refactor: use asUTF8() accessor and add comptime encoding guard

Address CodeRabbit review feedback:
- Use str.asUTF8() instead of str.latin1() for UTF-8 strings to use
  the semantically correct accessor
- Add comptime guard to writeQuotedString to reject unsupported
  encodings (only latin1 and utf8 are supported; use JSON path for UTF-16)

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

Co-Authored-By: Claude <noreply@anthropic.com>

src/bun.js/ConsoleObject.zig

@@ -2151,11 +2151,16 @@ pub const Formatter = struct {
                         writer.writeAll(Output.prettyFmt("<r>", true));
 
                     if (str.isUTF16()) {
+                        // Use JSON path for UTF-16 to properly handle surrogate pairs (emojis)
                         try this.printAs(.JSON, Writer, writer_, value, .StringObject, enable_ansi_colors);
                         return;
                     }
 
-                    JSPrinter.writeJSONString(str.latin1(), Writer, writer_, .latin1) catch unreachable;
+                    if (str.asUTF8()) |utf8| {
+                        JSPrinter.writeQuotedString(utf8, Writer, writer_, .utf8) catch unreachable;
+                    } else {
+                        JSPrinter.writeQuotedString(str.latin1(), Writer, writer_, .latin1) catch unreachable;
+                    }
 
                     return;
                 }
@@ -2170,9 +2175,12 @@ pub const Formatter = struct {
                     writer.print("[String: ", .{});
 
                     if (str.isUTF16()) {
+                        // Use JSON path for UTF-16 to properly handle surrogate pairs (emojis)
                         try this.printAs(.JSON, Writer, writer_, value, .StringObject, enable_ansi_colors);
+                    } else if (str.asUTF8()) |utf8| {
+                        JSPrinter.writeQuotedString(utf8, Writer, writer_, .utf8) catch unreachable;
                     } else {
-                        JSPrinter.writeJSONString(str.latin1(), Writer, writer_, .latin1) catch unreachable;
+                        JSPrinter.writeQuotedString(str.latin1(), Writer, writer_, .latin1) catch unreachable;
                     }
 
                     writer.print("]", .{});

src/js_printer.zig

@@ -353,6 +353,27 @@ pub fn writeJSONString(input: []const u8, comptime Writer: type, writer: Writer,
     try writer.writeAll("\"");
 }
 
+/// Writes a quoted string for console output, choosing the best quote character
+/// to minimize escaping. Supports latin1 and utf8 encodings (use JSON path for UTF-16).
+pub fn writeQuotedString(input: []const u8, comptime Writer: type, writer: Writer, comptime encoding: strings.Encoding) !void {
+    comptime {
+        if (encoding != .latin1 and encoding != .utf8) {
+            @compileError("writeQuotedString only supports latin1 and utf8 encodings; use JSON path for UTF-16");
+        }
+    }
+    // Reuse existing bestQuoteCharForString which counts quotes in a single pass
+    const quote_char = bestQuoteCharForString(u8, input, false);
+    if (quote_char == '\'') {
+        try writer.writeAll("'");
+        try writePreQuotedString(input, Writer, writer, '\'', false, false, encoding);
+        try writer.writeAll("'");
+    } else {
+        try writer.writeAll("\"");
+        try writePreQuotedString(input, Writer, writer, '"', false, false, encoding);
+        try writer.writeAll("\"");
+    }
+}
+
 pub const SourceMapHandler = struct {
     ctx: *anyopaque,
     callback: Callback,

test/regression/issue/25234.test.ts

@@ -0,0 +1,53 @@
+import { expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+// https://github.com/oven-sh/bun/issues/25234
+// Console output should use single quotes when string contains double quotes
+// to avoid ugly escaping like: "{\"test\":{\"pretty\":\"pretty\"}}"
+
+test("console.log uses single quotes for strings containing double quotes", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", 'console.log({ test: JSON.stringify({ test: { pretty: "pretty" } }) });'],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // Should use single quotes to avoid escaping double quotes
+  expect(stdout).toContain("'");
+  expect(stdout).not.toContain('\\"'); // no escaped double quotes
+  expect(exitCode).toBe(0);
+});
+
+test("console.log uses double quotes for strings containing single quotes", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", `console.log({ a: "hello 'world'" });`],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // Should use double quotes to avoid escaping single quotes
+  expect(stdout).toContain("\"hello 'world'\"");
+  expect(stdout).not.toContain("\\'"); // no escaped single quotes
+  expect(exitCode).toBe(0);
+});
+
+test("console.log uses double quotes by default", async () => {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), "-e", 'console.log({ a: "hello world" });'],
+    env: bunEnv,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+  // Default should be double quotes
+  expect(stdout).toContain('"hello world"');
+  expect(exitCode).toBe(0);
+});