Move CJS Extern Generation (#272)

* Move CJS Extern generation to its own transformer
* its 2020

src/transformers/cjs.ts

@@ -0,0 +1,48 @@
+/**
+ * Copyright 2020 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 { Transform } from '../types';
+import { OutputOptions } from 'rollup';
+
+const HEADER = `/**
+* @fileoverview Externs built via derived configuration from Rollup or input code.
+* This extern contains the cjs typing info for modules.
+* @externs
+*/
+
+/**
+* @typedef {{
+*   __esModule: boolean,
+* }}
+*/
+var exports;`;
+
+/**
+ * This Transform will apply only if the Rollup configuration is for a cjs output.
+ *
+ * In order to preserve the __esModules boolean on an Object, this typedef needs to be present.
+ */
+export default class CJSTransform extends Transform {
+  public name = 'CJSTransform';
+
+  public extern(options: OutputOptions): string {
+    if (options.format === 'cjs') {
+      return HEADER;
+    }
+
+    return '';
+  }
+}

src/transformers/exports.ts

@@ -39,13 +39,6 @@ const EXTERN_OVERVIEW = `/**
 * @externs
 */`;
 
-const CJS_EXTERN = `/**
- * @typedef {{
- *   __esModule: boolean,
- * }}
- */
-var exports;`;
-
 /**
  * This Transform will apply only if the Rollup configuration is for 'esm' output.
  *
@@ -111,9 +104,6 @@ export default class ExportTransform extends Transform implements TransformInter
 
   public extern(options: OutputOptions): string {
     let output = EXTERN_OVERVIEW;
-    if (options.format === 'cjs') {
-      output += CJS_EXTERN;
-    }
 
     for (const key of this.originalExports.keys()) {
       const value: ExportDetails = this.originalExports.get(key) as ExportDetails;

src/transforms.ts

@@ -17,6 +17,7 @@
 import { OutputOptions, PluginContext, InputOptions } from 'rollup';
 import { Transform } from './types';
 import IifeTransform from './transformers/iife';
+import CJSTransform from './transformers/cjs';
 import LiteralComputedKeys from './transformers/literal-computed-keys';
 import ExportTransform from './transformers/exports';
 import ImportTransform from './transformers/imports';
@@ -37,6 +38,7 @@ export const createTransforms = (
   return [
     new ConstTransform(context, options),
     new IifeTransform(context, options),
+    new CJSTransform(context, options),
     new LiteralComputedKeys(context, options),
     new StrictTransform(context, options),
     new ExportTransform(context, options),

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

@@ -28,10 +28,12 @@ test('generate extern for cjs export pattern', async t => {
   const transforms = createTransforms({});
   const options = defaults(outputOptions, [], transforms);
 
-  const contentMatch = options.externs.some(async externFilePath => {
+  for (const externFilePath of options.externs) {
     const fileContent = await fsPromises.readFile(externFilePath, 'utf8');
-    return fileContent === externFixtureContent;
-  });
-
-  t.is(contentMatch, true);
+    if (fileContent === externFixtureContent) {
+      t.pass();
+      return;
+    }
+  }
+  t.fail('None of the externs match the expected format');
 });

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

@@ -1,12 +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.
+* This extern contains the cjs typing info for modules.
 * @externs
 */
 
 /**
- * @typedef {{
- *   __esModule: boolean,
- * }}
- */
+* @typedef {{
+*   __esModule: boolean,
+* }}
+*/
 var exports;