strongly-typed decorators: infer return type of transform prop

[elijaholmos]

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

The strongly-typed decorator factories introduced in #12237 are really cool. Thanks for adding them.

I noticed a small problem that comes up with a slightly advanced usage. Currently, the return type of Refector.get(ReflectableDecorator<T>) is typed as whatever ReflectableDecorator<T> was initialized as, using Reflector.createDecorator. This is great for simple metadata storage, but if the transform method is used to alter the input value before it is stored, the return type does not update to match the transform method's return type.

As an example:

// roles.decorator.ts
import { Reflector } from '@nestjs/core';

export const Roles = Reflector.createDecorator<string | string[]>({
  key: 'roles',
  transform: (...roles: string[]): string[] => roles ?? [],
});

// decorator usage
import { Roles } from './roles.decorator';
@Roles('admin')

// roles.guard.ts
import { CanActivate, ExecutionContext, Injectable } from '@nestjs/common';
import { Reflector } from '@nestjs/core';
import { Roles } from './roles.decorator';

@Injectable()
export class PolicyGuard implements CanActivate {
  constructor(protected readonly reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const roles = this.reflector.get(Roles, context.getHandler()) || [];

    return roles.some((role) => role === 'admin'); // TypeError: Property 'some' does not exist on type 'string | string[]'.
  }
}

Minimum reproduction code

https://stackblitz.com/edit/nestjs-typescript-starter-yoxyxj?file=src%2Froles.guard.ts

Steps to reproduce

No response

Expected behavior

Ideally, the typing system would be able to detect the return value on the transform property, and use that as the return value of the Refector.get(ReflectableDecorator<T>) method.

Package

• I don't know. Or some 3rd-party package
• @nestjs/common
• @nestjs/core
• @nestjs/microservices
• @nestjs/platform-express
• @nestjs/platform-fastify
• @nestjs/platform-socket.io
• @nestjs/platform-ws
• @nestjs/testing
• @nestjs/websockets
• Other (see below)

Other package

No response

NestJS version

10.2.1

Packages versions

[System Information]
OS Version : Windows 10
NodeJS Version : v18.16.0
PNPM Version : 8.6.12

[Nest CLI]
Nest CLI Version : 10.1.12

[Nest Platform Information]
platform-express version : 10.2.1
mapped-types version : 2.0.2
schematics version : 10.0.2
passport version : 10.0.1
graphql version : 12.0.8
testing version : 10.2.1
apollo version : 12.0.7
common version : 10.2.1
config version : 3.0.0
core version : 10.2.1
jwt version : 10.1.0
cli version : 10.1.12

Node.js version

18.16.0

In which operating systems have you tested?

• macOS
• Windows
• Linux

Other

No response

[micalevisk]

another thing that we could improve (probably not feasible):

transform: (...roles: string[]): string[] => roles ?? []

it should error out at compiler time because of
Reflector.createDecorator<string | string[]>

that roles parameter will be string[][] when using @Roles(['foo']). And it will be string[] when using @Roles('foo')

[quangtran88]

I think we can add a second optional generic param on Reflector.createDecorator to describe the transformed value type as following:

const Roles = Reflector.createDecorator<string | string[], string[]>({
  key: 'roles',
  transform: (...roles: string[]): string[] => roles ?? [], // Must return type string[]
});


const roles = this.reflector.get(Roles, context.getHandler()) // Will resolve as type string[]

[micalevisk]

yeah, that should work. I just made a PoC

[quangtran88]

I created a PR #12276 for this.

[kamilmysliwiec]

Let's track this here #12276