[robobun]

Problem

• With --unhandled-rejections=warn (also warn-with-error-code, strict), the rejection warning takes reason.stack with a plain putDirect() (src/jsc/bindings/BunProcess.cpp:1469). Object.keys(warning) is ["stack"]. Node gives ["name"].
• The warning keeps its captured trace. While script waits for a macro, the first stack lookup formats it over the reason's stack.
• A reason with no string stack loses its text: a native ENOENT prints UnhandledPromiseRejectionWarning: [object Object].

Fix

• The warning keeps no trace and takes reason.stack with DontEnum. With no reason stack, its stack is its header, made on the first read. name is enumerable, as in Node.
• If reason.stack is not a string, the first warning has the reason's text, as in Node. Bun__noSideEffectsToString() gives name: message for an Error, in ERR_UNHANDLED_REJECTION too.
• Verified: test/js/node/process/process.test.js (10 reasons in 3 modes, same rows as Node v26.3.0) and a new source lint for DontEnum. Also reject-tostring.test.ts, module-graph.test.ts and Node's test-promise-unhandled-*.
• Self-reviewed: 11 concerns raised, 11 addressed.

Background

• JavaScriptCore formats an error's captured frames on the first lookup of stack (ErrorInstance::materializeErrorInfoIfNeeded()). putDirect() skips that lookup, so a later lookup writes over the property.
• Considered: ErrorInstance::put() in place of putDirect(). It formats the warning's own stack first. For an error with no frames, the property that it adds is enumerable.

Downsides

• A warning listener that lists or serializes the keys of the warning gets name and no stack, as in Node.
• For a reason with no string stack, stderr has one more warning line, or name: message where [object Object] was.
• Other objects still print [object Object]. Node prints #<Object>.

Notes

Reach. The function runs only with --unhandled-rejections=warn, with warn-with-error-code or strict when no unhandledRejection listener handles the rejection. The default mode does not reach it. No issue reports any of this. For an Error reason and no --trace-warnings, stderr is the same before and after.

The overwrite on main.

// bun --unhandled-rejections=warn x.mjs
process.on("warning", w => w.message.startsWith("Unhandled promise rejection.") && console.log(w.stack === reason.stack));
const reason = new Error("boom");
Promise.reject(reason);
(function caller() {
  new Bun.Transpiler().transformSync('import { sleep } from "bun" with { type: "macro" }; sleep(1);');
})();

main prints false: warning.stack is UnhandledPromiseRejectionWarning: Unhandled promise rejection. ... with at caller. This branch prints true. With the same script and a string reason, main's warning has the frames of caller. Here its stack is the header, as in Node.

The warning's own stack. Node makes the warning under Error.stackTraceLimit = 0, so its own stack is the header UnhandledPromiseRejectionWarning: Unhandled promise rejection. ..., made on the first read. Here the warning gets the lazy stack accessor that Error.captureStackTrace and the SQL errors use (m_lazyStackCustomGetterSetter). The first read formats an empty trace: the header, or what Error.prepareStackTrace returns for []. Before, warning.stack was undefined for a reason with no stack.

The text of an Error. As V8's NoSideEffectsErrorToString(): name and message count only when they are strings in data properties of the error or of a prototype. So an instance of class ConfigError extends TypeError {} prints TypeError: ..., a Symbol for name prints the message alone, and no getter, toString() or Proxy trap runs. Past the maximum string length the text is the message alone.

Earlier work. #34259 made stack non-enumerable at the Error.captureStackTrace sites. #34292 was the DontEnum half of this PR for this line. A cleanup of stale PRs closed it with no review decision. This PR adds the trace, name, the text of the first warning, and tests for each.

#43927 changes the same putDirect line and conflicts with this PR in one hunk. Its isUndefined() branch is covered here with Node's result: warning.stack is an own undefined, and the first warning has the reason's text. Its change to Bun__promises__isErrorLike is separate and composes. @dylan-conway, which PR do you want to carry this function?

oven-sh/WebKit#829 (closed) formatted the header of an error with an empty trace on the first lookup. On its preview build, main's putDirect() lost the reason's stack for every Error reason. This PR does not depend on an engine change: the warning has a null trace, which no lookup formats.

Not changed.

• The text for objects that are not Errors (a port of V8's NoSideEffectsToString).
• strict with nothing that handles the exception: Node emits no warning, Bun emits them.
• throw prints the rejection two times when nothing handles it.
• The warnings are plain Error objects, with no (rejection id: N) and no warning.id.
• name of the first warning is not enumerable (node errors: make code an own enumerable property on ERR_* errors and warnings #41737 changes Process::emitWarning).
• The warning is not marked as materialized. Bun__attachAsyncStackFromPromise can still attach frames to it if a listener passes it to a native rejection that an async function awaits. That is true on main for each error with no frames and an assigned stack.

Also considered. The sequence in SQLError.cpp that marks an error as materialized (setErrorInfoForEmbedderError with an empty string, then materializeErrorInfoIfNeeded(), then a delete of line and column). It leaves no open state, but each warning then has 3 Structures of its own: JavaScriptCore does not reuse the transition for a delete of a non-enumerable property. Only setStackPropertyProvidedByCapturedStackTrace(): a later Error.captureStackTrace(warning) then loses its frames. ErrorInstance::sanitizedToString() for the text: it reads name from one prototype only (Error: ... for a subclass of TypeError), throws for a Symbol, and its makeString() crashes past the maximum string length.

Each clause is needed by a test. With one clause removed and the rest in place:

removed	cells that fail
setErrorInfoForEmbedderError	stack of the Error and of the string reported under a frame
the lazy stack accessor	stack in 3 rows (none)
DontEnum on stack	keys in the rows that take the reason's stack
attributes 0 on name	keys in every row
the fallback after emitWarning throws	first warning in the rows with a stack that is not a string
the Error arm	first warning of the Error rows and of the native error, and the throw test
stringDataProperty() (with sanitizedNameString() in its place)	first warning of the subclass row and of the Symbol row

Measured. Every warning shares one Structure, before and after (1,000 warnings kept alive add fewer than 10 Structures, for Error reasons and for string reasons). The rows of the test are the same under Node v26.3.0 in warn, warn-with-error-code and strict (with the macro wait removed, which Node does not have). With a user Error.prepareStackTrace, a read of the warning's own stack calls it 1 time with [], as in Node.

Suites run on a debug build. test/js/node/process/process.test.js, test/js/node/promise/reject-tostring.test.ts, test/js/bun/module-graph/module-graph.test.ts, test/js/node/v8/capture-stack-trace.test.js, test-process-warnings.mjs, and 17 of Node's test-promise-unhandled-*, test-promises-unhandled-* and test-process-warning.js files. In the full runs of process.test.js and capture-stack-trace.test.js, 5 Worker and signal tests hit the 5 s default timeout on this machine and pass when run alone. The new tests also pass with BUN_JSC_validateExceptionChecks=1. Node's files pass before and after: this PR adds no passing Node test.

---

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/node/process/process.test.js

[robobun]

^{Updated 6:35 PM PT - Oct 20th, 2026}

✅ @robobun, your commit 0f4d07f2af60732d70f5892a7771f8fcf2df5312 passed in Build #129660! 🎉

---

🧪   To try this PR locally:

bunx bun-pr 45775

That installs a local version of the PR into your bun-45775 executable, so you can run:

bun-45775 --bun

[robobun]

Status: ready for review. CI passed on 0f4d07f2af (181 of 181 jobs), and all review threads are resolved.

How I reproduced it, with bun --unhandled-rejections=warn on main (017b2439a7) and on 1.4.3:

• Object.keys(warning) of the UnhandledPromiseRejectionWarning is ["stack"]. Node v26.3.0 gives ["name"].
• fs.promises.readFile() of a missing file prints UnhandledPromiseRejectionWarning: [object Object]. Node prints Error: ENOENT: no such file or directory, open '...'.
• A rejection that is reported while script waits for a macro gives warning.stack the warning's own frames. Node gives it the reason's stack.

The new tests in test/js/node/process/process.test.js and the new source lint fail on main and pass on this branch.

One question for a maintainer is in the notes of the PR body: #43927 changes the same putDirect line.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

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• src/jsc/bindings/BunProcess.cpp
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Walkthrough

The process binding now formats ErrorInstance rejection reasons and uses available stack values when emitting unhandled-rejection warnings. New tests cover warning modes, stack handling, and native stack property writes.

Changes

Unhandled Rejection Warnings

Layer / File(s)	Summary
Rejection reason formatting src/jsc/bindings/BunProcess.cpp	ErrorInstance reasons use string-valued data properties for name and message. Other object values retain generic formatting.
Warning stack and text emission src/jsc/bindings/BunProcess.cpp, test/js/node/process/process.test.js, test/internal/source-lints/error-stack-not-enumerable.test.ts	Warning emission uses an available rejection stack or falls back to formatted reason text. Subprocess tests cover warning modes, stack cases, and throw mode. The source lint checks native stack property writes.

Suggested reviewers: jarred-sumner

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to 0f4d0

Merge Risk: ⚪ Minimal · up to 0f4d0

The change makes unhandled-rejection warnings match Node's behavior more closely. No remaining concrete risk was found, and CI should confirm the new tests before merge.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: matching Node's two-warning behavior for unhandled rejections.
Description check		The description explains the problem, implementation, verification, behavioral scope, downsides, and test limitations. It does not use the exact template headings, but it provides the required informa…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Description check

Explanation

The description explains the problem, implementation, verification, behavioral scope, downsides, and test limitations. It does not use the exact template headings, but it provides the required information in equivalent sections.

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/jsc/bindings/BunProcess.cpp:
- Around line 1444-1452: In the ErrorInstance branch, handle conversion
exceptions from sanitizedNameString and sanitizedMessageString by clearing
ordinary exceptions and returning the generic object text; preserve termination
exceptions by returning without clearing them. Keep the existing
string-formatting path when both conversions succeed.

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📒 Files selected for processing: 3

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[claude]

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

[claude]

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

The latest push addresses the two concerns from my earlier pass that had a code fix (the trace-less warning now gets the lazy stack header via m_lazyStackCustomGetterSetter, and name/message are read only as string data properties, so the helper no longer enters user code). Beyond the inline finding, I also checked the new stringDataProperty helper for user-code reachability (VMInquiry slot rejects accessors and custom getters; toWTFString runs only on a JSString) and the unconditional setErrorInfoForEmbedderError against the lazy getter path (null trace formats an empty-frame header, the same shape SQLError.cpp uses) — neither raised anything new. A human look is still worthwhile given the ErrorInstance state interplay and the open overlap with the other PR touching this function.

Extended reasoning...

The diff rewrites Bun__promises__emitUnhandledRejectionWarning in src/jsc/bindings/BunProcess.cpp, adds an ErrorInstance branch to Bun__noSideEffectsToString (also reached from the ERR_UNHANDLED_REJECTION message in VirtualMachine.rs), and adds a source lint plus a 3-mode subprocess test. It touches no security-sensitive surface; the risk is ErrorInstance materialization state (setErrorInfoForEmbedderError, lazy stack getter) and Node-compat property attributes. Exception handling follows the file's existing CLEAR_IF_EXCEPTION/termination pattern and the tests drain pipes concurrently with await using. Deferring rather than approving because the change depends on JSC internals not visible in this checkout and the author notes a conflicting PR on the same hunk that a maintainer must arbitrate.

[robobun]

I ran f6268feb97 on a debug build with AddressSanitizer (Linux x64). The references are Node v26.3.0 and the same build with BunProcess.cpp from the base (017b2439a7).

Runs

• 161 rejection reasons in 21 configurations: 3,381 processes for each of the three. The configurations are the 6 modes of --unhandled-rejections, with and without uncaughtException and unhandledRejection listeners, --trace-warnings and --no-warnings.
• 784 processes in which a warning listener changes the second warning before it reads stack: freeze, delete, define, Error.captureStackTrace, structuredClone, a native rejection under an await, a user Error.prepareStackTrace.
• 264 processes with the rejection in a Worker.
• 805 processes with BUN_JSC_validateExceptionChecks=1.

Results

• No assertion, sanitizer report or crash comes from this PR. 3 processes abort, and the same 3 abort on the base: the error printer reads a message getter that throws (error printer: clear a throwing error.message getter's exception before the next property read #38586).
• The exit status is the same as on the base in 3,381 of 3,381.
• No process has fewer warnings than on the base. 315 have one more.
• A first warning that Node emits is missing in 280 processes on the base and in 0 here. On 71712152c3 it was missing for an Error with no string stack and a Symbol for name or message: 48 of the 756 processes that I ran there.
• Where Node emits a first warning (1,270 processes), its first line is the same as Node's in 848 here and in 432 on the base.
• A name and a message of 2^30 characters each (the overflow arm of tryMakeString()): the first warning is the message alone, and nothing crashes.

Differences from Node in these runs that this PR does not cause

• util.inspect(warning) shows name: 'UnhandledPromiseRejectionWarning' when reason.stack is a truthy value that is not a string (174 lines). util.inspect prints such a stack as String(stack). Node 20+ prints the header and then the value. With no rejection: new RangeError("foo") with stack = 42 prints [42] in Bun and [RangeError: foo\n 42] in Node.
• For class ConfigError extends TypeError {} the first warning is Error: ..., because reason.stack starts with Error:. Node has TypeError: .... ErrorInstance::sanitizedNameString() reads name from one prototype only (the closed error: read name/message via full [[Get]] for the .stack header; drop inspect.js workaround #34868 fixed it).
• An empty text, for example Promise.reject(""), prints Warning: Process::emitWarning() replaces an empty string.