在根目录创建 vue.config.js 文件；安装依赖    npm install --save-dev @intervolga/optimize-cssnano-plugin

vue.config.js 如下：

const OptimizeCssnanoPlugin = require('@intervolga/optimize-cssnano-plugin');
module.exports = {
    configureWebpack: config => {
        if (process.env.NODE_ENV === 'development') {
            config.plugins.splice(4, 0, new OptimizeCssnanoPlugin({
                cssnanoOptions: {
                    preset: ['default', {
                        discardComments: {
                            removeAll: true,
                        },
                    }],
                },
            }))
        }
    }
};

需要在 '@intervolga/optimize-cssnano-plugin' 中把 .css 改为 .wxss

https://github.com/intervolga/optimize-cssnano-plugin

uni-app使用的webpack版本应该是 webpack@^4.0.0