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node-fallbacks: bind process and Buffer to the sibling polyfills, fix util and process, fix CommonJS interop of the exports - #41533
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Included review availability: Your plan provides up to 10 included reviews per hour; 3 remain after this review. WalkthroughThe PR updates browser Node fallbacks for ESM and CommonJS interoperability, browser runtime shims, fallback building, linting, and bundler coverage. ChangesNode fallback compatibility
Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to This change improves browser Node-polyfill scheduling, global handling, and module interoperability. Reported bundler and browser checks cover the updated behavior, with no merge-blocking risk identified. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 2:55 AM PT - Sep 22nd, 2026
❌ @robobun, your commit 23c8ad3 has 2 failures in
🧪 To try this PR locally: bunx bun-pr 41533That installs a local version of the PR into your bun-41533 --bun |
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Status: this PR now consolidates the open browser polyfill fixes.
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…med exports
The polyfills for `bun build --target=browser` read the `process` and
`Buffer` globals, which a browser does not have. The build script now
renames every free use with `--define` and binds it to the sibling
polyfill. `timers.setImmediate` falls back to a zero-delay timeout.
util: `promisify` read an undeclared `kCustomPromisifiedSymbol`.
`callbackify`, `deprecate` and `debuglog` spread an array into `apply`,
so the callback got no arguments. `util.types` was an empty function.
The default export lacked `format`, `inspect` and most other members.
process: `nextTick` queued `args` but drained `array`, so callbacks got
no arguments. It also ran as a macrotask. It now uses `queueMicrotask`.
A default export is added so `import process from "process"` works.
zlib, punycode and console re-exported a CommonJS package with
`export *`, which yields no named exports. They now list the names.
assert is built as CommonJS so `require("assert")` and the default
import are the callable `assert` function. browserify-zlib calls it
directly, so `gzipSync` threw "assert is not a function" before.
…domain and constants
`require("events")` returned the ESM namespace, so `new EventEmitter()`
from a CommonJS dependency threw "EventEmitter is not a constructor".
The constructor already carries every named member, so the module is
now `module.exports = EventEmitter`, like assert and stream.
`import os from "os"`, `import domain from "domain"` and
`import constants from "constants"` failed the build with "doesn't have
a matching export named default". Every polyfilled builtin now accepts
a default import, checked by browser/NodePolyfillDefaultImports.
…yfills, leave util.types alone util.js and path.js read `globalThis.process` and `isBuffer()` checks the buffer methods, as the npm util browser build does. Neither file imports the process or buffer polyfill now. Before this, any import of util or assert pulled the whole buffer polyfill into the bundle (about 31 KB minified) for one `instanceof Buffer`. build-fallbacks.ts binds `process` and `Buffer` with `--define` entries (`process`, `global.process`, `Buffer`, `global.Buffer`) and no longer rewrites the text `globalThis.process`. It skips `Buffer` for assert.js, where only npm util's unused isBuffer() names it. oxlint checks src/node-fallbacks/*.js with `no-undef` under the browser environment, so an undeclared name or a node global in a polyfill source fails `bun lint`. The `util.types` implementation is removed. The stub stays as it is on main. Tests: browser/NodeUtilPolyfill, browser/NodeStreamWithoutGlobalProcess, browser/NodeProcessNextTickForwardsArgs and browser/NodeProcessNextTickOrder replace browser/NodeUtilPromisifyCallbackifyTypesNextTick.
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…file list A polyfill source that sets `module.exports` builds as CommonJS. The list of file names in build-fallbacks.ts is gone, so it cannot drift from the CommonJS override in oxlint.json. A new CommonJS polyfill that is not in that override now fails `bun lint` with no-undef on `module`. The built polyfills are byte-identical.
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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 finding on the nextTick drain, I checked the new source-based format detection in src/node-fallbacks/build-fallbacks.ts (added in the latest push): the ^module\.exports\s*= regex matches exactly assert.js, events.js and stream.js and no other polyfill source, so it agrees with the commonjs env override in oxlint.json and the build/lint lists can no longer drift.
Extended reasoning...
The latest push replaced the hardcoded CJS file list in build-fallbacks.ts with a regex over the polyfill source; I verified the regex selects the same three files the oxlint.json commonjs override names, and nothing else under src/node-fallbacks matches. The remaining inline finding (nextTick drain retaining callbacks for the length of a synchronous chain) is posted separately.
…ollected drainQueue walked one array that also received the ticks queued during the pass, and trimmed it only after the loop. A serial chain, where each tick queues the next, kept every finished callback reachable until the chain ended. Each batch is now a fresh array, as on main. A throw still drops only the tick that threw, and the rest of its batch runs before the ticks that the batch queued. Test: browser/NodeProcessNextTickChainReleasesCallbacks runs a chain of 5000 ticks and counts the live payloads after a full GC inside the last tick. The old drain kept all 5000.
…rwards its payload `process.nextTick(nonFunction)` throws a TypeError with `code: "ERR_INVALID_ARG_TYPE"` and a `Received <value>` tail, the same shape that the events polyfill uses. For `null` the message is identical to node. browser/NodeProcessNextTickChainReleasesCallbacks now passes its counter and payload through the nextTick arguments. It fails on main too, where nextTick drops its arguments, so it no longer guards only a revision of this branch.
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Actionable comments posted: 1
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Inline comments:
In `@src/node-fallbacks/process.js`:
- Line 36: Update the invalid-callback error construction near the TypeError in
the process fallback so it never coerces or stringifies fun; derive a safe
description using null handling and typeof, then use that description in the
error message while preserving the existing ERR_INVALID_ARG_TYPE assignment.
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src/node-fallbacks/process.jstest/bundler/bundler_browser.test.ts
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`String(fun)` throws for a value with no primitive conversion, such as `Object.create(null)`, before the error gets its code. The message now uses `null` or `typeof fun`. For `null` and `undefined` it is still identical to node.
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Problem
--target=browserpolyfills readprocess,BufferandsetImmediate, which browsers lack.Readable.from(['a'])throwsTypeError: Cannot read properties of undefined (reading 'nextTick').path.resolve()throwsReferenceError: process is not defined.util.promisifythrowsReferenceError: kCustomPromisifiedSymbol is not defined.callbackifyandprocess.nextTickdrop their arguments.zlib,punycodeandconsolehave no named exports.require("assert")andrequire("events")return a namespace object, soassert(x),new EventEmitter()andzlib.gzipSync()throw.Fix
build-fallbacks.tsrenames freeprocessandBufferin the npm packages with--defineand imports them from the sibling polyfill.util.jsandpath.jsreadglobalThis.process.zlib,punycode,consolelist their names.assertandeventsbuild as CommonJS (overlaps bundler: return themodule.exportsexport from require() of an ES module #42092). Every polyfill has a default export.bun lintchecks the polyfill sources withno-undef. node-fallbacks: implement util.types in the util polyfill #43727 ownsutil.types.test/bundler/bundler_browser.test.ts(eight new tests, all fail on main),bundler_loader,esbuild/default, Chromium 153.Background
build-fallbacks.tsbundles each file ofsrc/node-fallbacks/. A browser build mapsnode:x, and a polyfill's ownrequire("process"), to those files.--define=A:Brewrites only unbound references toA, so a localvar Bufferis safe.require()of an ES module in a bundle returns its namespace object, never a function.Downsides
assert+1.1 KB,stream+1.2 KB,events+0.7 KB,utildefault import +0.7 KB.require("events").defaultandrequire("assert").defaultare nowundefined, as in Node. Workarounds that used them break.process.nextTickchain now blocks timers, as in Node. Each tick was asetTimeout.Notes
This PR consolidates #35484 and #40072:
util.jsandpath.jschanges, the oxlint override, andbrowser/NodeUtilPolyfill, all unchanged. The lint also needs the CommonJS environment forassert.jsandevents.js.browser/NodeStreamWithoutGlobalProcessandbrowser/NodeProcessNextTickForwardsArgs, unchanged. Its two fixes are covered by the--definebinding and thenextTickrewrite.util.types: an earlier revision of this PR had an implementation based onObject.prototype.toStringtags. #43727 uses brand checks on internal slots and matches Node far better (43 predicates over 61 values: 3 differences against 26). That revision is removed, andexport const typesis byte-identical to main, so the two PRs merge cleanly in either order. A trial merge of #43727 on this branch has no conflicts, passesbun lint, and passesbrowser/NodeUtilTypes,browser/NodeUtilPolyfillandbrowser/NodePolyfills.Bundle size,
bun build --target=browser --minify, bytes, bun 1.4.3 against this branch:An earlier revision bound
Bufferinutil.jsandassert.jstoo. Oneinstanceof BufferinisBuffer()then pulled the whole buffer polyfill into every util or assert bundle (+31 KB minified).util.jsnow checks the buffer methods, as the npmutilbrowser build does.assert.jsis skipped forBuffer: only npm util'sisBuffer()names it there, and assert never calls it.Headless Chromium 153 smoke (bundle loaded with
<script type=module>from a local server,http.gethits the same server). Left: bun 1.4.3. Right: this branch.Other details:
build-fallbacks.tspasses--define=process:__bun_process,--define=global.process:__bun_process, and the same pair forBuffer. Theglobal.processentry covers readable-stream'sprocessshim (module.exports = global.process). The old text replacement ofglobal.processnever matched, because--define=global:globalThisran first.process.nextTickdrains withqueueMicrotask. A tick runs after already queued microtasks and beforesetTimeout(fn, 0). Ticks queued during a drain run in the same drain. A throw in one tick drops only that tick. The order matches node for a mixed case with throws (a,caught:boom,c,a2,c2,caught:last,later).browser/NodeProcessNextTickChainReleasesCallbacksguards this: it keeps fewer than 100 of 5000 payloads alive after a full GC inside the last tick. It forwards its payload through thenextTickarguments, so it fails on main too, where the arguments are dropped.process.nextTick(nonFunction)throws aTypeErrorwithcode: "ERR_INVALID_ARG_TYPE". The message ends withReceived nullorReceived <typeof value>and never coerces the value, soObject.create(null)gets the same error. Fornullandundefinedthe message is identical to Node.js. Main did not validate the callback.util.deprecate()keeps the behaviour main intends: with noprocessglobal it returns the function as it is.Related, not changed here:
module.exportsexport from require() of an ES module #42092 makesrequire()of an ES module return itsmodule.exportsexport in the bundler. If it lands,assertandeventscould stay ES modules. A maintainer can choose.target: "browser"to fix the crypto-browserify cycle (createHash2 is not a function). Both changebuild-fallbacks.tsand would need a merge.Not fixed here:
crypto.createHash(the cycle above), theurlpolyfill throwing on empty hosts,string_decoderwith a plainUint8Array,Buffer.from(s, "hex")on malformed input, and the resolver's different diagnostics forvmandnode:vm.[policy-decision:dep] gate passed · iteration 5 · 16 files touched
fails on main (without fix)
passes on PR (with fix)
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gate history · 6 passed · 0 rejected · iteration 5
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