feat: add option for interop default in cjs

docs/README.md

@@ -145,16 +145,12 @@ Provide the following configuration in your `.vscode/settings.json` (or global)
   "json.schemas": [
     {
       "url": "https://cdn.jsdelivr.net/npm/tsup/schema.json",
-      "fileMatch": [
-        "package.json",
-        "tsup.config.json"
-      ]
+      "fileMatch": ["package.json", "tsup.config.json"]
     }
   ]
 }
 ```
 
-
 ### Multiple entrypoints
 
 Beside using positional arguments `tsup [...files]` to specify multiple entrypoints, you can also use the cli flag `--entry`:
@@ -164,7 +160,7 @@ Beside using positional arguments `tsup [...files]` to specify multiple entrypoi
 tsup --entry src/a.ts --entry src/b.ts
 ```
 
-The associated output file names can be defined as follows: 
+The associated output file names can be defined as follows:
 
 ```bash
 # Outputs `dist/foo.js` and `dist/bar.js`.
@@ -350,6 +346,14 @@ tsup src/index.ts --env.NODE_ENV production
 
 When an entry file like `src/cli.ts` contains hashbang like `#!/bin/env node` tsup will automatically make the output file executable, so you don't have to run `chmod +x dist/cli.js`.
 
+### Interop with CommonJS
+
+By default, esbuild will transform `export default x` to `module.exports.default = x` in CommonJS, but you can change this behavior by using the `--cjsInterop` flag: If there are only default exports and no named exports, it will be transformed to `module.exports = x` instead.
+
+```bash
+tsup src/index.ts --cjsInterop
+```
+
 ### Watch mode
 
 ```bash

src/cli-main.ts

@@ -95,6 +95,7 @@ export async function main(options: Options = {}) {
       '--killSignal <signal>',
       'Signal to kill child process, "SIGTERM" or "SIGKILL"'
     )
+    .option('--cjsInterop', 'Enable cjs interop')
     .action(async (files: string[], flags) => {
       const { build } = await import('.')
       Object.assign(options, {

src/index.ts

@@ -21,6 +21,7 @@ import { sizeReporter } from './plugins/size-reporter'
 import { treeShakingPlugin } from './plugins/tree-shaking'
 import { copyPublicDir, isInPublicDir } from './lib/public-dir'
 import { terserPlugin } from './plugins/terser'
+import { cjsInterop } from './plugins/cjs-interop'
 
 export type { Format, Options, NormalizedOptions }
 
@@ -254,6 +255,7 @@ export async function build(_options: Options) {
                       silent: options.silent,
                     }),
                     cjsSplitting(),
+                    cjsInterop(),
                     es5(),
                     sizeReporter(),
                     terserPlugin({

src/options.ts

@@ -224,6 +224,11 @@ export type Options = {
    */
   publicDir?: string | boolean
   killSignal?: KILL_SIGNAL
+  /**
+   * Interop default within `module.exports` in cjs
+   * @default false
+   */
+  cjsInterop?: boolean
 }
 
 export type NormalizedOptions = Omit<

src/plugins/cjs-interop.ts

@@ -0,0 +1,26 @@
+import { Plugin } from '../plugin'
+
+export const cjsInterop = (): Plugin => {
+  return {
+    name: 'cjs-interop',
+
+    async renderChunk(code, info) {
+      if (
+        !this.options.cjsInterop ||
+        this.format !== 'cjs' ||
+        info.type !== 'chunk' ||
+        !/\.(js|cjs)$/.test(info.path) ||
+        !info.entryPoint ||
+        info.exports?.length !== 1 ||
+        info.exports[0] !== 'default'
+      ) {
+        return
+      }
+
+      return {
+        code: code + '\nmodule.exports = exports.default',
+        map: info.map,
+      }
+    },
+  }
+}

src/rollup.ts

@@ -132,6 +132,25 @@ const getRollupConfig = async (
     },
   }
 
+  const fixCjsExport: Plugin = {
+    name: 'tsup:fix-cjs-export',
+    renderChunk(code, info) {
+      if (
+        info.type !== 'chunk' ||
+        !/\.(ts|cts)$/.test(info.fileName) ||
+        !info.isEntry ||
+        info.exports?.length !== 1 ||
+        info.exports[0] !== 'default'
+      )
+        return
+
+      return code.replace(
+        /(?<=(?<=[;}]|^)\s*export\s*){\s*([\w$]+)\s*as\s+default\s*}/,
+        `= $1`
+      )
+    },
+  }
+
   return {
     inputConfig: {
       input: dtsOptions.entry,
@@ -179,7 +198,7 @@ const getRollupConfig = async (
         ...(options.external || []),
       ],
     },
-    outputConfig: options.format.map((format) => {
+    outputConfig: options.format.map((format): OutputOptions => {
       const outputExtension =
         options.outExtension?.({ format, options, pkgType: pkg.type }).dts ||
         defaultOutExtension({ format, pkgType: pkg.type }).dts
@@ -190,6 +209,9 @@ const getRollupConfig = async (
         banner: dtsOptions.banner,
         footer: dtsOptions.footer,
         entryFileNames: `[name]${outputExtension}`,
+        plugins: [
+          format === 'cjs' && options.cjsInterop && fixCjsExport,
+        ].filter(Boolean),
       }
     }),
   }