[commonjs] Add defaultIsModuleExports option to match Node.js behav…

…ior (#838)

* [commonjs] Add `nodeDefaultImport` option to match Node.js behavior

* Undo unrelated changes

* Undo editor autoformatter

* Change option

packages/commonjs/README.md

@@ -174,6 +174,49 @@ If you set `esmExternals` to `true`, this plugins assumes that all external depe
 
 You can also supply an array of ids to be treated as ES modules, or a function that will be passed each external id to determine if it is an ES module.
 
+### `defaultIsModuleExports`
+
+Type: `boolean | "auto"`<br>
+Default: `"auto"`
+
+Controls what is the default export when importing a CommonJS file from an ES module.
+
+- `true`: The value of the default export is `module.exports`. This currently matches the behavior of Node.js when importing a CommonJS file.
+  ```js
+  // mod.cjs
+  exports.default = 3;
+  ```
+  ```js
+  import foo from './mod.cjs';
+  console.log(foo); // { default: 3 }
+  ```
+- `false`: The value of the default export is `exports.default`.
+  ```js
+  // mod.cjs
+  exports.default = 3;
+  ```
+  ```js
+  import foo from './mod.cjs';
+  console.log(foo); // 3
+  ```
+- `"auto"`: The value of the default export is `exports.default` if the CommonJS file has an `exports.__esModule === true` property; otherwise it's `module.exports`. This makes it possible to import
+  the default export of ES modules compiled to CommonJS as if they were not compiled.
+  ```js
+  // mod.cjs
+  exports.default = 3;
+  ```
+  ```js
+  // mod-compiled.cjs
+  exports.__esModule = true;
+  exports.default = 3;
+  ```
+  ```js
+  import foo from './mod.cjs';
+  import bar from './mod-compiled.cjs';
+  console.log(foo); // { default: 3 }
+  console.log(bar); // 3
+  ```
+
 ### `requireReturnsDefault`
 
 Type: `boolean | "namespace" | "auto" | "preferred" | ((id: string) => boolean | "auto" | "preferred")`<br>

packages/commonjs/src/generate-exports.js

@@ -20,7 +20,8 @@ export function rewriteExportsAndGetExportsBlock(
   isRestorableCompiledEsm,
   code,
   uses,
-  HELPERS_NAME
+  HELPERS_NAME,
+  defaultIsModuleExports
 ) {
   const namedExportDeclarations = [`export { ${moduleName} as __moduleExports };`];
   const moduleExportsPropertyAssignments = [];
@@ -75,19 +76,29 @@ export function rewriteExportsAndGetExportsBlock(
 
   // Generate default export
   const defaultExport = [];
-  if (isRestorableCompiledEsm) {
-    defaultExport.push(`export default ${deconflictedDefaultExportName || moduleName};`);
-  } else if (
-    (wrapped || deconflictedDefaultExportName) &&
-    (defineCompiledEsmExpressions.length > 0 || code.indexOf('__esModule') >= 0)
-  ) {
-    // eslint-disable-next-line no-param-reassign
-    uses.commonjsHelpers = true;
-    defaultExport.push(
-      `export default /*@__PURE__*/${HELPERS_NAME}.getDefaultExportFromCjs(${moduleName});`
-    );
-  } else {
+  if (defaultIsModuleExports === 'auto') {
+    if (isRestorableCompiledEsm) {
+      defaultExport.push(`export default ${deconflictedDefaultExportName || moduleName};`);
+    } else if (
+      (wrapped || deconflictedDefaultExportName) &&
+      (defineCompiledEsmExpressions.length > 0 || code.includes('__esModule'))
+    ) {
+      // eslint-disable-next-line no-param-reassign
+      uses.commonjsHelpers = true;
+      defaultExport.push(
+        `export default /*@__PURE__*/${HELPERS_NAME}.getDefaultExportFromCjs(${moduleName});`
+      );
+    } else {
+      defaultExport.push(`export default ${moduleName};`);
+    }
+  } else if (defaultIsModuleExports === true) {
     defaultExport.push(`export default ${moduleName};`);
+  } else if (defaultIsModuleExports === false) {
+    if (deconflictedDefaultExportName) {
+      defaultExport.push(`export default ${deconflictedDefaultExportName};`);
+    } else {
+      defaultExport.push(`export default ${moduleName}.default;`);
+    }
   }
 
   return `\n\n${defaultExport

packages/commonjs/src/index.js

@@ -59,6 +59,8 @@ export default function commonjs(options = {}) {
       : Array.isArray(esmExternals)
       ? ((esmExternalIds = new Set(esmExternals)), (id) => esmExternalIds.has(id))
       : () => esmExternals;
+  const defaultIsModuleExports =
+    typeof options.defaultIsModuleExports === 'boolean' ? options.defaultIsModuleExports : 'auto';
 
   const { dynamicRequireModuleSet, dynamicRequireModuleDirPaths } = getDynamicRequirePaths(
     options.dynamicRequireTargets
@@ -145,7 +147,8 @@ export default function commonjs(options = {}) {
       dynamicRequireModuleSet,
       disableWrap,
       commonDir,
-      ast
+      ast,
+      defaultIsModuleExports
     );
   }
 

packages/commonjs/src/transform-commonjs.js

@@ -53,7 +53,8 @@ export default function transformCommonjs(
   dynamicRequireModuleSet,
   disableWrap,
   commonDir,
-  astCache
+  astCache,
+  defaultIsModuleExports
 ) {
   const ast = astCache || tryParse(parse, code, id);
   const magicString = new MagicString(code);
@@ -470,7 +471,8 @@ export default function transformCommonjs(
         isRestorableCompiledEsm,
         code,
         uses,
-        HELPERS_NAME
+        HELPERS_NAME,
+        defaultIsModuleExports
       );
 
   const importBlock = rewriteRequireExpressionsAndGetImportBlock(

packages/commonjs/test/fixtures/form/defaultIsModuleExports-auto-__esModule/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: 'auto'
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-auto-__esModule/input.js

@@ -0,0 +1,3 @@
+exports.__esModule = true;
+exports.default = 2;
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-auto-__esModule/output.js

@@ -0,0 +1,11 @@
+var _default = 2;
+var named = 3;
+
+var input = /*#__PURE__*/Object.defineProperty({
+	default: _default,
+	named: named
+}, '__esModule', {value: true});
+
+export default _default;
+export { input as __moduleExports };
+export { named };

packages/commonjs/test/fixtures/form/defaultIsModuleExports-auto-no-__esModule/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: 'auto'
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-auto-no-__esModule/input.js

@@ -0,0 +1,2 @@
+exports.default = 2;
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-auto-no-__esModule/output.js

@@ -0,0 +1,11 @@
+var _default = 2;
+var named = 3;
+
+var input = {
+	default: _default,
+	named: named
+};
+
+export default input;
+export { input as __moduleExports };
+export { named };

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-__esModule/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: false
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-__esModule/input.js

@@ -0,0 +1,3 @@
+exports.__esModule = true;
+exports.default = 2;
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-__esModule/output.js

@@ -0,0 +1,11 @@
+var _default = 2;
+var named = 3;
+
+var input = /*#__PURE__*/Object.defineProperty({
+	default: _default,
+	named: named
+}, '__esModule', {value: true});
+
+export default _default;
+export { input as __moduleExports };
+export { named };

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-no-__esModule/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: false
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-no-__esModule/input.js

@@ -0,0 +1,2 @@
+exports.default = 2;
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-no-__esModule/output.js

@@ -0,0 +1,11 @@
+var _default = 2;
+var named = 3;
+
+var input = {
+	default: _default,
+	named: named
+};
+
+export default _default;
+export { input as __moduleExports };
+export { named };

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-no-default/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: false
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-no-default/input.js

@@ -0,0 +1 @@
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-false-no-default/output.js

@@ -0,0 +1,9 @@
+var named = 3;
+
+var input = {
+	named: named
+};
+
+export default input.default;
+export { input as __moduleExports };
+export { named };

packages/commonjs/test/fixtures/form/defaultIsModuleExports-true-__esModule/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: true
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-true-__esModule/input.js

@@ -0,0 +1,3 @@
+exports.__esModule = true;
+exports.default = 2;
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-true-__esModule/output.js

@@ -0,0 +1,11 @@
+var _default = 2;
+var named = 3;
+
+var input = /*#__PURE__*/Object.defineProperty({
+	default: _default,
+	named: named
+}, '__esModule', {value: true});
+
+export default input;
+export { input as __moduleExports };
+export { named };

packages/commonjs/test/fixtures/form/defaultIsModuleExports-true-no-__esModule/_config.js

@@ -0,0 +1,5 @@
+module.exports = {
+  options: {
+    defaultIsModuleExports: true
+  }
+};

packages/commonjs/test/fixtures/form/defaultIsModuleExports-true-no-__esModule/input.js

@@ -0,0 +1,2 @@
+exports.default = 2;
+exports.named = 3;

packages/commonjs/test/fixtures/form/defaultIsModuleExports-true-no-__esModule/output.js

@@ -0,0 +1,11 @@
+var _default = 2;
+var named = 3;
+
+var input = {
+	default: _default,
+	named: named
+};
+
+export default input;
+export { input as __moduleExports };
+export { named };