升級(jí)webpack4.0后，打包報(bào)如下錯(cuò)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(dǎo)致的 ，
解決方法：
1.package.json中設(shè)置：

”scripts": {
    "dev": "webpack --mode development",  // 開發(fā)環(huán)境
     "build": "webpack --mode production",  // 生產(chǎn)環(huán)境
  }

2.webpack.config.js中設(shè)置：

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