feat: add cjsDefault option

sxzz · sxzz · commit 03f8fc4c1066 · 2025-08-08T13:24:32.000+08:00
diff --git a/README.md b/README.md
@@ -86,6 +86,14 @@ Resolve external types used in `.d.ts` files from `node_modules`.
 - If `true`, all external types are resolved.
 - If an array, only types matching the provided strings or regular expressions are resolved.
 
+#### `cjsDefault`
+
+Determines how the default export is emitted.
+
+If set to `true`, and you are only exporting a single item using `export default ...`,
+the output will use `export = ...` instead of the standard ES module syntax.
+This is useful for compatibility with CommonJS.
+
 ### `tsc` Options
 
 > [!NOTE]
diff --git a/src/fake-js.ts b/src/fake-js.ts
@@ -40,7 +40,8 @@ type NamespaceMap = Map<
 export function createFakeJsPlugin({
   dtsInput,
   sourcemap,
-}: Pick<OptionsResolved, 'dtsInput' | 'sourcemap'>): Plugin {
+  cjsDefault,
+}: Pick<OptionsResolved, 'dtsInput' | 'sourcemap' | 'cjsDefault'>): Plugin {
   let symbolIdx = 0
   const identifierMap: Record<string, number> = Object.create(null)
   const symbolMap = new Map<number /* symbol id */, SymbolInfo>()
@@ -264,7 +265,12 @@ export function createFakeJsPlugin({
     program.body = program.body
       .map((node) => {
         if (isHelperImport(node)) return null
-        if (patchImportExport(node, typeOnlyIds)) return node
+
+        const newNode = patchImportExport(node, typeOnlyIds, cjsDefault)
+        if (newNode) {
+          return newNode
+        }
+
         if (node.type !== 'VariableDeclaration') return node
 
         const [decl] = node.declarations
@@ -523,7 +529,11 @@ function isHelperImport(node: t.Node) {
 }
 
 // patch `.d.ts` suffix in import source to `.js`
-function patchImportExport(node: t.Node, typeOnlyIds: string[]) {
+function patchImportExport(
+  node: t.Node,
+  typeOnlyIds: string[],
+  cjsDefault: boolean,
+): t.Statement | undefined {
   if (
     isTypeOf(node, [
       'ImportDeclaration',
@@ -546,7 +556,22 @@ function patchImportExport(node: t.Node, typeOnlyIds: string[]) {
 
     if (node.source?.value && RE_DTS.test(node.source.value)) {
       node.source.value = filename_dts_to(node.source.value, 'js')
-      return true
+      return node
+    }
+
+    if (
+      cjsDefault &&
+      node.type === 'ExportNamedDeclaration' &&
+      !node.source &&
+      node.specifiers.length === 1 &&
+      node.specifiers[0].type === 'ExportSpecifier' &&
+      resolveString(node.specifiers[0].exported) === 'default'
+    ) {
+      const defaultExport = node.specifiers[0] as t.ExportSpecifier
+      return {
+        type: 'TSExportAssignment',
+        expression: defaultExport.local,
+      }
     }
   }
 }
diff --git a/src/options.ts b/src/options.ts
@@ -62,6 +62,15 @@ export interface GeneralOptions {
    * Resolve external types used in `.d.ts` files from `node_modules`.
    */
   resolve?: boolean | (string | RegExp)[]
+
+  /**
+   * Determines how the default export is emitted.
+   *
+   * If set to `true`, and you are only exporting a single item using `export default ...`,
+   * the output will use `export = ...` instead of the standard ES module syntax.
+   * This is useful for compatibility with CommonJS.
+   */
+  cjsDefault?: boolean
 }
 
 //#region tsc Options
@@ -194,6 +203,7 @@ export function resolveOptions({
   compilerOptions = {},
   sourcemap,
   resolve = false,
+  cjsDefault = false,
 
   // tsc
   build = false,
@@ -280,6 +290,7 @@ export function resolveOptions({
     tsconfigRaw,
     sourcemap,
     resolve,
+    cjsDefault,
 
     // tsc
     build,
diff --git a/tests/__snapshots__/index.test.ts.snap b/tests/__snapshots__/index.test.ts.snap
@@ -38,6 +38,23 @@ var Cls = class {
 export { Cls, Enum, fn, foo };"
 `;
 
+exports[`cjs exports 1`] = `
+"// cjs-exports.js
+
+//#region tests/fixtures/cjs-exports.ts
+var cjs_exports_default = 42;
+
+//#endregion
+module.exports = cjs_exports_default;"
+`;
+
+exports[`cjs exports 2`] = `
+"// cjs-exports.d.ts
+//#region tests/fixtures/cjs-exports.d.ts
+declare const _default: number;
+export = _default;"
+`;
+
 exports[`dts input 1`] = `
 "// index.d.ts
 //#region tests/fixtures/dts-input.d.ts
diff --git a/tests/fixtures/cjs-exports.ts b/tests/fixtures/cjs-exports.ts
@@ -0,0 +1 @@
+export default 42
diff --git a/tests/index.test.ts b/tests/index.test.ts
@@ -144,3 +144,23 @@ test('type-only export', async () => {
   )
   expect(snapshot).toMatchSnapshot()
 })
+
+test.only('cjs exports', async () => {
+  {
+    const { snapshot } = await rolldownBuild(
+      [path.resolve(dirname, 'fixtures/cjs-exports.ts')],
+      [],
+      {},
+      { format: 'cjs', exports: 'auto' },
+    )
+    expect(snapshot).toMatchSnapshot()
+  }
+
+  {
+    const { snapshot } = await rolldownBuild(
+      [path.resolve(dirname, 'fixtures/cjs-exports.ts')],
+      [dts({ emitDtsOnly: true, cjsDefault: true })],
+    )
+    expect(snapshot).toMatchSnapshot()
+  }
+})
