feat: add patchCjsDefaultExport feature

src/core/options.ts

@@ -10,6 +10,8 @@ export type Options = {
   extraOutdir?: string
   /** Automatically add `.js` extension to resolve in `Node16` + ESM mode. */
   autoAddExts?: boolean
+  /** Patch `export default` in `.d.cts` to `export = ` */
+  patchCjsDefaultExport?: boolean
 } & (
   | {
       /**
@@ -45,5 +47,6 @@ export function resolveOptions(options: Options): OptionsResolved {
     ignoreErrors: options.ignoreErrors || false,
     extraOutdir: options.extraOutdir,
     autoAddExts: options.autoAddExts || false,
+    patchCjsDefaultExport: options.patchCjsDefaultExport || false,
   }
 }

src/index.ts

@@ -55,13 +55,17 @@ export const IsolatedDecl: UnpluginInstance<Options | undefined, false> =
           entryFileNames = path.join(options.extraOutdir, entryFileNames)
         }
 
-        for (const [filename, source] of Object.entries(outputFiles)) {
+        for (let [outname, source] of Object.entries(outputFiles)) {
+          const fileName = entryFileNames.replace(
+            '[name]',
+            path.relative(outBase, outname),
+          )
+          if (options.patchCjsDefaultExport && fileName.endsWith('.d.cts')) {
+            source = patchCjsDefaultExport(source)
+          }
           this.emitFile({
             type: 'asset',
-            fileName: entryFileNames.replace(
-              '[name]',
-              path.relative(outBase, filename),
-            ),
+            fileName,
             source,
           })
         }
@@ -246,13 +250,16 @@ export const IsolatedDecl: UnpluginInstance<Options | undefined, false> =
         }
 
         const textEncoder = new TextEncoder()
-        for (const [filename, source] of Object.entries(outputFiles)) {
+        for (let [filename, source] of Object.entries(outputFiles)) {
           const outDir = build.initialOptions.outdir
           let outFile = `${path.relative(outBase, filename)}.d.${outExt}`
           if (options.extraOutdir) {
             outFile = path.join(options.extraOutdir, outFile)
           }
           const filePath = outDir ? path.resolve(outDir, outFile) : outFile
+          if (options.patchCjsDefaultExport && filePath.endsWith('.d.cts')) {
+            source = patchCjsDefaultExport(source)
+          }
           if (write) {
             await mkdir(path.dirname(filePath), { recursive: true })
             await writeFile(filePath, source)
@@ -292,6 +299,13 @@ function stripExt(filename: string) {
   return filename.replace(/\.(.?)[jt]s$/, '')
 }
 
+function patchCjsDefaultExport(source: string) {
+  return source.replace(
+    /(?<=(?:[;}]|^)\s*export\s*)(?:\{\s*([\w$]+)\s*as\s+default\s*\}|default\s+([\w$]+))/,
+    (_, s1, s2) => `= ${s1 || s2}`,
+  )
+}
+
 export function lowestCommonAncestor(...filepaths: string[]): string {
   if (filepaths.length === 0) return ''
   if (filepaths.length === 1) return path.dirname(filepaths[0])

tsdown.config.ts

@@ -8,6 +8,7 @@ export default defineConfig({
   plugins: [
     IsolatedDecl({
       autoAddExts: true,
+      patchCjsDefaultExport: true,
     }),
   ],
 })