js_parser: attach the generated default name to an anonymous default function only in a bundle

src/js_parser/visit/visit_stmt.rs

@@ -660,10 +660,14 @@ impl<'a, const TYPESCRIPT: bool, const SCAN_ONLY: bool> P<'a, TYPESCRIPT, SCAN_O
                 match s2_data {
                     StmtData::SFunction(mut func_ref) => {
                         let func: &mut S::Function = &mut *func_ref;
+                        // Leave anonymous so SetFunctionName assigns `.name = "default"`;
+                        // React Fast Refresh injects a name for its `$RefreshReg$` binding.
                         let name: &'a [u8] = if let Some(func_loc) = func.func.name {
                             p.load_name_from_ref(func_loc.ref_)
                         } else {
-                            func.func.name = Some(data.default_name);
+                            if p.options.features.react_fast_refresh {
+                                func.func.name = Some(data.default_name);
+                            }
                             js_ast::ClauseItem::DEFAULT_ALIAS
                         };
 

src/jsc/RuntimeTranspilerCache.rs

@@ -77,7 +77,8 @@ bun_core::declare_scope!(cache, visible);
 /// Version 42: The body of a class with lowered class decorators names the class by the
 /// binding that holds what the decorators return.
 /// Version 43: A `bun:` specifier that names no builtin is printed as it is written.
-const EXPECTED_VERSION: u32 = 43;
+/// Version 44: An anonymous `export default function` is printed without a generated name.
+const EXPECTED_VERSION: u32 = 44;
 
 /// Source files smaller than this are not written to / read from the on-disk
 /// transpiler cache. Originally 50 KiB, which excluded almost every file in a

test/bundler/transpiler/export-default-name.test.ts

@@ -0,0 +1,39 @@
+import { describe, expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+
+describe("anonymous export default .name", () => {
+  test.concurrent.each([
+    ["function", "export default function () {}"],
+    ["async function", "export default async function () {}"],
+    ["function*", "export default function* () {}"],
+    ["async function*", "export default async function* () {}"],
+    ["class", "export default class {}"],
+    ["arrow", "export default () => 1;"],
+  ])("%s: .name is 'default'", async (_label, decl) => {
+    using dir = tempDir("export-default-name", {
+      "mod.mjs": `${decl}\nimport self from "./mod.mjs";\nconsole.log(JSON.stringify(self.name));\n`,
+    });
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "mod.mjs"],
+      env: bunEnv,
+      cwd: String(dir),
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    expect({ stdout, stderr, exitCode }).toEqual({ stdout: '"default"\n', stderr: "", exitCode: 0 });
+  });
+
+  test.concurrent("named function keeps its own name", async () => {
+    using dir = tempDir("export-default-named", {
+      "mod.mjs": `export default function Foo() {}\nimport self from "./mod.mjs";\nconsole.log(JSON.stringify(self.name));\n`,
+    });
+    await using proc = Bun.spawn({
+      cmd: [bunExe(), "mod.mjs"],
+      env: bunEnv,
+      cwd: String(dir),
+      stderr: "pipe",
+    });
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+    expect({ stdout, stderr, exitCode }).toEqual({ stdout: '"Foo"\n', stderr: "", exitCode: 0 });
+  });
+});

test/regression/issue/31401.test.ts

@@ -44,13 +44,14 @@ describe.concurrent("issue 31401: anonymous default export from digit-named modu
     expect(exitCode).toBe(0);
   });
 
-  test("transpile-only output uses a valid identifier for the generated default name", async () => {
-    using dir = tempDir("issue-31401-transpile", {
+  test("bundled output uses a valid identifier for the generated default name", async () => {
+    using dir = tempDir("issue-31401-bundle", {
       "1.ts": `export default function () {}\n`,
+      "entry.ts": `import f from "./1.ts"; console.log(typeof f);\n`,
     });
 
     await using proc = Bun.spawn({
-      cmd: [bunExe(), "build", "--no-bundle", join(String(dir), "1.ts")],
+      cmd: [bunExe(), "build", join(String(dir), "entry.ts")],
       env: bunEnv,
       stderr: "pipe",
       stdout: "pipe",
@@ -60,7 +61,7 @@ describe.concurrent("issue 31401: anonymous default export from digit-named modu
 
     expect(stderr).not.toContain("No identifiers allowed directly after numeric literal");
     // Must be a valid identifier: the leading digit gets an underscore prefix.
-    expect(normalizeBunSnapshot(stdout)).toContain("export default function _1_default() {}");
+    expect(normalizeBunSnapshot(stdout)).toContain("function _1_default() {}");
     expect(exitCode).toBe(0);
   });
 });