Export CJS auto generate extern (#235)

Author: kristoferbaxter

src/transformers/exports.ts

@@ -22,7 +22,7 @@ import {
   Node,
   ClassDeclaration,
 } from 'estree';
-import { TransformSourceDescription } from 'rollup';
+import { TransformSourceDescription, OutputOptions } from 'rollup';
 import { NamedDeclaration, DefaultDeclaration } from './parsing-utilities';
 import { isESMFormat } from '../options';
 import {
@@ -34,6 +34,20 @@ import {
 import MagicString from 'magic-string';
 import { parse, walk } from '../acorn';
 
+const CJS_EXTERN = `/**
+* @fileoverview Externs built via derived configuration from Rollup or input code.
+* This extern contains the export global object so Closure doesn't get confused by its presence.
+* @externs
+*/
+
+/**
+ * @typedef {{
+ *   __esModule: boolean,
+ * }}
+ */
+let exports;
+`;
+
 /**
  * This Transform will apply only if the Rollup configuration is for 'esm' output.
  *
@@ -76,6 +90,14 @@ export default class ExportTransform extends Transform implements TransformInter
     return originalExports;
   }
 
+  public extern(options: OutputOptions): string {
+    if (options.format === 'cjs') {
+      return CJS_EXTERN;
+    }
+
+    return '';
+  }
+
   /**
    * Before Closure Compiler modifies the source, we need to ensure it has window scoped
    * references to the named exports. This prevents Closure from mangling their names.
@@ -225,9 +247,7 @@ export default class ExportTransform extends Transform implements TransformInter
 
                     if (originalExports[exportName].alias !== null) {
                       collectedExportsToAppend.push(
-                        `${ancestor.expression.left.property.name} as ${
-                          originalExports[exportName].alias
-                        }`,
+                        `${ancestor.expression.left.property.name} as ${originalExports[exportName].alias}`,
                       );
                     } else {
                       collectedExportsToAppend.push(ancestor.expression.left.property.name);
@@ -250,9 +270,7 @@ export default class ExportTransform extends Transform implements TransformInter
 
                     if (ancestor.expression.right.type === 'Identifier') {
                       collectedExportsToAppend.push(
-                        `${ancestor.expression.right.name} as ${
-                          ancestor.expression.left.property.name
-                        }`,
+                        `${ancestor.expression.right.name} as ${ancestor.expression.left.property.name}`,
                       );
                     }
                     break;

test/export-cjs/export-cjs-extern.test.js

@@ -0,0 +1,40 @@
+/**
+ * Copyright 2019 The AMP HTML Authors. All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS-IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import test from 'ava';
+import { createTransforms } from '../../transpile/transforms';
+import { defaults } from '../../transpile/options';
+import * as fs from 'fs';
+import { promisify } from 'util';
+
+const readFile = promisify(fs.readFile);
+
+test('generate extern for cjs export pattern', async t => {
+  const externFixtureContent = await readFile('test/export-cjs/fixtures/export.extern.js', 'utf8');
+  const outputOptions = {
+    format: 'cjs',
+  };
+
+  const transforms = createTransforms({});
+  const options = defaults(outputOptions, [], transforms);
+
+  const contentMatch = options.externs.some(async externFilePath => {
+    const fileContent = await readFile(externFilePath, 'utf8');
+    return fileContent === externFixtureContent;
+  });
+
+  t.is(contentMatch, true);
+});

test/export-cjs/fixtures/export.extern.js

@@ -0,0 +1,12 @@
+/**
+* @fileoverview Externs built via derived configuration from Rollup or input code.
+* This extern contains the export global object so Closure doesn't get confused by its presence.
+* @externs
+*/
+
+/**
+ * @typedef {{
+ *   __esModule: boolean,
+ * }}
+ */
+let exports;