参考官方文档：https://webpack.docschina.org/plugins/mini-css-extract-plugin/
webpack5以上可用，用mini-css-extract-plugin 插件，支持js和css打包分离。
webpack.pro.config.js 中新增如下：

const path = require('path');
const { merge } = require('webpack-merge');
const { CleanWebpackPlugin } = require('clean-webpack-plugin');
const HtmlWebpackPlugin = require('html-webpack-plugin');
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const MiniCssExtractPlugin = require('mini-css-extract-plugin');
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const webpackConfigBase = require('./webpack.base.config');

const webpackConfigProd = {
    mode: 'production',
    plugins: [
        new CleanWebpackPlugin({
            protectWebpackAssets: true,
        }),
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        new MiniCssExtractPlugin({
            filename: '[name].[fullhase:4].css',
        }),
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        new HtmlWebpackPlugin({
            inject: 'body',
            title: 'React APP',
            filename: 'index.html',
            template: path.join(__dirname, '../src/index.html'),
        }),
    ],
};

module.exports = merge(webpackConfigBase, webpackConfigProd);

webpack.base.config.js 中

module: {
        rules: [
            {
                test: /\.js[x]/,
                use: 'babel-loader',
            },
            {
                test: /\.ts[x]/,
                use: 'ts-loader',
            },
            {
                test: /\.(sc|c)ss/,
                use: [
                    devMode ? 'style-loader' : MiniCssExtractPlugin.loader,
                    'css-loader',
                    'sass-loader',
                ],
            },
        ],
    },

根据情况使用不同的loader插件
最后package.json 中增加如下：

"scripts": {
    "dev": "webpack server --progress --config config/webpack.dev.config.js",
    "build": "NODE_ENV=production webpack --progress --config config/webpack.prod.config.js",
    "test": "echo \"Error: no test specified\" && exit 1"
  },

这样，build完之后，js和css将分离。