delegate_decorators.ts

interface User {
  id: string;
  name: string;
  roles: string[];
}

function withRoles(roles: string[]) {
  return function (_target: Selector, _propertyName: string, propertyDescriptor: PropertyDescriptor) {
    let originalMethod = propertyDescriptor.value!;
    propertyDescriptor.value = function () {
      const ctx = this as Selector;
      console.log('Checking roles...');
      if(roles.some(role => ctx.user.roles.includes(role))) {
        return originalMethod.apply(this, arguments);
      }
      throw new Error('Unauthorized', {
        cause: `User does not have the required roles. ${roles.join(', ')} required. Was ${ctx.user.roles.join(', ')}`,
      });
    }
  }
}

class Selector {
  user: User
  constructor(user: User) {
    this.user = user;
  }
  select() {
    return `SELECT * FROM "appointments"`;
  }
}

class AdminSelector extends Selector {
  @withRoles(['admin'])
  select() {
    return super.select();
  }
}

class UserSelector extends Selector {
  @withRoles(['user'])
  select() {
    return super.select() + ` WHERE "user_id" = ${this.user.id}`;
  }
}

class SelectorDelegate {
  private selector: Selector;
  constructor(user: User) {
    switch (true) {
      case user.roles.includes('admin'):
        this.selector = new AdminSelector(user);
        break;
      case user.roles.includes('user'):
        this.selector = new UserSelector(user);
        break;
      default:
        throw new Error('Unauthorized', {
          cause: 'User does not have any roles assigned'
        });
    }
  }
  select() {
    return this.selector.select();
  }
}

const selector = new SelectorDelegate({ id: '1', name: 'John Doe', roles: ['user'] });

try {
  console.log(selector.select());
} catch (e: unknown) {
  const error = e as Error;
  console.error(error.message, error.cause);
}

remb to put "experimentalDecorators": true to your compilerOptions of tsconfig.json