webpack4下webpack-dev-server热更新缓慢问题【已解决：webpack4 mode设置出现问题】

  webpack4下有production(生产)和development（开发）两种模式，若不设置，则会在终端下warn个不停。css   warn内容以下：webpack The ‘mode’ option has not been set. Set ‘mode’ option to ‘development’ or ‘production’ to enable defaults for t

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