined` after `bun build` when the CommonJS package sets `__esModule` and has no `default` property. Affected packages include rxjs, redis, mobx, path-to-regexp, react-router-dom, yup and formik. `bun run`, Node and the bundler before #41150 give `module.exports`. - Cause: `__toESM` (`src/runtime.js:59`) returns `mod.default` when `__esModule` is truthy and the importer is not in node mode. #41150 took `.js` and `.ts` importers out of node mode. ### Fix - `__toESM` returns `mod.default` only when `mod` has its own `default` property. Otherwise the default import is `mod`. This is the `bun run` rule (`populateESMExports`, `src/jsc/bindings/JSCommonJSModule.cpp:998`). - The #41150 fix stays. A module with `__esModule` and `default`, such as styled-components, still gives `exports.default`. - `lifted_default_import_needs_wrapper` (`src/bundler/LinkerContext.rs:4232`) applies the same rule to the static binding of a lifted module. - Verified: `test/bundler/bundler_cjs.test.ts` and `bundler_cjs2esm.test.ts` (7 cases fail on main). The other suites are in the notes. ### Background - `__toESM(mod, isNodeMode)` is the runtime helper that builds the ES module view of a CommonJS module in a bundle. `isNodeMode` is 1 for an `.mjs`, `.mts` or `"type": "module"` importer. - TypeScript and Babel set `exports.__esModule = true` when they compile an ES module to CommonJS. A file with only named exports gets the marker and no `default`. - A lifted module is a CommonJS file of the React family. The bundler turns its `exports.x = ...` assignments into ES exports (#41162). <details><summary>Notes</summary> Repro, with a package that sets `__esModule` and has no `default`: ``` # node_modules/nodefault/index.js: # "use strict"; Object.defineProperty(exports, "__esModule", { value: true }); exports.foo = "foo"; # entry.ts: import D from "nodefault"; console.log(JSON.stringify(D)); bun entry.ts # {"foo":"foo"} bun build entry.ts --outfile=out.mjs && bun out.mjs # undefined on canary, {"foo":"foo"} with this change ``` Real packages, `import D from "<pkg>"` in a `.ts` file, bundled and run. Canary gives `undefined` and this branch gives the exports object for: - rxjs, path-to-regexp, esprima, redis, mobx, mobx-react-lite, react-router-dom (`--target=bun` and `--target=node`) - yup, history, class-validator, class-transformer, formik (`--target=node`). With `--target=bun` these resolve to an ES module entry with no default export, and the build fails on every version. esbuild's `__toESM` gives `undefined` for this shape. This change follows `bun run` and Node instead, so a bundle behaves like the unbundled code. Expectation updates: the minified helper is 22 bytes longer. That moves the line 1 columns and the file size in `npm/ReactSSR`, the first mapping in `edgecase/EmitInvalidSourceMap2`, the `BundledReactPreservesImportRefs` snapshot, and the two bundler entries in `bundler_bytecode_portable.test.ts` (the `js` hash, and the bytecode built from it). Unchanged: an `.mjs` importer still gets Node's interop, the whole `module.exports` even when `default` exists. `bun run` gives `exports.default` there. That difference predates #41150. Suites run with the debug build: `bundler_cjs`, `bundler_cjs2esm`, `transpiler/react-compiler`, `esbuild/{importstar,importstar_ts,default,splitting,extra,dce}`, `bundler_{npm,edgecase,splitting,barrel,regressions,bun,browser,loader,plugin}`, `bun-build-api`, `bundler_bytecode_portable`, `regression/issue/03844`. A few bytecode tests in the last two exceed the 5 s default timeout in a debug build. They pass with `--timeout`. </details>
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…ed or not (#41232) ### Problem - Regression on main from #41150. No release has it. `bun build` reads `"type"` from the wrong package.json, so it misses a `{ "type": "module" }` that has no `"name"`. Two common places: a project root, and a dual package's `dist/esm/package.json`. - A `.js` or `.ts` file there gets `exports.default` for the default import of a CommonJS module with `__esModule`. Node, esbuild and bun 1.4.0 give the whole `module.exports`. The bundle has `__toESM(require_tsdep())`, without `, 1`. - Cause: `finalize_result` (`src/resolver/resolver.rs:1669`) read `"type"` from the package root, or else from `enclosing_package_json`. `dir_info_uncached` (`src/resolver/resolver.rs:6357`) sets that field only for a named package.json (#229). ### Fix - `DirInfo` gets `package_json_for_module_type`: the nearest package.json in the directory or above it, named or not. `finalize_result` reads `"type"` from it for the primary path. The extension still wins. - Correct because esbuild uses this rule, and Node ignores `"name"` too. - Only the bundler reads `Result.module_type`. `enclosing_package_json` does not change. The four runtime lookups in `src/runtime/jsc_hooks.rs` are out of scope. - Verified: `test/bundler/bundler_cjs.test.ts`, 10 new cases, 9 fail on main. Self-reviewed: 3 concerns raised, 3 addressed. Other suites in Notes. ### Background - `__toESM(mod, isNodeMode)` builds the ESM view of a CommonJS module. With `, 1` (Node mode), `default` is the whole `module.exports`. Without it, `default` is `mod.default` when `__esModule` is set. - `DirInfo` is the resolver's cached record for one directory. Its "enclosing" fields come from the parent. - Open PRs in this area: #33883, #33807, #33890, #40940. This PR supersedes none. Notes cover #40940. <details><summary>Notes</summary> Found by comparing `bun build` on main with bun 1.4.0, Node 26 and esbuild 0.25. No issue is open for it. Repro for the dual-package face: ```sh D=$(mktemp -d); cd $D; mkdir -p node_modules/pkg/dist/esm node_modules/tsdep echo '{"name":"tsdep","version":"1.0.0","main":"index.js"}' > node_modules/tsdep/package.json echo 'Object.defineProperty(exports,"__esModule",{value:true}); exports.default=function styled(){}; exports.css="css";' > node_modules/tsdep/index.js echo '{"name":"pkg","version":"1.0.0","main":"./dist/esm/index.js"}' > node_modules/pkg/package.json echo '{"type":"module"}' > node_modules/pkg/dist/esm/package.json echo 'import styled from "tsdep"; export const seen = typeof styled + "/" + typeof styled.default;' > node_modules/pkg/dist/esm/index.js echo 'import { seen } from "pkg"; console.log(seen);' > app.mjs node app.mjs # object/function bun build ./app.mjs --target=node --outfile=out.mjs && node out.mjs # main: function/undefined, this PR: object/function ``` For the project-root face, put `{ "type": "module" }` (no `"name"`) in the project's package.json and bundle a `.js` file that imports `tsdep`. Faces of the bug on main. Each has a test: - A project package.json with `"type"` and no `"name"` (case 58). - The nested marker reached through `"main"`, `"module"` or a relative path (cases 53, 55, 56). Through an exports map it worked, because `handle_esm_resolution` reads the file's own directory. - A nested package.json with a `"name"`, reached through `"main"` (case 54). `result.package_json` was the package root, so the nested file was not read at all. - A file in a subdirectory of the marker (case 57). Why a new field instead of widening `enclosing_package_json`: that field also names the package for `sideEffects`, the auto-install version gate and `bun run` script discovery. #33883 widens it for every consumer and had to rework the `sideEffects` loop in `finalize_result` to keep the DCE tests passing. Four cases pin the lookup rule. Each result matches esbuild 0.25.1: - Case 59: a nameless `{ "type": "commonjs" }` below a `"type": "module"` package wins, because it is the nearest. - Case 60: a nearest package.json without `"type"` is the scope. The lookup does not continue to a typed package root, so the importer is not ESM by type. Main read the root's `"type"` here. Node prints `object/function` for this shape, but only because it detects ESM syntax in a file with no `"type"`. #41150 chose the esbuild rule for such files. - Case 61: the lookup does not stop at a `node_modules` directory. A package without a package.json of its own takes the `"type"` above it. Node prints the same result. - Case 62: only the primary path decides. With the default target, `"module"` is the primary path and `"main"` is the fallback for `require()`. The fallback's package.json does not count. esbuild has the same check. The case fails when the check is removed. Overlap with #40940: it adds a field with the same name, but its lookup stops at `node_modules`, and the runtime reads it too. If #40940 lands after this PR, it must choose one rule for the field. Case 61 pins the crossing for the bundler. Node's stop can still apply at the runtime read sites. #40940 also calls the lookup for the fallback path, which case 62 rejects. Out of scope: the runtime's four lookups (`src/runtime/jsc_hooks.rs` lines 1474, 2972, 3220 and 4071) keep `package_json().or(enclosing_package_json)`. So `bun run` still skips a nameless package.json above the file's own directory. That gap predates #41150. For this import, `bun run` 1.4.1 gives `exports.default` for every importer, even `.mjs`. Self-review, the three concerns and what changed: - Document the `node_modules` rule on the field. Done in `src/resolver/dir_info.rs`. - Add a default-target case with both `"main"` and `"module"`. That is case 62. - Say in this body that no release has the bug, lead with the project-root face, and name the runtime lookups that stay. Suites run with the fix on a debug ASAN build, after a rebase on main: `bundler_cjs` (62), `esbuild/packagejson`, `esbuild/dce`, `esbuild/default`, `bundler_cjs2esm`, `bundler_npm`, `bundler_edgecase`, `bundler_regressions`, `bundler_splitting`, `bundler_barrel`, `cli/run/run-cjs`, `test/js/bun/resolve`. All pass except the second case of `test/js/bun/resolve/load-same-js-file-a-lot.test.ts`. It times out at 5 s on this build with and without this change (back-to-back runs on the same machine). </details> <!-- robobun:evidence:begin --> --- **[human-review]** gate passed · iteration 0 · 4 files touched <details><summary>fails on main (without fix)</summary> ```console ASAN without fix: 9 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_cjs.test.ts bun test v1.4.1 (a6c4cc2) test/bundler/bundler_cjs.test.ts: (pass) bundler > cjs/__toESM_import_syntax_with_esModule [945.38ms] (pass) bundler > cjs/__toESM_import_syntax_without_esModule [445.45ms] (pass) bundler > cjs/__toESM_import_syntax_function [371.94ms] (pass) bundler > cjs/__toESM_import_syntax_primitive [387.72ms] (pass) bundler > cjs/__toESM_import_syntax_named_and_default [361.78ms] (pass) bundler > cjs/__toESM_import_syntax_namespace [367.76ms] (pass) bundler > cjs/__toESM_target_node [455.64ms] (pass) bundler > cjs/__toESM_target_browser [413.38ms] (pass) bundler > cjs/__toESM_target_bun [455.22ms] (pass) bundler > cjs/__toESM_format_esm [462.40ms] (pass) bundler > cjs/__toESM_format_cjs_with_import [377.93ms] (pass) bundler > cjs/__toESM_mjs_reexport [431.60ms] (pass) bundler > cjs/__toESM_mjs_reexport_with_esModule [432.63ms] (pass) bundler > cjs/__toESM_deep_reexport_chain [368.70ms] (pass) bundler > cjs/__toESM_reexport_with_rename [443.84ms] (pass) bundler > cjs/__toESM_default_prop ... (truncated) release without fix: 20 FAILED bun test v1.4.1-canary.1 (a6c4cc2) test/bundler/bundler_cjs.test.ts: runtime failed file: /tmp/bun-build-tests/bun-4t1lr0/cjs/__toESM_import_syntax_with_esModule/out.js stdout output: {"__esModule":true,"default":{"value":"default export"},"named":"named export"} --- expected stdout: {"value":"default export"} --- 1913 | console.log(`---`); 1914 | console.log(`expected ${name}:`); 1915 | console.log(expected); 1916 | console.log(`---`); 1917 | } 1918 | expect(result).toBe(expected); ^ error: expect(received).toBe(expected) Expected: "{"value":"default export"}" Received: "{"__esModule":true,"default":{"value":"default export"},"named":"named export"}" at <anonymous> (/workspace/bun/test/bundler/expectBundled.ts:1918:28) (fail) bundler > cjs/__toESM_import_syntax_with_esModule [29.17ms] (pass) bundler > cjs/__toESM_import_syntax_without_esModule [11.71ms] (pass) bundler > cjs/__toESM_import_syntax_function [9.90ms] (pass) bundler > cjs/__toESM_import_syntax_primitive [9.43ms] (pass) bundler > cjs/__toESM_import_syntax_named_and_default [9.97ms] ... (truncated) ``` </details> <details><summary>passes on PR (with fix)</summary> ```console 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_cjs.test.ts bun test v1.4.1 (a6c4cc2) test/bundler/bundler_cjs.test.ts: (pass) bundler > cjs/__toESM_import_syntax_with_esModule [1024.06ms] (pass) bundler > cjs/__toESM_import_syntax_without_esModule [473.45ms] (pass) bundler > cjs/__toESM_import_syntax_function [489.71ms] (pass) bundler > cjs/__toESM_import_syntax_primitive [494.25ms] (pass) bundler > cjs/__toESM_import_syntax_named_and_default [388.89ms] (pass) bundler > cjs/__toESM_import_syntax_namespace [381.97ms] (pass) bundler > cjs/__toESM_target_node [389.92ms] (pass) bundler > cjs/__toESM_target_browser [453.17ms] (pass) bundler > cjs/__toESM_target_bun [481.14ms] (pass) bundler > cjs/__toESM_format_esm [415.53ms] (pass) bundler > cjs/__toESM_format_cjs_with_import [376.77ms] (pass) bundler > cjs/__toESM_mjs_reexport [451.67ms] (pass) bundler > cjs/__toESM_mjs_reexport_with_esModule [380.06ms] (pass) bundler > cjs/__toESM_deep_reexport_chain [453.02ms] (pass) bundler > cjs/__toESM_reexport_with_rename [430.08ms] (pass) bundler > cjs/__toESM_default_pro ... (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 822e3b2 features baseline 23 deps, 131 codegen, 1172 objects in 647ms ninja: Entering directory `/workspace/bun/build/release' [1/1244] install /workspace/bun bun install v1.4.1-canary.1 (a6c4cc2) Checked 25 installs across 62 packages (no changes) [10.00ms] [2/1244] gen bindgenv2 [3/1244] gen ErrorCode+*.h [4/1244] install /workspace/bun/packages/bun-error bun install v1.4.1-canary.1 (a6c4cc2) Checked 1 install across 2 packages (no changes) [3.00ms] [5/1244] fetch libjpeg-turbo [libjpeg-turbo] up to date [6/1217] gen ProcessBindingConstants.lut.h Generating /workspace/bun/build/release/codegen/ProcessBindingConstants.lut.h from /workspace/bun/src/jsc/bindings/ProcessBindingConstants.cpp [7/1217] gen bake.{client,server,error}.js -> bake.client.js, bake.server.js, bake.error.js [8/1217] fetch tinycc [tinycc] up to date [9/1216] install /workspace/bun/src/node-fallbacks bun install v1.4.1-canary.1 (a6c4cc2) Checked 111 installs across 104 packages (no changes) [15.00 ... (truncated) ``` </details> <details><summary>diff hotspot</summary> ``` src/resolver/dir_info.rs | 9 ++ src/resolver/resolver.rs | 20 +++-- src/resolver/result.rs | 15 +--- test/bundler/bundler_cjs.test.ts | 182 ++++++++++++++++++++++++++++++++++++++- 4 files changed, 207 insertions(+), 19 deletions(-) ``` </details> **gate history** · 1 passed · 0 rejected · iteration 0 <details><summary>evidence per changed file</summary> ``` file reads edits tests src/resolver/dir_info.rs 3 4 33 src/resolver/resolver.rs 8 5 34 src/resolver/result.rs 1 1 33 test/bundler/bundler_cjs.test.ts 3 11 33 ``` </details> <!-- robobun:evidence:end -->
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Problem
bun buildreturns the wholemodule.exportsfor a default import of a CommonJS module that sets__esModule, when the importer is a.ts,.tsxor.jsfile.bun run, esbuild and Rolldown returnexports.defaultfor the same files. They use Node's interop (the wholemodule.exports) only for a.mjs,.mtsor"type": "module"importer. TypeScript and Babel emit that marker, so this breaks the default import of many packages (styled-components, react-bootstrap). Found while bundling Outline: a React invalid hook call at boot.print_code_for_file_in_chunk_js(src/bundler/LinkerContext.rs:2290) derived the printer'sinput_module_typefromast.exports_kind. Any file with ESM syntax counted as ESM, so every__toESMcall gotisNodeMode = 1, which ignores__esModule.module_typefrom the matched"import"or"require"export condition. A "fake ESM" file in a package without"type", reached through"exports": { "import": ... }, was ESM to the printer too. Fixes react-bootstrap andbun buildnot playing together #7709. Fixes Default import gets wrapped in extra default #18615.Fix
BundledAstcarries the resolver'smodule_type(the extension, else the nearest package.json"type"). The linker passes that to the printer, soisNodeMode = 1only for an ESM-by-type importer. The externalrequire()path now prints, 1under the same rule, as esbuild does..mjs/.mts/.cjs/.ctsnow win over any package.json"type". Node decides a file's format from its extension and the nearest package.json only. A task that bypasses the resolver (a plugin result) falls back to the extension.ModuleTypeData, set from the extension or the enclosing package.json) and Node's. Checked against esbuild 0.21.5 for every importer kind below.test/bundler/bundler_cjs.test.ts(45 tests, 17 fail on the released bun). Alsotest/bundler/esbuild/*,bundler_cjs2esm,bundler_splitting,bundler_npm,bundler_edgecase,bundler_regressions,bundler_barrel,transpiler/*,test/js/bun/resolve/*,cli/run/run-cjs.Background
__toESM(mod, isNodeMode)is the runtime helper that builds the ESM view of a CommonJS module. WithisNodeMode = 0it honors__esModule:defaultismod.default. WithisNodeMode = 1it copies Node:defaultismoditself.ExportsKindis what the parser found in a file (import/exportsyntax versusexports/moduleuse).ModuleTypeis what the file system says: the extension or package.json"type". Node's interop follows the second, never the first.Notes
Repro from the report (1.3.13, 1.4.0, canary):
esbuild 0.21.5 output for each importer, matched by the new tests:
.ts:__toESM(require_dep())..mts:__toESM(require_dep(), 1)."type": "module":, 1."type": "commonjs": no, 1. External CJS in CJS output:__toESM(require("ext"))for.ts,__toESM(require("ext"), 1)for.mjs. Dynamicimport()of a bundled or split CJS module: the same rule.History: #23803 (Oct 2025) moved
isNodeModefrom the output format toexports_kind. Before that, the default--format=esmalways usedisNodeMode = 1, sobun buildnever honored__esModulefor.tsimporters.Test expectation updates for the dropped
, 1(2 bytes minified, 3 bytes otherwise):EmitInvalidSourceMap2mapping column,npm/ReactSSRmapping columns and file size (6 bytes: three, 1), and theBundledReactPreservesImportRefssnapshot. Six tests inbundler_cjs.test.tsasserted the old behavior for a.jsentry and now assertexports.default.regression/issue/03844still sees, 1: the importer there isws/wrapper.mjs.Resolver change, what else it touches:
module_typeis also the parser's hint for a file with noimport/exportand noexports/moduleuse. Such a file reached through an"import"condition was ESM by the condition. It now follows theUnknownrules (ESM unless it usesrequire,__dirnameor__filename). A.cjstarget of an"import"condition is now CommonJS.Known gap, unchanged here: the resolver's
enclosing_package_jsonskips a package.json without a"name"(#229). A nameless{ "type": "module" }above the importer does not make it ESM for this decision, except when it sits next to a file resolved through an exports map (handle_esm_resolutionreads the file's own directory). Node and esbuild use the nearest package.json whatever its name.Unrelated, found on the way:
bun build test/regression/issue/03844/03844.fixture.tsfrom the repo root fails withEISDIR reading file: "test"and tripsassertion failed: crate::is_absolute(self.text)in a debug build. Handed off separately.