webpack4.0报错The 'mode' option has not been set, webpack will fallback to 'production' for thisvalue.

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/

webpack官网更新日志有说明：webpack升级4.0新增mode属性

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
}

下载指定webpack指定版本：

npm i -D webpack@3 // 3： webpack版本3最新