Understanding Express.js middleware function

Express middleware can be very confusing at first if you are not familiar with callbacks functions. Today we will show you how a middleware works, providing a basic usage example with user authentication.

Based on this express guide, middleware are functions that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle, let’s describe these elements one by one below:

  • The “req” object represents the HTTP request and has properties for the request query string, parameters, body, HTTP headers, and so on.
  • The “res” object represents the HTTP response that an Express app sends when it gets an HTTP request.
  • The “next” function is a function of the Express router which, when invoked, executes the middleware succeeding the current middleware.

// middleware function example:

function myMiddleware(req, res, next) {
  // your logic here


The req and res objects are pretty simple, but next function is a bit more complicated if you are new to express, you can do a whole application without even using it, this is a very bad practice, you can’t make a real express application without properly using the next function, understanding it is the key to make a modular and scalable express applications. We will make a basic user authentication app to help you understand the next function, so, let’s start coding!

Modern web and mobile applications use jwt to authenticate the users, that’s what we are using today for our example, the following code is a middleware that sends a token to the user’s client, and to verify the token sent by the user from the client side.

// auth.middleware.js

Notice that we export two functions (module.exports), these are the middleware functions, but now, how can we use it in our application?

Let´s make an example of it with a user login and Sign up API rest, you need to send a token to the user after the sign up/login, to handle future HTTP requests from that user, also we have to verify the token sent in those HTTP request, that’s what our middleware’s do. The User model below is just a basic schema made with mongoose.

// auth.routes.js

We have two routes here:

  1. ‘/login’ takes two callbacks, login, and auth.signToken functions, auth.signToken is the middleware we defined before on auth.middleware.js, we used require(‘fileRouteHere’) to be able to use the middleware, auth.signToken  will be called only if we call next() on login, notice that I call next() only if I successfully find the user, also notice that I modified the req.user in login, this is the purpose of the req object, not just to get the data from the client but to share data between middleware, req.user will be used in auth.signToken to sign and sent the token to the user on the client side.
  2. /register’ takes only one callback function as a parameter, the register function, this means no more function will be called after this middleware function if we use next(), so, can we remove it from register function and only pass (req, res) as parameters? The answer is no, you should always use the next function even if there are no more chained middleware, because you can use it to handle errors, notice that I call next(err) if I get an error saving the user, this won’t call the next middleware, but the next error handler middleware. If you want to know more about error handling I suggest you go to my next post “Understanding Express.js error handling”.

Now we will handle a basic HTTP request from the user to update his profile, this time the auth.verifyToken middleware will protect the update route, if the token is valid it will call next() to update the user, otherwise, it will send an error 403 to the client (unauthorized).

// user.routes.js

This time the auth.verifyToken is first because we don’t want the user to access to the server APIs if they have an invalid token or no token at all, ‘/user/’  has a patch method, this will call auth.verifyToken decode the token and assign it to req.user then the findByIdAndUpdate function will have access to the user’s _id to update the user, the req.body is the data sent from the client side to update the user.

We are done here! I hope I could have helped you to understand more about express middleware, stay online to learn more about node.js, express.js and MongoDB!




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