ValidationPipe.ts

import { Pipe, PipeTransform, ArgumentMetadata, BadRequestException } from '@nestjs/common';
import { validate } from 'class-validator';
import { plainToClass } from 'class-transformer';

function isNil(obj: any): boolean {
  return typeof obj === 'undefined' || obj === null;
}

@Pipe()
export class ValidationPipe<T, V> implements PipeTransform<any> {
  public async transform(value: V, metadata: ArgumentMetadata): Promise<T> {
    const { metatype } = metadata;
    if (!metatype || !this.toValidate(metatype)) {
      throw new Error('Unable to validate type');
    }
    // this rates in the top 10 most annoying bugs I've ever had to debug
    if (typeof value === 'object' && !(value instanceof Object)) {
      value = Object.assign({}, value);
    }
    const entity = plainToClass<T, V>(metatype, value);
    const errors = await validate(entity);
    if (errors.length > 0) {
      throw new BadRequestException(errors);
    }
    return entity;
  }

  private toValidate(metatype: any): boolean {
    const types = [String, Boolean, Number, Array, Object];
    return !types.find((type) => metatype === type) && !isNil(metatype);
  }
}