fix(console): send console.trace to stderr with a "Trace:" prefix

src/jsc/ConsoleObject.rs

@@ -454,7 +454,8 @@ 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 {
@@ -477,7 +478,9 @@ fn message_with_type_and_level_(
         return Ok(());
     }
 
-    let enable_colors = if matches!(level, MessageLevel::Warning | MessageLevel::Error) {
+    let enable_colors = if matches!(level, MessageLevel::Warning | MessageLevel::Error)
+        || message_type == MessageType::Trace
+    {
         Output::enable_ansi_colors_stderr()
     } else {
         Output::enable_ansi_colors_stdout()
@@ -496,7 +499,7 @@ 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 matches!(level, MessageLevel::Warning | MessageLevel::Error) || message_type == MessageType::Trace {
             (*console).error_writer()
         } else {
             (*console).writer()
@@ -578,6 +581,19 @@ fn message_with_type_and_level_(
         }
     }
 
+    // console.trace prints "Trace: <message>" (or just "Trace" with no args),
+    // matching Node, with the stack trace appended below. The label goes through
+    // FormatOptions so it lands after the console.group indent rather than
+    // before it.
+    if message_type == MessageType::Trace {
+        if print_length > 0 {
+            print_options.prefix = b"Trace: ";
+        } else {
+            let _ = formatter::write_indent_n(u32::from(default_indent), writer);
+            let _ = writer.write_all(b"Trace\n");
+        }
+    }
+
     if print_length > 0 {
         format2(
             level,
@@ -1298,6 +1314,9 @@ pub struct FormatOptions {
     pub single_line: bool,
     pub default_indent: u16,
     pub error_display_level: ErrorDisplayLevel,
+    /// Label emitted immediately after the indent, before the formatted values
+    /// (used by `console.trace` for its "Trace: " prefix).
+    pub prefix: &'static [u8],
 }
 
 impl Default for FormatOptions {
@@ -1312,6 +1331,7 @@ impl Default for FormatOptions {
             single_line: false,
             default_indent: 0,
             error_display_level: ErrorDisplayLevel::Full,
+            prefix: b"",
         }
     }
 }
@@ -1484,6 +1504,7 @@ pub fn format2(
         if fmt.write_indent(writer).is_err() {
             return Ok(());
         }
+        let _ = writer.write_all(options.prefix);
 
         if matches!(tag.tag, TagPayload::String) {
             if options.enable_colors {
@@ -1548,6 +1569,7 @@ pub fn format2(
     if fmt.write_indent(writer).is_err() {
         return Ok(());
     }
+    let _ = writer.write_all(options.prefix);
 
     let mut any = false;
     if options.enable_colors {

test/js/node/console/console.test.ts

@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test";
 import { Console } from "node:console";
+import { bunEnv, bunExe } from "harness";
 
 import { Writable } from "node:stream";
 
@@ -88,3 +89,52 @@ test("console._stderr", () => {
     configurable: true,
   });
 });
+
+// console.trace writes to stderr with a "Trace:" prefix, matching Node.
+// https://github.com/oven-sh/bun/issues/19952
+describe("console.trace", () => {
+  async function run(code: string) {
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "-e", code],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    return { stdout, stderr, exitCode };
+  }
+
+  test("goes to stderr, not stdout", async () => {
+    const { stdout, stderr, exitCode } = await run(`console.trace("hi")`);
+    expect(stdout).toBe("");
+    expect(stderr).toStartWith("Trace: hi\n");
+    expect(stderr).toContain("at ");
+    expect(exitCode).toBe(0);
+  });
+
+  test("no arguments prints bare 'Trace'", async () => {
+    const { stdout, stderr, exitCode } = await run(`console.trace()`);
+    expect(stdout).toBe("");
+    expect(stderr).toStartWith("Trace\n");
+    expect(stderr).toContain("at ");
+    expect(exitCode).toBe(0);
+  });
+
+  test("applies format specifiers", async () => {
+    const { stdout, stderr, exitCode } = await run(`console.trace("x=%d", 5)`);
+    expect(stdout).toBe("");
+    expect(stderr).toStartWith("Trace: x=5\n");
+    expect(exitCode).toBe(0);
+  });
+
+  test("label goes after the console.group indent", async () => {
+    const { stdout, stderr, exitCode } = await run(
+      `console.group("G"); console.trace("x"); console.trace(); console.groupEnd(); console.trace("top");`,
+    );
+    // The group indent precedes the label, and the bare header is indented too.
+    const headers = stderr.match(/^(?:  Trace: x|  Trace|Trace: top)$/gm);
+    expect(headers).toEqual(["  Trace: x", "  Trace", "Trace: top"]);
+    expect(stdout).toBe("G\n");
+    expect(exitCode).toBe(0);
+  });
+});