babelとは?

ブラウザにまだサポートされていないようなJSの次世代標準機能をブラウザで使えるようにするNode.jsトランスパイラ
ES6, ES7など(ECMAScript)
Bableを使う際に書くのはAltJSではなく、純粋なJavaScript

Bable ES6の構文サポート
– Class, Template Strings, Arrow Function, Default Parameter, Rest Parameter, Property Literals, Modules

class Code {
    constructor(item){
        this.item = item;
    }
    showItem(){
        console.log(`you are buying ${this.item}`);
    }
}

class Sec extends Code {
    constructor(item = 'コロプラ'){
        super(item);
    }
}

var app = new Sec();
app.showItem();

[vagrant@localhost front]$ node entry.js
you are buying コロプラ

では、早速Babelをインストールしましょう。
$ npm install -g babel-cli
$ babel entry.js –out-file output.js

あれ、全然変わってない？
presetをインストールして、.babelrcを書く必要がある？？
$ npm install –save-dev babel-preset-env

.babelrc

{
   "presets": ["env"]
}

再度実行
$ babel entry.js –out-file output.js

output.js

'use strict';

var _createClass = function () { function defineProperties(target, props) { for (var i = 0; i < props.length; i++) { var descriptor = props&#91;i&#93;; descriptor.enumerable = descriptor.enumerable || false; descriptor.configurable = true; if ("value" in descriptor) descriptor.writable = true; Object.defineProperty(target, descriptor.key, descriptor); } } return function (Constructor, protoProps, staticProps) { if (protoProps) defineProperties(Constructor.prototype, protoProps); if (staticProps) defineProperties(Constructor, staticProps); return Constructor; }; }();

function _possibleConstructorReturn(self, call) { if (!self) { throw new ReferenceError("this hasn't been initialised - super() hasn't been called"); } return call && (typeof call === "object" || typeof call === "function") ? call : self; }

function _inherits(subClass, superClass) { if (typeof superClass !== "function" && superClass !== null) { throw new TypeError("Super expression must either be null or a function, not " + typeof superClass); } subClass.prototype = Object.create(superClass && superClass.prototype, { constructor: { value: subClass, enumerable: false, writable: true, configurable: true } }); if (superClass) Object.setPrototypeOf ? Object.setPrototypeOf(subClass, superClass) : subClass.__proto__ = superClass; }

function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }

var Code = function () {
    function Code(item) {
        _classCallCheck(this, Code);

        this.item = item;
    }

    _createClass(Code, &#91;{
        key: 'showItem',
        value: function showItem() {
            console.log('you are buying ' + this.item);
        }
    }&#93;);

    return Code;
}();

var Sec = function (_Code) {
    _inherits(Sec, _Code);

    function Sec() {
        var item = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 'コロプラ';

        _classCallCheck(this, Sec);

        return _possibleConstructorReturn(this, (Sec.__proto__ || Object.getPrototypeOf(Sec)).call(this, item));
    }

    return Sec;
}(Code);

var app = new Sec();
app.showItem();

なるほど、OK!
ES5への変換は、babel-preset-envが必要ってことね。
そんで、これ、webpackとどう連携するの？