Role of next(err) in Error Handling Middleware

In Express.js the next function is the callback function which is mainly used to pass the control to the next middleware for performing certain tasks. If we use the next function along with the error parameter as (err) then it handles the error which occurs during the processing of the request and passes the control to the next middleware function.

1. Pass Control to the Next Middleware: When the next(err) is called, the Express knows that an error has occurred, so it skips the control to the next middleware function which handles the error.
2. Error Propagation: The parameter err consists of the information about the error that has occurred. This parameter is mostly the instance of the “Error” object it can be a custom object also. We can also log the error information, which is sent as a response to the client.
3. Bypass the Regular Middleware: In the application, when the error has occurred and the next(Err) function is called, the regular middleware functions for the current route are skipped and the control is passed to the next middleware to handle the error.
4. Error Response: The middleware that is developed to handle these errors can give the correct response to the client. This can include the logging of errors or sending some specific HTTP status codes that give the information about the error.

Syntax:

app.use((err, req, res, next) => {
  // Handle the error or log it
  console.error(err);
  // Pass the error to the next error-handling middleware
  next(err);
});

Express is the most popular framework for Node.js which is used to build web-based applications and APIs. In application development, error handling is one of the important concepts that provides the aspect to handle the errors that occur in the middleware. The middleware functions have access to the request object, the response object, and the next middleware function which is defined as next.

In this article, we will see the concept of next(err) for error handling in Express. We will see its Syntax and practical example with output.