You are currently using minified code outside of NODE_ENV === ‘production’. This means that you are running a slower development build of Redux. You can use loose-envify (https://github.com/zertosh/loose-envify) for browserify or DefinePlugin for webpack (http://stackoverflow.com/questions/30030031) to ensure you have the correct code for your production build.

找了很多文章，都是說在webpack.config.js加上此句，

new webpack.DefinePlugin({
            'process.env': {
                NODE_ENV: JSON.stringify('development')
            }
        })

或是將react-router從v3升級v4，但我原本就使用4.0.0了，但還是嘗試升級看看，

反而我從4.0.0升到4.0.5又出現其他錯誤訊息，只好將回4.0.0版本，

不管怎麼在設定檔案上做NODE_ENV的參數，都會報錯，

後來看到此command設定：

"dev": "webpack --mode development",

    "build": "webpack --mode production"

好像可以解決我的問題，但我又發現我是使用webpack-dev-middleware，

並沒有地方可以設定command，幸好在原本的webpack.config.js可以加上此參數

mode:”development”

module.exports = {
    mode:"development",
    //入口文件
    entry:{
        'webpack-hot-middleware/client',
        path.resolve(__dirname, 'react')
    },
...
...

設定完後，重新啟動start:dev，就沒有在跑出此錯誤訊息了！

mode此參數為v4所新增的。

Reference

https://blog.csdn.net/jiang7701037/article/details/80682662