A beautiful way to add NestJS decorator vibes for NextJS app API route to check for authorisation or anything else

authGuard.ts

import { NextRequest, NextResponse } from 'next/server';
import { checkAuth } from './checkAuth'; // define it by yourself

const httpMethods = ['GET', 'POST', 'PUT', 'DELETE'] as const;

type MethodMap<T> = { GET?: T; POST?: T; PUT?: T; DELETE?: T };

export default function authGuard(options?: MethodMap<boolean>) {
  return function AuthGuard(given: Function) {
    const ctr = given as Omit<Function, 'prototype'> & {
      prototype: MethodMap<(req: NextRequest) => NextResponse | Promise<NextResponse>>;
    };
    for (const method of httpMethods) {
      if (options && options?.[method] === false) {
        continue;
      }
      const originalMethod = ctr.prototype[method];
      if (originalMethod) {
        ctr.prototype[method] = function (req: NextRequest) {
          if (!(await checkAuth(req))) return new NextResponse('Unauthorised', { status: 401 });
          return originalMethod.call(this, req);
        };
      }
    }
  };
}

route.ts

// app/api/.../route.ts
import { NextRequest, NextResponse } from 'next/server';
import authGuard from './authGuard';

// define your class with HTTP methods and authGuard decorator
@authGuard({ GET: false }) // disable auth check for GET HTTP method
class Route {
  #hello = 'World';
  
  GET = (req: NextRequest) => {
    return NextResponse.json({ hello: 'World 1' });
  };
  
  POST = (req: NextRequest) => {
    return NextResponse.json({ hello: this.#hello });
  };
}

// export your HTTP methods
export const { GET, POST } = new Route();