fix(console): route console.trace() to stderr instead of stdout by costajohnt · Pull Request #25791 · oven-sh/bun

costajohnt · 2026-01-01T21:08:49Z

Summary

Fixes #19952 - console.trace() now outputs to stderr instead of stdout, matching Node.js behavior.

Changes

Added message_type == .Trace checks alongside the existing .Warning, .Error, and .Assert checks in ConsoleObject.zig:

Mutex selection - Trace now uses the stderr mutex for thread safety
Writer selection - Trace now uses error_writer (stderr) instead of writer (stdout)
ANSI color settings - Trace now respects enable_ansi_colors_stderr

Test

Added a test case in console-log.test.ts that verifies:

stdout is empty when running console.trace()
stderr contains the trace output

Verification

Before (incorrect - goes to stdout):

$ bun trace.js >/dev/null
$ # no output (trace went to stdout which was redirected)

After (correct - goes to stderr):

$ bun trace.js >/dev/null
Trace: hello
    at <anonymous> (trace.js:1:9)

This matches Node.js behavior:

$ node trace.js >/dev/null
Trace: hello
    at Object.<anonymous> (trace.js:1:9)
    ...

coderabbitai · 2026-01-01T21:11:05Z

Walkthrough

Updated console output logic so Trace messages are treated like Warning/Error: mutex locking, ANSI color enabling, and writer selection now route console.trace output to stderr. Added a test and fixture to verify stderr output.

Changes

Cohort / File(s)	Summary
Console output handling `src/bun.js/ConsoleObject.zig`	Expanded conditional logic to include `Trace` alongside `Warning`/`Error` for mutex acquisition/release, ANSI color enabling, and selecting the `stderr` writer instead of `stdout`.
Test coverage `test/js/web/console/console-log.test.ts`, `test/js/web/console/console-trace.fixture.js`	Added a test asserting `console.trace()` outputs to `stderr` and a fixture that calls `console.trace("hello")`.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: routing console.trace() to stderr instead of stdout, which is the core objective of this PR.
Description check	✅ Passed	The description fully covers both required template sections: it explains what the PR does (fixes console.trace stderr routing) and how it was verified (test case and before/after comparison).
Linked Issues check	✅ Passed	The code changes directly address issue `#19952` by routing console.trace() to stderr through ConsoleObject.zig modifications and adding comprehensive test coverage.
Out of Scope Changes check	✅ Passed	All changes are scoped to the console.trace() stderr routing objective: ConsoleObject.zig modifications, test case addition, and test fixture—no unrelated changes present.

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📒 Files selected for processing (3)

src/bun.js/ConsoleObject.zig
test/js/web/console/console-log.test.ts
test/js/web/console/console-trace.fixture.js

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src/**/*.zig

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src/**/*.zig: Private fields in Zig are fully supported using the # prefix: struct { #foo: u32 };
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**/*.test.ts?(x): Never use bun test directly - always use bun bd test to run tests with debug build changes
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🧠 Learnings (21)

📓 Common learnings

Learnt from: Jarred-Sumner
Repo: oven-sh/bun PR: 24082
File: test/cli/test/coverage.test.ts:60-112
Timestamp: 2025-10-26T01:32:04.844Z
Learning: In the Bun repository test files (test/cli/test/*.test.ts), when spawning Bun CLI commands with Bun.spawnSync for testing, prefer using stdio: ["inherit", "inherit", "inherit"] to inherit stdio streams rather than piping them.

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: CLAUDE.md:0-0
Timestamp: 2025-12-16T00:21:32.179Z
Learning: Applies to **/*.test.ts?(x) : When spawning processes in tests, expect stdout before expecting exit code for more useful error messages on test failure

📚 Learning: 2025-10-16T02:17:35.237Z

Learnt from: markovejnovic
Repo: oven-sh/bun PR: 23710
File: src/analytics.zig:15-21
Timestamp: 2025-10-16T02:17:35.237Z
Learning: In src/analytics.zig and similar files using bun.EnvVar boolean environment variables: the new EnvVar API for boolean flags (e.g., bun.EnvVar.do_not_track.get(), bun.EnvVar.ci.get()) is designed to parse and return boolean values from environment variables, not just check for their presence. This is an intentional design change from the previous presence-based checks using bun.getenvZ().

Applied to files:

src/bun.js/ConsoleObject.zig

📚 Learning: 2025-10-30T21:52:04.707Z

Learnt from: pfgithub
Repo: oven-sh/bun PR: 24212
File: src/cli/publish_command.zig:782-788
Timestamp: 2025-10-30T21:52:04.707Z
Learning: In the Bun codebase (oven-sh/bun), `enable_ansi_colors` flags are used to gate both ANSI color codes and Unicode box-drawing characters/emoji. This is the established pattern across the codebase.

Applied to files:

src/bun.js/ConsoleObject.zig

📚 Learning: 2025-09-30T22:53:19.887Z

Learnt from: pfgithub
Repo: oven-sh/bun PR: 23117
File: src/bun.js/test/snapshot.zig:265-276
Timestamp: 2025-09-30T22:53:19.887Z
Learning: In Bun's snapshot testing (src/bun.js/test/snapshot.zig), multiple inline snapshots at the same line and column (same call position) must have identical values. However, multiple inline snapshots on the same line at different columns are allowed to have different values. The check is position-specific (line+col), not line-wide.

Applied to files:

src/bun.js/ConsoleObject.zig

📚 Learning: 2025-11-24T18:37:11.466Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: src/js/CLAUDE.md:0-0
Timestamp: 2025-11-24T18:37:11.466Z
Learning: Applies to src/js/{builtins,node,bun,thirdparty,internal}/**/*.{ts,js} : Use `$debug()` for debug logging and `$assert()` for assertions; both are stripped in release builds

Applied to files:

src/bun.js/ConsoleObject.zig
test/js/web/console/console-trace.fixture.js
test/js/web/console/console-log.test.ts

📚 Learning: 2025-10-19T03:01:29.084Z

Learnt from: theshadow27
Repo: oven-sh/bun PR: 23798
File: src/bun.js/telemetry.zig:286-289
Timestamp: 2025-10-19T03:01:29.084Z
Learning: In src/bun.js/telemetry.zig, the guard preventing double configuration (lines 213-216) is intentional. The telemetry API uses a single-shot configuration model where configure() is called once during application startup. Users must call Bun.telemetry.configure(null) to reset before reconfiguring. This design ensures: (1) predictable state—callbacks don't change mid-request, avoiding race conditions; (2) zero overhead when disabled—no checking for callback changes on every request; (3) clear ownership—one adapter (e.g., bun-otel) owns the telemetry config. Merge-style reconfiguration would require atomic updates during active requests, adding overhead to the hot path.
<!-- [/add_learning]

Applied to files:

src/bun.js/ConsoleObject.zig

📚 Learning: 2025-11-24T18:37:30.259Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: test/CLAUDE.md:0-0
Timestamp: 2025-11-24T18:37:30.259Z
Learning: Applies to test/**/*-fixture.ts : Test files that spawn Bun processes should end in `*-fixture.ts` to identify them as test fixtures and not tests themselves

Applied to files:

test/js/web/console/console-trace.fixture.js
test/js/web/console/console-log.test.ts

📚 Learning: 2025-12-16T00:21:32.179Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: CLAUDE.md:0-0
Timestamp: 2025-12-16T00:21:32.179Z
Learning: Applies to **/*.test.ts?(x) : When spawning processes in tests, expect stdout before expecting exit code for more useful error messages on test failure

Applied to files:

test/js/web/console/console-trace.fixture.js
test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-24T18:36:59.706Z

Learnt from: CR
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File: src/bun.js/bindings/v8/CLAUDE.md:0-0
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Learning: Applies to src/bun.js/bindings/v8/test/v8/v8.test.ts : Add corresponding test cases to test/v8/v8.test.ts using checkSameOutput() function to compare Node.js and Bun output

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-24T18:37:30.259Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: test/CLAUDE.md:0-0
Timestamp: 2025-11-24T18:37:30.259Z
Learning: Applies to test/**/*.test.{ts,js,jsx,tsx,mjs,cjs} : When spawning Bun processes in tests, use `bunExe` and `bunEnv` from `harness` to ensure the same build of Bun is used and debug logging is silenced

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-24T18:37:30.259Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: test/CLAUDE.md:0-0
Timestamp: 2025-11-24T18:37:30.259Z
Learning: Applies to test/**/*.test.{ts,js,jsx,tsx,mjs,cjs} : Always check exit codes and test error scenarios in error tests

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-12-16T00:21:32.179Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: CLAUDE.md:0-0
Timestamp: 2025-12-16T00:21:32.179Z
Learning: Applies to **/*.test.ts?(x) : Never use `bun test` directly - always use `bun bd test` to run tests with debug build changes

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test/js/web/console/console-log.test.ts

📚 Learning: 2025-10-26T01:32:04.844Z

Learnt from: Jarred-Sumner
Repo: oven-sh/bun PR: 24082
File: test/cli/test/coverage.test.ts:60-112
Timestamp: 2025-10-26T01:32:04.844Z
Learning: In the Bun repository test files (test/cli/test/*.test.ts), when spawning Bun CLI commands with Bun.spawnSync for testing, prefer using stdio: ["inherit", "inherit", "inherit"] to inherit stdio streams rather than piping them.

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-24T18:37:30.259Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: test/CLAUDE.md:0-0
Timestamp: 2025-11-24T18:37:30.259Z
Learning: Applies to test/**/*.test.{ts,js,jsx,tsx,mjs,cjs} : Use `-e` flag for single-file tests when spawning Bun processes

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-12-16T00:21:32.179Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: CLAUDE.md:0-0
Timestamp: 2025-12-16T00:21:32.179Z
Learning: Applies to **/*.test.ts?(x) : For multi-file tests, prefer `tempDir` and `Bun.spawn` over single-file tests

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-06T00:58:23.965Z

Learnt from: markovejnovic
Repo: oven-sh/bun PR: 24417
File: test/js/bun/spawn/spawn.test.ts:903-918
Timestamp: 2025-11-06T00:58:23.965Z
Learning: In Bun test files, `await using` with spawn() is appropriate for long-running processes that need guaranteed cleanup on scope exit or when explicitly testing disposal behavior. For short-lived processes that exit naturally (e.g., console.log scripts), the pattern `const proc = spawn(...); await proc.exited;` is standard and more common, as evidenced by 24 instances vs 4 `await using` instances in test/js/bun/spawn/spawn.test.ts.

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-08T04:06:33.198Z

Learnt from: Jarred-Sumner
Repo: oven-sh/bun PR: 24491
File: test/js/bun/transpiler/declare-global.test.ts:17-17
Timestamp: 2025-11-08T04:06:33.198Z
Learning: In Bun test files, `await using` with Bun.spawn() is the preferred pattern for spawned processes regardless of whether they are short-lived or long-running. Do not suggest replacing `await using proc = Bun.spawn(...)` with `const proc = Bun.spawn(...); await proc.exited;`.

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-12-16T00:21:32.179Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: CLAUDE.md:0-0
Timestamp: 2025-12-16T00:21:32.179Z
Learning: Applies to **/*.test.ts?(x) : Verify tests fail with `USE_SYSTEM_BUN=1 bun test <file>` and pass with `bun bd test <file>` - tests are invalid if they pass with USE_SYSTEM_BUN=1

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-10-19T02:44:46.354Z

Learnt from: theshadow27
Repo: oven-sh/bun PR: 23798
File: packages/bun-otel/context-propagation.test.ts:1-1
Timestamp: 2025-10-19T02:44:46.354Z
Learning: In the Bun repository, standalone packages under packages/ (e.g., bun-vscode, bun-inspector-protocol, bun-plugin-yaml, bun-plugin-svelte, bun-debug-adapter-protocol, bun-otel) co-locate their tests with package source code using *.test.ts files. This follows standard npm/monorepo patterns. The test/ directory hierarchy (test/js/bun/, test/cli/, test/js/node/) is reserved for testing Bun's core runtime APIs and built-in functionality, not standalone packages.

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-10-08T13:48:02.430Z

Learnt from: Jarred-Sumner
Repo: oven-sh/bun PR: 23373
File: test/js/bun/tarball/extract.test.ts:107-111
Timestamp: 2025-10-08T13:48:02.430Z
Learning: In Bun's test runner, use `expect(async () => { await ... }).toThrow()` to assert async rejections. Unlike Jest/Vitest, Bun does not require `await expect(...).rejects.toThrow()` - the async function wrapper with `.toThrow()` is the correct pattern for async error assertions in Bun tests.

Applied to files:

test/js/web/console/console-log.test.ts

📚 Learning: 2025-11-24T18:37:30.259Z

Learnt from: CR
Repo: oven-sh/bun PR: 0
File: test/CLAUDE.md:0-0
Timestamp: 2025-11-24T18:37:30.259Z
Learning: Applies to test/**/*.test.{ts,js,jsx,tsx,mjs,cjs} : Always use `await using` or `using` to ensure proper resource cleanup in tests for APIs like Bun.listen, Bun.connect, Bun.spawn, Bun.serve, etc

Applied to files:

test/js/web/console/console-log.test.ts

🧬 Code graph analysis (1)

test/js/web/console/console-log.test.ts (2)

test/js/node/stream/node-stream.test.js (2)

Bun (339-343)

Bun (448-454)

test/harness.ts (2)

bunExe (102-105)

bunEnv (49-64)

🔇 Additional comments (2)

src/bun.js/ConsoleObject.zig (1)

114-167: LGTM! Clean and consistent implementation.

The changes correctly extend console.trace() to use stderr by adding message_type == .Trace checks alongside .Warning, .Error, and .Assert in all four critical locations:

Mutex selection (lines 114, 129) - ensures thread-safe stderr access

ANSI color settings (line 159) - respects enable_ansi_colors_stderr

Writer selection (line 164) - routes output to error_writer/stderr

The condition is consistently applied across all locations, which is essential for correct behavior.

test/js/web/console/console-trace.fixture.js (1)

1-3: LGTM! Simple and appropriate test fixture.

The fixture is minimal and focused on testing console.trace() output behavior. The naming follows the established *-fixture.js pattern for test fixtures.

costajohnt · 2026-01-23T00:01:27Z

Hi! 👋 Just checking in on this PR. The CI appears to be blocked waiting for approval. Would it be possible to trigger the build so the tests can run? Happy to make any changes if needed. Thanks!

This fixes issue #19952 where console.trace() was outputting to stdout instead of stderr, which differs from Node.js behavior. The fix adds `message_type == .Trace` checks alongside the existing `.Warning`, `.Error`, and `.Assert` checks in: - Mutex selection for thread safety - Writer selection (error_writer vs writer) - ANSI color settings (enable_ansi_colors_stderr vs stdout) Added a test to verify console.trace() output goes to stderr.

Check stdout/stderr before exit code for better debugging on failure. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

coderabbitai Bot reviewed Jan 1, 2026

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Comment thread test/js/web/console/console-log.test.ts

costajohnt mentioned this pull request Jan 8, 2026

console.assert() with message should include "Assertion failed" alongside message #19953

Open

costajohnt and others added 2 commits February 6, 2026 17:29

fix(test): reorder assertions per CodeRabbit suggestion

18511e4

Check stdout/stderr before exit code for better debugging on failure. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

costajohnt force-pushed the fix-console-trace-stderr branch from 5dbb7d3 to 18511e4 Compare February 7, 2026 01:29

costajohnt closed this Apr 1, 2026

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fix(console): route console.trace() to stderr instead of stdout#25791

fix(console): route console.trace() to stderr instead of stdout#25791
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