Guards

Guards are classes that are decorated with the @Injectable() decorator and implement the CanActivate interface. A guard is responsible for determining if a request should be handled by a route handler or route. Guards are executed after every middleware, but before pipes. Unlike middleware, guards have access to the ExecutionContext object, so they know exactly what is going to evaluated.

In our blog application, we use the CheckLoggedInUserGuard in the UserController to only allow a user to access and access their own user information.

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';
import { Observable } from 'rxjs';

@Injectable()
export class CheckLoggedInUserGuard implements CanActivate {
    canActivate(
        context: ExecutionContext
    ): boolean | Promise<boolean> | Observable<boolean> {
        const req = context.switchToHttp().getRequest();
        return Number(req.params.userId) === req.user.id;
    ...