升級webpack4.0后，打包報如下錯誤：

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/

原因是沒有指定mode 導致的 ，
解決方法：
1.package.json中設置：

”scripts": {
    "dev": "webpack --mode development",  // 開發環境
     "build": "webpack --mode production",  // 生產環境
  }

2.webpack.config.js中設置：

module.exports = {
    entry: './src/app.js',
    output: {
        path: path.resolve(__dirname, 'dist'),
        filename: 'main.js'
    },
    mode: 'development' // 設置mode
}