How to list properties of a Nestjs DTO class?

I have following Nestjs DTO class:

// create-job-offer.dto.ts
import { IsOptional, IsNumber } from 'class-validator';

export class CreateJobOfferDto {
  @IsNumber()
  @IsOptional()
  mentorId: number;

  @IsNumber()
  @IsOptional()
  companyId: number;
}

I want to obtain the list of class properties: ['mentorId', 'companyId'].

I tried so far in a controller without success following methods:

Object.getOwnPropertyNames(new CreateJobOfferDto());
Object.getOwnPropertyNames(CreateJobOfferDto);

Object.getOwnPropertySymbols(new CreateJobOfferDto());
Object.getOwnPropertySymbols(CreateJobOfferDto);

Object.getOwnPropertyDescriptors(CreateJobOfferDto);
Object.getOwnPropertyDescriptors(new CreateJobOfferDto());

Object.getPrototypeOf(CreateJobOfferDto);
Object.getPrototypeOf(new CreateJobOfferDto());

If I add a method, or vars in a constructor, I can get them, but not the properties.

The reason why I want to achieve this is, I am using Prisma and React, and in my React app I want to receive the list of class properties so that I can generate a model form dynamically.

Solution

There is no easy direct way to get the list of properties of a DTO class, or more comprehensively, any type of class that has only properties.

The reason is that whenever you defined a property without any values, it becomes disappears after compiling to javascript.

Example:

// typescript
class A {
    private readonly property1: string;
    public readonly property2: boolean;
}

it compiles to this:

// javascript
"use strict";
class A {}

In order to achieve this goal, you need to write a custom decorator. like this:

const properties = Symbol('properties');

// This decorator will be called for each property, and it stores the property name in an object.
export const Property = () => {
  return (obj: any, propertyName: string) => {
    (obj[properties] || (obj[properties] = [])).push(propertyName);
  };
};

// This is a function to retrieve the list of properties for a class
export function getProperties(obj: any): [] {
  return obj.prototype[properties];
}

Imagine I have a DTO class for a user, like this:

import { getProperties } from './decorators/property.decorator';

export class UserDto {
  @Property()
  @IsNotEmpty()
  firstName: string;

  @Property()
  @IsEmail()
  @IsOptional()
  public readonly email: string;
}

and I want to get all properties in a list, so we need to call the getProperties method which we defined earlier, so:

import { UserDto } from './dtos/user.dto';

getProperties(UserDto); // [ 'firstName', 'email' ]

You can also use an npm module ts-transformer-keys.

A TypeScript custom transformer which enables to obtain keys of given type.

How to use:

import { keys } from 'ts-transformer-keys';

interface Props {
  id: string;
  name: string;
  age: number;
}
const keysOfProps = keys<Props>();

console.log(keysOfProps); // ['id', 'name', 'age']