Webpack+React+Hot控制台报错：Cannot read property ‘NODE_ENV’ of undefined

解决办法如下：

找到webpack.config.js文件：

var webpack = require('webpack');
var path = require('path');

module.exports = {
  // entry: './js/entry.js',
  entry: [
    'webpack-dev-server/client?http://127.0.0.1:3000', // WebpackDevServer host and port
    'webpack/hot/only-dev-server',
    './js/index.js' // Your app entry point
  ],
  output: {
    path: __dirname + '/js/',
    filename: 'bundle.js',//定义热插拔文件名字
    publicPath: '/js/'
  },
  module: {
    loaders: [
      { test: /\.css$/, loader: 'style!css' },
      { test: /\.(png|jpg)$/, loader: 'url-loader?limit=8192'},
      //{ test: /\.jsx?$/, loaders: ['react-hot', 'babel'], exclude:'/node_modules/' }
      { test: /\.jsx?$/, loaders: ['react-hot', 'babel'], include: path.join(__dirname, 'src') }
    ]
  },
  resolve: {
    extensions: ['', '.js', '.jsx']
  },
    postLoaders: [
        {test: /intents\//, loader: 'webworker'},
        {test: /components\//, loader: 'react-hot', except: /node_modules/}
    ]
},
plugins: [
    new webpack.NoErrorsPlugin(),
    new webpack.DefinePlugin({//添加此块代码
        'process.env.NODE_ENV': '"development"'
    }),
    new webpack.ContextReplacementPlugin(
        /transitions\//,
        path.join(__dirname, "webapp")
    ),
    new webpack.ContextReplacementPlugin(
        /intents\//,
        path.join(__dirname, "webapp")
    )
]

注意'"不能直接写字符串，要字符串外再包装一层。
也可以这么写：

new webpack.DefinePlugin({
      'process.env.NODE_ENV': JSON.stringify('production')
    }),