feat(babel-transpiler): support .js babel config files (#1422)

The babel transpiler will now also look for babel.config.js files. New algoritm:

* Check if the provided optionsFile is a `.babelrc.js`-file. If so, require and return it.
* Check if the provided optionsFile is a `babel.config.js`. If so, require it, call it with an optionsApi object provided by the user and return the result

packages/babel-transpiler/src/BabelConfigReader.ts

@@ -8,6 +8,7 @@ export interface StrykerBabelConfig {
   extensions: ReadonlyArray<string>;
   options: babel.TransformOptions;
   optionsFile: string | null;
+  optionsApi?: Partial<babel.ConfigAPI>;
 }
 
 export const CONFIG_KEY = 'babel';
@@ -33,21 +34,27 @@ export class BabelConfigReader {
       ...strykerOptions[CONFIG_KEY]
     };
     babelConfig.options = {
-      ...this.readBabelOptionsFromFile(babelConfig.optionsFile),
+      ...this.readBabelOptionsFromFile(babelConfig.optionsFile, babelConfig.optionsApi),
       ...babelConfig.options
     };
     this.log.debug(`Babel config is: ${JSON.stringify(babelConfig, null, 2)}`);
     return babelConfig;
   }
 
-  private readBabelOptionsFromFile(relativeFileName: string | null): babel.TransformOptions {
+  private readBabelOptionsFromFile(relativeFileName: string | null, optionsApi?: Partial<babel.ConfigAPI>): babel.TransformOptions {
     if (relativeFileName) {
       const babelrcPath = path.resolve(relativeFileName);
       this.log.debug(`Reading .babelrc file from path "${babelrcPath}"`);
       if (fs.existsSync(babelrcPath)) {
         try {
-          const config: babel.TransformOptions = JSON.parse(fs.readFileSync(babelrcPath, 'utf8'));
-          return config;
+          if (path.basename(babelrcPath) === '.babelrc.js') {
+            return require(babelrcPath) as babel.TransformOptions;
+          }
+          if (path.basename(babelrcPath) === 'babel.config.js') {
+            const config: babel.ConfigFunction = require(babelrcPath);
+            return config(optionsApi as babel.ConfigAPI);
+          }
+          return JSON.parse(fs.readFileSync(babelrcPath, 'utf8')) as babel.TransformOptions;
         } catch (error) {
           this.log.error(`Error while reading "${babelrcPath}" file: ${error}`);
         }

packages/babel-transpiler/test/integration/BabelProjects.it.spec.ts

@@ -6,11 +6,12 @@ import { expect } from 'chai';
 import { testInjector } from '@stryker-mutator/test-helpers';
 import { CONFIG_KEY, StrykerBabelConfig } from '../../src/BabelConfigReader';
 import { commonTokens } from '@stryker-mutator/api/plugin';
+import { ConfigAPI } from '@babel/core';
 
 function describeIntegrationTest(projectName: string, babelConfig: Partial<StrykerBabelConfig> = {}) {
 
   const projectDir = path.resolve(__dirname, '..', '..', 'testResources', projectName);
-  babelConfig.optionsFile = path.join(projectDir, '.babelrc');
+  babelConfig.optionsFile = path.join(projectDir, babelConfig.optionsFile || '.babelrc');
   let projectFiles: File[] = [];
   let resultFiles: File[] = [];
   let babelTranspiler: BabelTranspiler;
@@ -65,3 +66,17 @@ describe('Project with binary files', () => {
 describe('Different extensions', () => {
   describeIntegrationTest('differentExtensions');
 });
+describe('A Babel project with babel.config.js config file', () => {
+  const noop = () => {};
+  describeIntegrationTest('babelProjectWithBabelConfigJs', {
+    extensions: ['.ts'],
+    optionsApi: { cache: { forever: noop }} as ConfigAPI,
+    optionsFile: 'babel.config.js',
+  });
+});
+describe('A Babel project with .babelrc.js config file', () => {
+  describeIntegrationTest('babelProjectWithBabelRcJs', {
+    extensions: ['.ts'],
+    optionsFile: '.babelrc.js'
+  });
+});

packages/babel-transpiler/testResources/babelProjectWithBabelConfigJs/babel.config.js

@@ -0,0 +1,7 @@
+module.exports = api => {
+  api.cache.forever();
+
+  return {
+    presets: ["@babel/preset-typescript"]
+  }
+}

packages/babel-transpiler/testResources/babelProjectWithBabelConfigJs/expectedResult/foo.js

@@ -0,0 +1 @@
+export const foo = 4;

packages/babel-transpiler/testResources/babelProjectWithBabelConfigJs/expectedResult/main.js

@@ -0,0 +1 @@
+const x = 0;

packages/babel-transpiler/testResources/babelProjectWithBabelConfigJs/source/foo.ts

@@ -0,0 +1 @@
+export const foo: number = 4;

packages/babel-transpiler/testResources/babelProjectWithBabelConfigJs/source/main.ts

@@ -0,0 +1 @@
+const x: number = 0;

packages/babel-transpiler/testResources/babelProjectWithBabelRcJs/.babelrc.js

@@ -0,0 +1,3 @@
+module.exports = { 
+  presets: ["@babel/preset-typescript"]
+}

packages/babel-transpiler/testResources/babelProjectWithBabelRcJs/expectedResult/foo.js

@@ -0,0 +1 @@
+export const foo = 4;

packages/babel-transpiler/testResources/babelProjectWithBabelRcJs/expectedResult/main.js

@@ -0,0 +1 @@
+const x = 0;

packages/babel-transpiler/testResources/babelProjectWithBabelRcJs/source/foo.ts

@@ -0,0 +1 @@
+export const foo: number = 4;

packages/babel-transpiler/testResources/babelProjectWithBabelRcJs/source/main.ts

@@ -0,0 +1 @@
+const x: number = 0;