使用 webpack 打包项目出现“The 'mode' option has not been set, webpack will fallback to 'production' for this value.”错误。
错误描述：
WARNING in configuration
The 'mode' option has not been set, webpack will fallback to 'production' for this value. 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/
错误原因：
上述错误是因为我们没有设置 webpack 的模式（生产-production，开发-development）
解决办法：
打开 DOS 窗口，执行 webpack --mode=production 或者 webpack --mode=development 命令，例如：
$ ./node_modules/.bin/webpack --mode=development
Hash: 310a9d712d610fa38fef
Version: webpack 4.17.2
Time: 743ms
Built at: 2018-09-05 16:06:05
Asset Size Chunks Chunk Names
main.js 3.81 KiB main [emitted] main
Entrypoint main = main.js
[./src/index.js] 37 bytes {main} [built]
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