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Published June 3, 2022

Express middleware: A complete guide

Express middleware: A complete guide

In this guide, we’ll explore the basics of using Express.js middleware. We’ll create a simple Express API from scratch, then add the middleware to it and demonstrate how to use each tool.

The Express middleware tools we’re going to discuss are must-haves for your initial Express.js app setup. We’ll show you how to get started with them, and you can further configure them according to your application’s unique needs.

We’ll go over the following:

  • What is Node.js?
  • What is Express.js?
  • What is Express middleware?
  • How does middleware work?
  • Setting up an Express.js API
    • Using Express middleware
    • morgan
    • Helmet
    • cors
    • Express Rate Limit
    • serve-favicon

For simplicity’s sake, we’ll create only one endpoint in the example Express API. The complete code is available on GitHub.

What is Node.js?
Node.js is an open-source JavaScript runtime environment built on top of Chrome’s V8 JavaScript engine.

While Node.js can handle elementary tasks such as creating a simple server, more complex tasks, such as separately handling requests at different routes or serving static files, are more difficult.

What is Express.js?
Express.js is one of the most popular and widely used Node web frameworks. In fact, the “E” in MERN, MEVN, and MEAN Stack stands for “Express.”

According to the official Express.js documentation, “Express is a fast, unopinionated, minimalist web framework for Node.js.” Although Express is minimalist, it is also very flexible, which has led to the development of various middlewares that can be used with Express.js to address almost any task or problem you can think of.

What is Express middleware?
Middleware is software containing functions that execute during the request-response cycle and have access to both the request object (req) and the response object (res). Middleware is executed during the window between when a server receives a request and when it sends a response.

Express middleware includes application-level, router-level, and error handling functionality and can be built-in or from a third party. Since Express.js has limited functionality of its own, an Express app is largely comprised of multiple middleware function calls.

You could write your own middleware for Express.js, but most developers prefer to use and configure built-in and third-party tools for common tasks. In this guide, we’ll show you how to use five of the most popular Express middlewares. But first, a short overview of how middleware functions within an app.

How does middleware work?
To understand how middleware works, imagine you own a lemonade stand where customers bring their own lemons and you make the lemonade. You’re responsible for evaluating the lemons’ origin and freshness, discarding any subpar lemons, and, finally, making the lemonade.

To reduce your workload, you hire a worker — we’ll call him Larry — to make sure the lemons were grown organically and without any harmful chemicals. In this analogy, Larry is the middleware that functions between you and your customers’ lemons.

Now you’re making a profit, so you hire two other employees, Curly and Moe. Larry checks the lemons’ origin and passes the organically grown lemons to Curly, who discards the rotten lemons and hands the good ones to Moe. Moe verifies their freshness and hands the fresh lemons to you.

Now you can focus on making the lemonade and increasing your profits.

Think of the lemons as your HTTP requests and your lemonade stand as the server. You check a lemon’s origin just as you would with an HTTP request before accepting or rejecting it. Not all requests from a trusted origin are good, so they still need to be filtered. Your employees — Larry, Curly, and Moe — are like middleware for your lemonade program. If at any stage a middleware determines that a request is bad, it has the ability to terminate the request-response cycle.

Once a request has passed all the middleware present in your app, it reaches the controller function — which, in the case of our example, is you (or, more specifically, the act of making the lemonade).

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