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在 Webpack 5 中，核心节点模块的 polyfill 被移除，而是需要在 resolve.fallback 属性中列出所需的包。下面是 webpack.config.client.js 文件的解析属性。

resolve: {
  fallback: {
    path: require.resolve("path-browserify"),
    crypto: require.resolve("crypto-browserify"),
    "babel-polyfill": require.resolve("@babel/polyfill"),
    buffer: require.resolve("buffer/"),
    stream: require.resolve("stream-browserify"),
  }
}

我使用 webpack-dev-middleware 和 webpack-hot-middleware 以及 express 来为我的 ssr 应用程序实现 HMR 的好处。

import express from "express";
import webpack from "webpack";
...
const app = express();
...
const webpackDevConfig = require("../webpack/webpack.config.client");
const compiler = webpack(webpackDevConfig);
app.use(
  require("webpack-dev-middleware")(compiler, {
    publicPath: webpackDevConfig.output.publicPath,
  }),
);
app.use(require("webpack-hot-middleware")(compiler));
...

导入配置文件时，后备属性中的那些模块将被解析并以数字形式返回。当配置对象被传递给它的构造函数时，webpack 会抛出一个错误，因为configuration.resolve.fallback属性应该是非空字符串，但是给出了数字。

下面是传递的实际 resolve.fallback 属性，并返回了错误。

{
  path: 79936 (which suppose to be "/some path/node_module/path/index.js",
  crypto: 32640,
  'babel-polyfill': 71202,
  buffer: 30816,
  stream: 78787
}

ValidationError: Invalid configuration object. Webpack has been initialized using a configuration object that does not match the API schema.
 - configuration.resolve.fallback should be one of these:
   [object { alias, name, onlyModule? }, ...] | object { <key>: [non-empty string, ...] | false | non-empty string }
   -> Redirect module requests. 
.....
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