Error Handling Middlewares

Error handling is a crucial part of any web application. Node.js and Express make it easy to create error-handling middlewares that can intercept, manage, and respond to errors centrally. In this module, you will learn how to configure and use middlewares to handle errors efficiently.

What is an Error Handling Middleware?

An error-handling middleware is similar to any other middleware but is distinguished by having four parameters in its function: err, req, res, and next. Having the err parameter at the beginning indicates to Express that this function is specifically for error handling.

Basic Structure

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Capturing Errors in Routes

To capture an error in a route and pass it to the error-handling middleware, use the next function, passing the error as an argument.

Example

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In this example, any request to the /error route will generate an intentional error and pass it to the error-handling middleware, which will then log the error and send a 500 status response.

Different Types of Errors

It's possible to handle different types of errors specifically. For example, validation errors, authentication errors, database connection errors, etc., can be handled differently based on their characteristics.

Example: Differentiated Error Handling

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In this example, validation errors send a response with a 400 status, and authentication errors send a response with a 401 status. Other errors are handled generically.

[Insert image here: Diagram showing the flow of a request that encounters an error and how it is passed through the error-handling middleware.]

Handling errors correctly ensures a smoother experience for your users and a simpler way for you, as a developer, to debug and resolve issues. In the next module, we will discuss the utility and methods of logging in middlewares to improve the observability of your application.