Validation with `class-validator` & Decorators

Install

npm i class-validator class-transformer reflect-metadata
npm i -D @types/class-validator

Add import 'reflect-metadata'; at the entry point.

DTO with decorators

import { IsInt, IsString, Length, validate } from 'class-validator';

export class CreateUserDto {
  @IsString()
  @Length(2, 20)
  name!: string;

  @IsInt()
  age!: number;
}

async function validateInput(input: any) {
  const dto = Object.assign(new CreateUserDto(), input);
  const errors = await validate(dto);
  if (errors.length) throw new Error('Invalid');
  return dto;
}

Use this in an Express route to enforce runtime validation while keeping compile‑time types.