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: 'fist-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!');
                },
            });

            middlewares.push((req, res) => {
                res.send('Hello World!');
            });

            return middlewares;
        },
    },
};