Data Validation and Sanitization

Every time a web application accepts user input—whether from a sign-up form or a search box—it opens the door to unexpected and sometimes dangerous data. Without proper validation and sanitization, bad input can crash the server, corrupt stored data, or introduce critical security vulnerabilities like SQL injection or cross-site scripting (XSS).

Fortunately, Express.js provides powerful tools to ensure that only well-formed data enters the system.

In this lesson, we’ll explore how to validate user input using middleware and the express-validator library. We’ll also cover sanitization techniques to clean input data before processing.

Note: While input validation plays a key role in security, we’ll save an in-depth discussion of threats like SQL injection and XSS attacks for a later lesson on securing Express applications.

Understanding validation and sanitization

Validation ensures that incoming data follows the expected format. This includes checking required fields, enforcing data types, and setting constraints such as minimum and maximum values. Without proper validation, we may store incorrect or missing data, leading to application errors.

Sanitization, on the other hand, modifies input to remove unwanted characters or formats. It helps standardize data, ensuring consistency in our application. While validation rejects bad input, sanitization cleans it before use.

Validating user input using middleware

We can manually validate input using middleware functions. These functions check if the request data meets our requirements before proceeding to the next middleware or route handler.