解决了，必须要在 plugins 里加上：

new webpack.DefinePlugin({
  'process.env': {
    'NODE_ENV': '"production"'
  }
})

才是正确姿势

什么warning？

参考：undefined is not an object (evaluating 'process.env.NODE_ENV')

Note: by default, React will be in development mode. To use React in production mode, set the environment variable NODE_ENV to production (using envify or webpack's DefinePlugin). A minifier that performs dead-code elimination such as UglifyJS is recommended to completely remove the extra code present in development mode.

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