webpack4 打包时报错：Invalid configuration object.

报错信息：

Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration.mode should be one of these:
   "development" | "production" | "none"
   -> Enable production optimizations or development hints.

 

在webpack4发生这个错误是因为在config/webpack.dev.js文件中未指定打包方式；

如果是在开发环境下，则配置：development

如果是在生产环境下，则配置：production

代码示例：

webpack.dev.js

const path = require("path")
module.exports = {
    // 入口文件可以有多个，多个的时候用数组的形式写
    entry: {
        main: "./src/main.js"
    },

    // 打包环境：开发/生产
    mode: "development",

    // 出口，只能有一个
    output: {
        filename: "[name]-bundle.js",
        path: path.resolve(__dirname, "../dist")
    }
}