build.js

const webpack = require("webpack");

const plugins = [];

if (require.main == module) {
    plugins.push(
        new webpack.DefinePlugin({
            "process.env": {
                NODE_ENV: JSON.stringify("production")
            }
        }),
        new webpack.optimize.UglifyJsPlugin({
            compress: {
                warnings: false
            }
        })
    );
}

const conf = {
    entry: ["babel-polyfill", __dirname + "/src/start.js"],
    output: {
        path: __dirname,
        filename: "bundle.js"
    },
    plugins: plugins,
    module: {
        loaders: [
            {
                test: /\.js$/,
                loader: "babel-loader",
                query: {
                    presets: [["es2015"], ["react"]],
                    plugins: [
                        "transform-async-to-generator",
                        "transform-object-rest-spread"
                    ]
                }
            }
        ]
    }
};

if (require.main == module) {
    webpack(conf, function(err, info) {
        if (err) {
            console.log(err);
        }
        if (info && info.compilation.errors.length) {
            console.log(info.compilation.errors);
        }
    });
} else {
    module.exports = require("webpack-dev-middleware")(webpack(conf), {
        watchOptions: {
            aggregateTimeout: 300
        },
        publicPath: "/"
    });
}