build(rollup): enable babel transform

package.json

@@ -79,6 +79,7 @@
     "babel-cli": "^6.14.0",
     "babel-core": "^6.14.0",
     "babel-loader": "^6.2.5",
+    "babel-plugin-external-helpers": "^6.8.0",
     "babel-plugin-transform-es2015-modules-commonjs": "^6.14.0",
     "babel-plugin-transform-inline-environment-variables": "^6.8.0",
     "babel-plugin-transform-node-env-inline": "^6.8.0",
@@ -103,6 +104,7 @@
     "postcss-custom-properties": "^5.0.1",
     "postcss-import": "^8.1.2",
     "rollup": "^0.35.14",
+    "rollup-plugin-babel": "^2.6.1",
     "rollup-plugin-commonjs": "^5.0.3",
     "rollup-plugin-node-resolve": "^2.0.0",
     "stylelint": "^7.2.0",

rollup.config.js

@@ -6,6 +6,7 @@ const path = require('path');
 
 const nodeResolve = require('rollup-plugin-node-resolve');
 const commonjs = require('rollup-plugin-commonjs');
+const babel = require('rollup-plugin-babel');
 
 const isRelease = process.env.NODE_ENV === 'production';
 
@@ -15,6 +16,42 @@ assert.ok(!!KAREN_ENTRY_POINT, 'not found process.env.KAREN_ENTRY_POINT');
 const KAREN_CLIENT_DIST_DIR = process.env.KAREN_CLIENT_DIST_DIR;
 assert.ok(!!KAREN_CLIENT_DIST_DIR, 'not found process.env.KAREN_CLIENT_DIST_DIR');
 
+const babelPresets = [
+];
+
+let babelPlugins = [
+    // For our target browsers, we need not some transforms.
+
+    // es2016 level
+    'babel-plugin-transform-exponentiation-operator',
+    // es2017 level
+    'babel-plugin-syntax-trailing-function-commas',
+    'babel-plugin-transform-async-to-generator',
+
+    // for React
+    'syntax-jsx',
+    'transform-react-jsx',
+
+    // some utilitis
+    'transform-inline-environment-variables',
+    'transform-node-env-inline',
+
+    // to remove duplicated helper methods inserted by babel.
+    'external-helpers',
+];
+
+if (isRelease) {
+    babelPlugins = babelPlugins.concat([
+        'transform-react-constant-elements',
+        'transform-react-inline-elements',
+    ]);
+}
+else {
+    babelPlugins = babelPlugins.concat([
+        'transform-react-jsx-source',
+    ]);
+}
+
 // https://github.com/rollup/rollup/wiki/JavaScript-API
 // https://github.com/rollup/rollup/wiki/Command-Line-Interface
 module.exports = {
@@ -43,5 +80,12 @@ module.exports = {
         commonjs({
             ignoreGlobal: true,
         }),
+
+        // https://github.com/rollup/rollup-plugin-babel
+        babel({
+            exclude: 'node_modules/**',
+            presets: babelPresets,
+            plugins: babelPlugins,
+        }),
     ]
 };