Код

...

const app = express();

if (process.env.NODE_ENV != 'production') {

 const webpack = require('webpack');

 const webpackDevMiddleware = require('webpack-dev-middleware');

 const config = require('./webpack.config.js');

 const compiler = webpack(config);

 app.use(

  webpackDevMiddleware(compiler, {

   publicPath: config.output.publicPath,

  })

 );

}