From 0f7494569e81a411ba64d11fcc503e3939e28fad Mon Sep 17 00:00:00 2001
From: robobun <robobun@oven.sh>
Date: Fri, 28 Nov 2025 22:57:55 -0800
Subject: [PATCH] fix(console): implement %j format specifier for JSON output
 (#25195)
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## Summary
- Implements the `%j` format specifier for `console.log` and related
console methods
- `%j` outputs the JSON stringified representation of the value
- Previously, `%j` was not recognized and was left as literal text in
the output

## Test plan
- [x] Run `bun bd test test/regression/issue/24234.test.ts` - all 5
tests pass
- [x] Verify tests fail with system Bun (`USE_SYSTEM_BUN=1`) to confirm
fix validity
- [x] Manual verification: `console.log('%j', {foo: 'bar'})` outputs
`{"foo":"bar"}`

## Example

Before (bug):
```
$ bun -e "console.log('%j %s', {foo: 'bar'}, 'hello')"
%j [object Object] hello
```

After (fixed):
```
$ bun -e "console.log('%j %s', {foo: 'bar'}, 'hello')"
{"foo":"bar"} hello
```

Closes #24234

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
---
 src/bun.js/ConsoleObject.zig                 | 12 ++++
 test/js/web/console/console-log.expected.txt |  2 +-
 test/regression/issue/24234.test.ts          | 72 ++++++++++++++++++++
 3 files changed, 85 insertions(+), 1 deletion(-)
 create mode 100644 test/regression/issue/24234.test.ts

diff --git a/src/bun.js/ConsoleObject.zig b/src/bun.js/ConsoleObject.zig
index 20e1de9898..b9a24edf18 100644
--- a/src/bun.js/ConsoleObject.zig
+++ b/src/bun.js/ConsoleObject.zig
@@ -1425,6 +1425,7 @@ pub const Formatter = struct {
         o, // o
         O, // O
         c, // c
+        j, // j
     };
 
     fn writeWithFormatting(
@@ -1466,6 +1467,7 @@ pub const Formatter = struct {
                         'O' => .O,
                         'd', 'i' => .i,
                         'c' => .c,
+                        'j' => .j,
                         '%' => {
                             // print up to and including the first %
                             const end = slice[0..i];
@@ -1625,6 +1627,16 @@ pub const Formatter = struct {
                         .c => {
                             // TODO: Implement %c
                         },
+
+                        .j => {
+                            // JSON.stringify the value
+                            var str = bun.String.empty;
+                            defer str.deref();
+
+                            try next_value.jsonStringify(global, 0, &str);
+                            this.addForNewLine(str.length());
+                            writer.print("{f}", .{str});
+                        },
                     }
                     if (this.remaining_values.len == 0) break;
                 },
diff --git a/test/js/web/console/console-log.expected.txt b/test/js/web/console/console-log.expected.txt
index 882b76de63..9a6cdf569c 100644
--- a/test/js/web/console/console-log.expected.txt
+++ b/test/js/web/console/console-log.expected.txt
@@ -236,7 +236,7 @@ Hello World 123
 Hello %vWorld 123
 Hello NaN %i
 Hello NaN % 1
-Hello NaN %j 1
+Hello NaN 1
 Hello \5 6,
 Hello %i 5 6
 %d 1
diff --git a/test/regression/issue/24234.test.ts b/test/regression/issue/24234.test.ts
new file mode 100644
index 0000000000..5bf23a5d21
--- /dev/null
+++ b/test/regression/issue/24234.test.ts
@@ -0,0 +1,72 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+test("console.log with %j should format as JSON", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "console.log('%j', {foo: 'bar'})"],
+    env: bunEnv,
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toBe('{"foo":"bar"}\n');
+  expect(exitCode).toBe(0);
+});
+
+test("console.log with %j should handle arrays", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "console.log('%j', [1, 2, 3])"],
+    env: bunEnv,
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toBe("[1,2,3]\n");
+  expect(exitCode).toBe(0);
+});
+
+test("console.log with %j should handle nested objects", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "console.log('%j', {a: {b: {c: 123}}})"],
+    env: bunEnv,
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toBe('{"a":{"b":{"c":123}}}\n');
+  expect(exitCode).toBe(0);
+});
+
+test("console.log with %j should handle primitives", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "console.log('%j %j %j %j', 'string', 123, true, null)"],
+    env: bunEnv,
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toBe('"string" 123 true null\n');
+  expect(exitCode).toBe(0);
+});
+
+test("console.log with %j and additional text", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", "console.log('Result: %j', {status: 'ok'})"],
+    env: bunEnv,
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  expect(stderr).toBe("");
+  expect(stdout).toBe('Result: {"status":"ok"}\n');
+  expect(exitCode).toBe(0);
+});