Note--Middleware

Middleware is commonly used to implement shared functional requirements in web applications. It is an integral part of web applications, just like libraries, as it helps developers avoid code duplication and encourages separation of concerns.

This guide focuses on middleware used in the context of client and server applications communicating over HTTP, as discussed in this blog post. The author describes middleware as code that wraps around a web application object to do tasks before and/or after the web application is called. These tasks typically include exporting observability data, performing application-wide authentication and authorization, and caching results.

Axios:

Interpreter

TypeScript

export const requestInterceptor = function (requestConfig) {
  requestConfig.startTime = Date.now();
  console.log("url=%s method=%s", requestConfig.url, requestConfig.method);
  return requestConfig;
};

export const responseInterceptor = function (response) {
  console.log(
    "url=%s method=%s status=%s latency=%s",
    response.config.url,
    response.config.method,
    response.status,
    Date.now() - response.config.startTime
  );
  return response;
};

export const errorInterceptor = function (error) {
  if (error.response) {
    console.log(
      "url=%s method=%s error=%s status=%s",
      error.response.config.url,
      error.response.config.method,
      error.response.data,
      error.response.status
    );
  } else if (error.request) {
    console.log(error.request);
  } else {
    console.log("Error", error.message);
  }
  return Promise.reject(error);
};

Usage

TypeScript

import axios from "axios";

const axiosGithub = axios.create();

axiosGithub.interceptors.request.use(requestInterceptor, errorInterceptor);
axiosGithub.interceptors.response.use(responseInterceptor, errorInterceptor);

Express

Application-level middleware

TypeScript

function logOriginalUrl (req, res, next) {
  console.log('Request URL:', req.originalUrl)
  next()
}

function logMethod (req, res, next) {
  console.log('Request Type:', req.method)
  next()
}

const logStuff = [logOriginalUrl, logMethod]
app.get('/user/:id', logStuff, (req, res, next) => {
  res.send('User Info')
})

Router-level middleware

TypeScript

const express = require('express')
const app = express()
const router = express.Router()

// a middleware function with no mount path. This code is executed for every request to the router
router.use((req, res, next) => {
  console.log('Time:', Date.now())
  next()
})

// a middleware sub-stack shows request info for any type of HTTP request to the /user/:id path
router.use('/user/:id', (req, res, next) => {
  console.log('Request URL:', req.originalUrl)
  next()
}, (req, res, next) => {
  console.log('Request Type:', req.method)
  next()
})

// a middleware sub-stack that handles GET requests to the /user/:id path
router.get('/user/:id', (req, res, next) => {
  // if the user ID is 0, skip to the next router
  if (req.params.id === '0') next('route')
  // otherwise pass control to the next middleware function in this stack
  else next()
}, (req, res, next) => {
  // render a regular page
  res.render('regular')
})

// handler for the /user/:id path, which renders a special page
router.get('/user/:id', (req, res, next) => {
  console.log(req.params.id)
  res.render('special')
})

// mount the router on the app
app.use('/', router)

Error-handling middleware

TypeScript

app.use((err, req, res, next) => {
  console.error(err.stack)
  res.status(500).send('Something broke!')
})

Other resources

Middleware Patterns in Go, a discussion of server-side middleware patterns

Writing HTTP Client Middleware in Go, a blog post by the author that shows how to avoid external HTTP API interactions using client-side middleware

The book Practical Go by the author, which describes in detail how to implement middleware in client and server applications

Github/go-fault, which demonstrates how to use server-side middleware for chaos engineering

Javascript

Axios library documentation on Interceptors

Writing Middleware and Using Middleware from the Express documentation, which are useful for learning more about middleware in Express

Python

Requests-Middleware, a package that implements various middleware for the Requests library

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Framework