Role-based authorization in NestJS - Step-by-Step Execution

Concept Flow - Role-based authorization

User sends request with JWT token

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Extract user roles from token

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Check required roles for route

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Compare user roles with required roles

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Allow access

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Send response

The system checks the user's roles from their token and compares them to the roles required by the route to decide access.

Execution Sample

NestJS

import { CanActivate, ExecutionContext } from '@nestjs/common';
import { Reflector } from '@nestjs/core';

export class RolesGuard implements CanActivate {
  constructor(private reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const requiredRoles = this.reflector.get<string[]>('roles', context.getHandler());
    if (!requiredRoles) return true;

    const { user } = context.switchToHttp().getRequest();
    return requiredRoles.some(role => user.roles?.includes(role));
  }
}

This guard checks if the user has any of the roles required by the route to allow access.

Execution Table

Step	Action	Data Extracted	Check	Result
1	Request received with JWT token	JWT token with roles ['user', 'admin']	N/A	Proceed
2	Extract roles from token	User roles: ['user', 'admin']	N/A	Proceed
3	Get required roles from route metadata	Required roles: ['admin']	N/A	Proceed
4	Compare user roles with required roles	User roles: ['user', 'admin']	Does user have 'admin'?	Yes
5	Allow access	N/A	N/A	Access granted
6	Send response	N/A	N/A	Response sent
7	If user lacked role	User roles: ['user']	Does user have 'admin'?	No
8	Deny access	N/A	N/A	403 Forbidden sent

💡 Execution stops after access is granted or denied based on role check.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
user.roles	undefined	['user', 'admin']	['user', 'admin']	['user', 'admin']	['user', 'admin']
requiredRoles	undefined	undefined	['admin']	['admin']	['admin']
accessAllowed	undefined	undefined	undefined	true	true

Key Moments - 3 Insights

Why does the guard allow access if no roles are required on the route?

If requiredRoles is undefined (no roles set), the guard returns true immediately (see execution_table step 3), meaning open access.

What happens if the user has multiple roles but none match the required roles?

The guard checks all user roles against required roles (step 4). If none match, it returns false and denies access (steps 7-8).

How does the guard get the user's roles?

The guard extracts the user object from the request (step 2) and reads the roles array from it.

Visual Quiz - 3 Questions

Look at the execution table, what roles does the user have at step 2?

A ['admin']

B ['user', 'admin']

C ['guest']

D undefined

💡 Hint

Check the 'Data Extracted' column at step 2 in the execution_table.

At which step does the guard decide to deny access?

A Step 3

B Step 5

C Step 8

D Step 1

💡 Hint

Look for the step where '403 Forbidden sent' appears in the 'Result' column.

If the route had no required roles, what would the guard return?

A true

B throws error

C false

D undefined

💡 Hint

See the guard's behavior when requiredRoles is undefined in execution_table step 3.

Concept Snapshot

Role-based authorization in NestJS:
- Use a RolesGuard implementing CanActivate.
- Extract required roles from route metadata.
- Extract user roles from request.
- Allow access if user roles include any required role.
- Deny access with 403 if no match.
- If no roles required, allow access by default.

Full Transcript

Role-based authorization in NestJS works by checking the user's roles against roles required by a route. When a request arrives, the guard extracts the user's roles from the JWT token in the request. It then reads the required roles set on the route handler. If no roles are required, access is allowed immediately. Otherwise, the guard compares the user's roles with the required roles. If the user has at least one matching role, access is granted. If not, the guard denies access by sending a 403 Forbidden response. This process ensures only users with proper roles can access protected routes.