NestJS - Transformer for Query params to be transformed into array before validation of dto's (or undefined)

to-array.transformer.decorator.ts

import { Transform } from 'class-transformer';

export const ToArray = () => {
  const toPlain = Transform(
    ({value}) => {
      return value;
    },
    {
      toPlainOnly: true,
    }
  );
  const toClass = (target: any, key: string) => {
    return Transform(
      ({obj}) => {
        return valueToArray(obj[key]);
      },
      {
        toClassOnly: true,
      }
    )(target, key);
  };
  return function (target: any, key: string) {
    toPlain(target, key);
    toClass(target, key);
  };
};

const valueToArray = (value: any) => {
  if (value instanceof Array) return value;
  
  if (typeof value === 'string') {
    if (value === '') return [];
    
    return value.split(',');
  }
  
  return undefined;
};