Managing Program Flow Using Guards

A guard is a widely used pattern for managing the flow of a program. It checks if certain conditions are met before allowing the code to run further. Guards make our code easier to read and maintain by ensuring that it follows the right path. In this lesson, we’ll learn how to implement a guard to protect the routes of our NestJS application.

Guards overview

In NestJS, a guard is a fundamental component responsible for controlling access to specific routes within our application.

A guard is a class decorated with @Injectable() and must implement the CanActivate interface. The CanActivate interface, fundamental to guards in NestJS, consists of a single method: canActivate. Guards must implement this method, which allows the guard to make decisions based on specific criteria, such as user roles, authentication status, or custom conditions. The simplicity of this interface allows guards to focus on the primary task of evaluating conditions and returning a boolean result.

Guards are essential for authentication and authorization, ensuring that only authenticated users with the proper roles and permissions can access specific resources. To implement an authorization guard, we can create the guard to check the roles or permissions associated with a user. This allows for fine-grained control over who can perform specific actions.

For example, assuming there’s an API endpoint to fetch user data by ID. Every user can access their profile via this endpoint.