webpack.config.js

module.exports = {
  // ...
  devServer: {
    setupMiddlewares: (middlewares, devServer) => {
      if (!devServer) {
        throw new Error("webpack-dev-server is not defined");
      }

      devServer.app.get("/setup-middleware/some/path", (_, response) => {
        response.send("setup-middlewares option GET");
      });

      // Use the `unshift` method if you want to run a middleware before all other middlewares
      // or when you are migrating from the `onBeforeSetupMiddleware` option
      middlewares.unshift({
        name: "first-in-array",
        // `path` is optional
        path: "/foo/path",
        middleware: (req, res) => {
          res.send("Foo!");
        },
      });

      // Use the `push` method if you want to run a middleware after all other middlewares
      // or when you are migrating from the `onAfterSetupMiddleware` option
      middlewares.push({
        name: "hello-world-test-one",
        // `path` is optional
        path: "/foo/bar",
        middleware: (req, res) => {
          res.send("Foo Bar!");
        },
      });

        res.send("Hello World!");
      });

      return middlewares;
    },
  },
};