console: route console.trace() output to stderr

src/jsc/ConsoleObject.rs

@@ -408,7 +408,8 @@ fn message_with_type_and_level_(
     // Lock/unlock a mutex incase two JS threads are console.log'ing at the same
     // time. We do this the slightly annoying way to avoid assigning a pointer.
     let use_stderr = matches!(level, MessageLevel::Warning | MessageLevel::Error)
-        || message_type == MessageType::Assert;
+        || message_type == MessageType::Assert
+        || message_type == MessageType::Trace;
     let _stream_lock = ConsoleStreamLock::acquire(use_stderr);
 
     if message_type == MessageType::Clear {
@@ -431,7 +432,7 @@ fn message_with_type_and_level_(
         return Ok(());
     }
 
-    let enable_colors = if matches!(level, MessageLevel::Warning | MessageLevel::Error) {
+    let enable_colors = if use_stderr {
         Output::enable_ansi_colors_stderr()
     } else {
         Output::enable_ansi_colors_stdout()
@@ -450,7 +451,7 @@ fn message_with_type_and_level_(
     // long-lived `&mut ConsoleObject` across the re-derive in the empty-`Log`
     // arm below.
     let raw_writer: &mut bun_core::io::Writer = unsafe {
-        if matches!(level, MessageLevel::Warning | MessageLevel::Error) {
+        if use_stderr {
             (*console).error_writer()
         } else {
             (*console).writer()

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

@@ -0,0 +1,43 @@
+import { expect, it } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+// https://github.com/oven-sh/bun/issues/19952
+it.concurrent("console.trace() writes to stderr, not stdout", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", `console.trace("marker");`],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+  expect(stdout).toBe("");
+  expect(stderr).toContain("marker");
+  expect(stderr).toContain("at ");
+  expect(exitCode).toBe(0);
+});
+
+it.concurrent("console.trace() with no arguments writes the stack to stderr", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", `console.trace();`],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+  expect(stdout).toBe("");
+  expect(stderr).toContain("at ");
+  expect(exitCode).toBe(0);
+});
+
+it.concurrent("console.trace() does not interleave with console.log() on stdout", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", `console.log("before"); console.trace("traced"); console.log("after");`],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+  expect(stdout).toBe("before\nafter\n");
+  expect(stderr).toContain("traced");
+  expect(exitCode).toBe(0);
+});