升级webpack4.0后，打包报如下错误：
WARNING in configuration
The 'mode' option has not been set, webpack will fallback to 'production' for thisvalue. Set 'mode' option to 'development' or 'production' to enable defaults for each environment.
You can also set it to 'none' to disable any default behavior. Learn more: https://webpack.js.org/concepts/mode/
原因是没有指定mode 导致的 ，
解决方法：
1.package.json中设置：
”scripts": {"dev": "webpack --mode development", // 开发环境"build": "webpack --mode production", // 生产环境}
2.webpack.config.js中设置：
module.exports = {entry: './src/app.js',output: {path: path.resolve(__dirname, 'dist'),filename: 'main.js'},mode: 'development' // 设置mode
}