nestjs-guards-interceptors

# NestJS Guards and Interceptors

Master NestJS guards and interceptors for implementing authentication,
authorization, logging, and request/response transformation.

## Guards Fundamentals

Understanding CanActivate and ExecutionContext.

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

@Injectable()
export class BasicGuard implements CanActivate {
  canActivate(
    context: ExecutionContext,
  ): boolean | Promise<boolean> | Observable<boolean> {
    const request = context.switchToHttp().getRequest();
    return this.validateRequest(request);
  }

  private validateRequest(request: any): boolean {
    // Simple validation logic
    return !!request.headers.authorization;
  }
}

// ExecutionContext provides context about current request
@Injectable()
export class ContextAwareGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    // Get HTTP context
    const httpContext = context.switchToHttp();
    const request = httpContext.getRequest();
    const response = httpContext.getResponse();

    // Get handler and class information
    const handler = context.getHandler();
    const controller = context.getClass();

    console.log(`Handler: ${handler.name}`);
    console.log(`Controller: ${controller.name}`);

    return true;
  }
}

// Usage in controller
import { Controller, Get, UseGuards } from '@nestjs/common';

@Controller('users')
@UseGuards(BasicGuard)
export class UserController {
  @Get()
  findAll() {
    return [];
  }

  @Get('profile')
  @UseGuards(ContextAwareGuard)  // Method-level guard
  getProfile() {
    return { name: 'John' };
  }
}
```

## Authentication Guards

JWT, session, and API key authentication patterns.

```typescript
import { Injectable, UnauthorizedException } from '@nestjs/common';
import { JwtService } from '@nestjs/jwt';

@Injectable()
export class JwtAuthGuard implements CanActivate {
  constructor(private jwtService: Jw