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Express Middleware

A Comprehensive Guide to Express.js Middleware

Express.js is a popular Node.js web application framework that provides a robust set of features for building web applications. One of the key concepts in Express is middleware, which allows you to intercept and modify requests and responses before they reach your application's routes.

Express middleware is a function that has access to the request object (req), the response object (res), and the next middleware function in the request-response cycle. Middleware functions can perform various tasks, such as:

  • Logging requests and responses
  • Adding headers to responses
  • Authenticating users
  • Validating data
  • Handling errors

There are three main types of middleware in Express:

  • Application-level middleware: This type of middleware applies to all routes in your application.
  • Router-level middleware: This type of middleware applies only to specific routes defined within a router.
  • Error-handling middleware: This type of middleware is used to handle errors that occur during request processing.

To create middleware, you simply define a function that takes three parameters: req, res, and next.

function logger(req, res, next) {
  console.log(`Request URL: ${req.url}`);
  next();
}

You can use middleware in Express by calling the use() method on your application or router object.

app.use(logger); // Application-level middleware
router.use(logger); // Router-level middleware

Middleware functions are executed in the order they are defined. This means that if you have multiple middleware functions registered, the first one will be called first, followed by the second, and so on.

Express comes with several built-in middleware functions that you can use to perform common tasks, such as:

  • express.static(): Serves static files from a directory.
  • express.json(): Parses JSON request bodies.
  • express.urlencoded(): Parses URL-encoded request bodies.
Do You Know?

Express also supports asynchronous middleware, which allows you to perform tasks that take time, such as database queries.

Error handling middleware is special middleware that is called only when an error occurs during request processing. You can identify error handling middleware by its four parameters: err, req, res, and next.

app.use((err, req, res, next) => {
  console.error(err.stack);
  res.status(500).send('Something went wrong!');
});
Important Note

It's important to call next() in your middleware functions to allow the request to continue to the next middleware or route handler.

  • Express middleware allows you to intercept and modify requests and responses.
  • There are three types of middleware: application-level, router-level, and error-handling middleware.
  • You can create your own middleware functions or use Express's built-in middleware.
  • Middleware functions are executed in the order they are defined.
  • Error handling middleware is used to handle errors during request processing.

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