Express.js - middleware

In this article, we would like to show you how to create and use middleware functions in Express.js.

Middleware functions are functions that have access to the request object (req), the response object (res), and the next middleware function (next) in express.js application’s request-response cycle.

They are used to:

• execute any code,
• modify req and res objects for tasks like parsing request bodies, adding response headers, authentication etc.
• end the request-response cycle,
• call the next middleware function in the stack.

Note:

If the current middleware function doesn't end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

In this example, we create a simple request tracker that prints every request date in the console and then calls the next() function.

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const express = require('express');

const app = express();

​

app.use((req, res, next) => {

  // get and print current date

  const date = new Date();

  console.log('A new request registered at ' + date);

​

  // call the next middleware function in the stack

  next();

});

​

app.listen(3000);

Now when you run the application and send any request (e.g just open http://localhost:3000/) you should get the following result in the console:

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A new request registered at Fri Aug 27 2021 16:41:27 GMT+0200 (Central European Summer Time)

To assign the middleware to a specific route and all its subroutes, you need to specify that route as the first argument of the app.use().

Example:

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const express = require('express');

const app = express();

​

// request tracker

app.use('/users', (req, res, next) => {

  // get and print current date

  const date = new Date();

  console.log('A new request received at ' + date);

​

  // call the next middleware function in the stack

  next();

});

​

app.listen(3000);

Now the middleware won't work for localhost:3000/. You need to request any subroute of '/users', only then it will print the time.

Request:

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http://localhost:3000/users

Console:

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A new request received at Fri Aug 27 2021 16:51:11 GMT+0200 (Central European Summer Time)

• Node.js / Express.js - middleware calls order

• Express.js documentation - middleware