Today we'll explore how to create a custom decorator in a NestJS project. Custom decorators are useful to simplify logic and make your code cleaner and more reusable.

What is a Decorator?

A decorator in NestJS is a special function that can be applied to classes, methods, or properties. It allows you to modify behaviors, add metadata, or apply reusable logic in an expressive and declarative way.

Step-by-Step Guide to Creating a Custom Decorator

Step 1: Create a Decorator Logic

In decorators/time.decorator.ts lets create a decorator, which will log the controller execution time onto console:

export function Time(): MethodDecorator {
  return (target: any, key: string | symbol, descriptor: PropertyDescriptor) => {
    const originalMethod = descriptor.value

    descriptor.value = function (...args: any[]) {
      console.time(key.toString())
      const result = originalMethod.apply(this, args)
      console.timeEnd(key.toString())

      return result
    }

    return descriptor
  }
}

Step 2: Apply Decorator on a Route

import { Time } from './decorators/time.decorator'

@Controller()
export class AppController {
  constructor(private readonly appService: AppService) {}

  @Get()
  @Time()
  getHello(): string {
    return this.appService.getHello()
  }
}