function arrayDifference(firstArray: number[], secondArray: number[]): number[] {
  const notRepeat = [];
  firstArray.forEach( (element) => {
    const isRepeat = secondArray.filter( (num) => num === element );
    if (isRepeat.length === 0){
      notRepeat.some( (num) => num === element) ? '' : notRepeat.push(element);
    } 
  });
  return notRepeat;
}

// No modificar: necesario para evaluar el resultado.
export { arrayDifference };