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…odule Node.js (nodejs/node#54563) and Bun's runtime return the export named "module.exports" from require() of an ES module, when the module has one, instead of the namespace object. The linker already kept that export alive for require() targets and refused to bind destructured names through it, and a split require() (--splitting --target=bun) already honored it at run time, but an in-chunk require() still printed __toCommonJS(exports_foo) and returned the namespace copy. The linker now flags a require() whose target resolves an export named "module.exports" and the printer emits __toCommonJS(exports_foo, 1), which returns that export. While the export is still unset inside a require() cycle the caller gets the namespace copy as before. The dev server's HMR require() follows the same rule. The browser polyfills for node builtins use it: assert, console, events, http, https, net, path, punycode, querystring, string_decoder, sys, tty, url, util and zlib export their Node-shaped module.exports value under that name, so require("events") is the EventEmitter class, require("assert") is callable, require("zlib").gzipSync exists and require("path").posix === require("path"). assert's default export is now the assert function rather than its namespace, and util's default export lists every util export.
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Updated 3:06 PM PT - Sep 9th, 2026
❌ @robobun, your commit 5c09958 has 3 failures in
🧪 To try this PR locally: bunx bun-pr 42092That installs a local version of the PR into your bun-42092 --bun |
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Reproduced on bun 1.4.3-canary (f42e980) with the program from the report: // ev.cjs
const EE = require("events");
console.log(JSON.stringify({ type: typeof EE, EventEmitter: typeof EE.EventEmitter, default: typeof EE.default }));
try { class Bus extends EE {}; new Bus().on("x", v => console.log("got", v)).emit("x", 1); }
catch (e) { console.log("THROWN:", e.constructor.name + ": " + e.message); }
The same inconsistency without any polyfill: a module with CI: build 113580 passed on cee2baf. The branch then merged
Every lane that ran the new and updated tests passed in both builds. The diff is ready for review; a rerun of 113587's failed jobs should clear it. |
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Included review availability: Your plan provides up to 10 included reviews per hour; 1 remains after this review. WalkthroughChangesThe bundler now supports Node.js-style ESM and CommonJS interop
Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to This change adds coverage for bundled require() interoperability with ES module "module.exports" values and Node browser polyfills. No remaining merge-readiness risk is identified. 🚥 Pre-merge checks | ✅ 3 | ❌ 1❌ Failed checks (1 warning)
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In `@src/runtime/bake/hmr-module.ts`:
- Around line 868-869: Update the module.exports handling in toCommonJS to
safely guard the property read against TDZ exceptions during cyclic require()
evaluation, matching the established behavior in src/runtime.js; preserve
returning the entry wrapper when the binding is uninitialized. Add a dev-server
regression covering this cycle and run the specified ESM test suite.
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docs/bundler/index.mdxsrc/ast/import_record.rssrc/bundler/linker_context/scanImportsAndExports.rssrc/js_printer/lib.rssrc/node-fallbacks/README.mdsrc/node-fallbacks/assert.jssrc/node-fallbacks/console.jssrc/node-fallbacks/events.jssrc/node-fallbacks/http.jssrc/node-fallbacks/https.jssrc/node-fallbacks/net.jssrc/node-fallbacks/path.jssrc/node-fallbacks/punycode.jssrc/node-fallbacks/querystring.jssrc/node-fallbacks/string_decoder.jssrc/node-fallbacks/sys.jssrc/node-fallbacks/tty.jssrc/node-fallbacks/url.jssrc/node-fallbacks/util.jssrc/node-fallbacks/zlib.jssrc/runtime.jssrc/runtime/bake/hmr-module.tstest/bake/dev/esm.test.tstest/bundler/bundler_browser.test.tstest/bundler/bundler_cjs.test.ts
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…e the bytecode snapshot The bundled __toCommonJS helper text changed, so the four ESM entries in the bytecode portability corpus that are built with --format=cjs hash differently. The new values are the ones every CI platform produced.
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`const E = require("./events.mjs"); typeof E` printed
`var E = (init_events(), {})`: with no property read off the result,
the linker marked the namespace object unused. When the module exports
`module.exports` the result is that export, so `typeof` gave
"object" for a class and `if (!flag)` never saw a `false` export.
One predicate, `require_returns_module_exports_export`, now decides
all three sites: import binding, the unused-namespace pass, and the
printer flag.
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Consolidated the two narrower PRs into this one and closed them: #41328 (built
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#42002 changed the cache format, so the four `--format=cjs` ESM entries whose `__toCommonJS` helper text differs here get new `.jsc` hashes.
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Fixes #19808. Fixes #29985.
Problem
require()of an ES module always gave a copy of the namespace object. With--target=browser,class Bus extends require("events")throwsTypeError: Class extends value #<Object> is not a constructor or null, andrequire("assert")(true)throws too (bun buildbundling of CJSrequire("node:assert")does not properly expose exports. and returns an empty{}object #19808).export { b as "module.exports" }did not change the result (bun build should support to return the named export "module.exports" whenrequire(esm)#29985).bun run(src/js/builtins/CommonJS.ts:123) return the export named"module.exports"instead. The printer's in-chunkrequire()path (src/js_printer/lib.rs:2873) never did.Fix
require()(LinkerContext.rs:4324). The printer emits__toCommonJS(exports_x, 1), which returns the export unless it isnullorundefined(an unfinished cycle). A result that is only feature-tested (typeof E) keeps its value too. The dev server does too.export { x as "module.exports" }.require(x)now matchesbun runand Node, whileimportstill sees the namespace.bundler_cjs.test.ts(8 new cases),bundler_browser.test.ts(2 new, 1 updated),bake/dev/esm.test.ts(2 new). All but thenullcase fail on bun 1.4.3. Self-reviewed: 5 concerns, 2 addressed, 3 kept (see notes).Background
bun build --target=browserreplaces Node builtins with the ES modules insrc/node-fallbacks/.require()of an ES module prints as(init_x(), __toCommonJS(exports_x)), where__toCommonJS(src/runtime.js) copies the namespace objectexports_xand adds__esModule: true."module.exports"here is a string export name (ES2022). Node (>= 20.19, 22.12) documents it as the way an ES module picks itsrequire()value.Notes
Review of this PR raised five concerns. Two are addressed: a
require()result that is only feature-tested (const E = require("./events.mjs"); typeof E) printedvar E = (init_events(), {}), because the unused-namespace pass did not know the result is the export (fixed, withcjs/require_esm_module_exports_export_tree_shaken_uses), and the docs named every browser polyfill where only 15 export the name (narrowed). Three are kept as choices: eachrequire()call reads the export live (next paragraph), anullorundefinedexport falls back to the namespace copy (thehttppolyfill cycle below needs it, andbun rundoes the same), andrequire()of the 15 polyfills changes shape on purpose (no__esModule, no.default, which is the fix for #19808).A split
require()(--splitting --target=bun) already honored the export at run time, so before this change one source gaverightwith--splittingandWRONGwithout.Supersedes #41328 (built the
assertandeventspolyfills as CommonJS) and #29993 (changed only the__toCommonJShelper). Both were narrower and are closed in favor of this PR. Their test programs are folded in ascjs/require_esm_module_exports_export_primitive,cjs/require_esm_module_exports_export_null, and the #19808 checks inbrowser/NodePolyfillRequireCallable.Known difference from Node and
bun run: eachrequire()call reads the export live. Node andbun runkeep the value from the first load. The two differ only when the module later reassigns the exportedletbinding. The live read is what lets arequire()inside a cycle fall back to the namespace copy while a later call gets the export, with no second cache in the runtime helper.The linker already assumed this interop: it keeps the
module.exportsexport alive forrequire()targets (scanImportsAndExports.rs:272) and refuses to bind destructured names through it (LinkerContext.rs:4314).Before and after,
ev.cjsfrom the report built with--target=browserand run under node:afteris byte-identical tobun ev.cjsandnode ev.cjs. The printed call isvar EE = (init_events(), __toCommonJS(exports_events, 1));.The #19808 program (
const Assert = require("node:assert"); Assert.equal(1,1); Assert(true),--target=browser) throwsTypeError: Assert.equal is not a functionon bun 1.4.3 and runs here under bun and node. The #29985 program (require("./m.js")wherem.jshasexport { b as "module.exports" },b = 2) prints the namespace object on 1.4.3 and2here, asbun main.jsand node do.Node docs example (
point.mjswithexport { Point as "module.exports" }, required from a.cjsfile), one line per check:node,bun run, and the new bundle all printfunction true 5 undefined / true false / star: function true / falsy: false / import(): true function default,midpoint,module.exports. bun 1.4.3's bundle throws atclass Point3D extends Point. This coversexport *propagation of the string name, afalsevalue, identity across calls, no__esModuleon the result, and an unchangedimport()namespace.Why the non-null fallback instead of Node's strict own-property rule: Node throws
ERR_REQUIRE_CYCLE_MODULEfor arequire()inside the module's own evaluation, andbun runhands outnamespace["module.exports"] ?? namespace. The prebuilthttppolyfill has such a cycle (stream-http requiresbuiltin-status-codes, whose node entry doesrequire("http").STATUS_CODES). A strictexports_http["module.exports"]madeimport http from "node:http"throw at load. With the fallback the inner call sees the namespace copy, as it does today, and every laterrequire("http")gets the stream-http object. An explicitnullorundefined"module.exports"export therefore yields the namespace copy (Node yields the value).bun rundoes the same.Deliberately unchanged: the statement
module.exports = __toCommonJS(exports)that a--format=cjsbuild puts at the top of an ESM entry point. It runs before the module body, so it keeps the one-argument form and a--format=cjsbundle exposes the same shape to an externalrequire()as before.Polyfills changed: assert, console, events, http, https, net, path, punycode, querystring, string_decoder, sys, tty, url, util, zlib. Default exports that change value:
assert(namespace of the CJSassertpackage -> the assert function),httpsandquerystring(import-star namespace -> the package'smodule.exportsobject, same members),sys(namespace ofutil->util's default object),util(8 members -> all 28 named exports, sorequire("util")keepsformatandinspect).string_decoderkeepsStringDecoderas its default and exports the{ StringDecoder }package object asmodule.exports.bufferandcryptoare unchanged: their wrapper is already object-shaped like Node'smodule.exports.bundler_bytecode_portable's snapshot moves for the four--format=cjsESM entries because the__toCommonJShelper text changed. The values were regenerated after merging main (#42002 changed the cache format), and CI checks them on every platform.Not in this PR, checked on 1.4.3 and unchanged here:
require("crypto").createHashisundefinedbecause the prebuild resolvesrandombytesand friends to their node entries, whichrequire("crypto")back into the polyfill mid-evaluation (#41524 builds the polyfills for the browser target).import { gzipSync } from "zlib"andimport { toASCII } from "punycode"are still build errors because those polyfillsexport *from a CommonJS package (#41533).os,process,timers,domainandconstantshave only named exports, so theirrequire()result was already Node-shaped apart from__esModule.Other open PRs from this account that touch the same files: #41533 (also converts
assert, while its util, zlib, punycode, console, process and Buffer changes are separate), #40072 (util default export and lint), #41524 and #41315 (build-fallbacks.ts, not touched here).Found while testing, handed off separately:
bun build --format=iifewith a CommonJS entry point definesrequire_entryand never calls it, so the entry does not run.Suites run with the debug build:
bundler_browser,bundler_cjs,bundler_cjs2esm,bundler_edgecase,bundler_dynamic_import_dce,bundler_splitting,bundler_minify,esbuild/default,esbuild/importstar,bake/dev/esm.no test proof · iteration 1 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/bundler/bundler_bytecode_portable.test.ts