const surname = document.getElementById('contact_surname'); const cartnext = document.querySelector('input.re-button.re-step-next.re-button-blue') const cartnext = document.getElementsByName('cartnext'); console.log(cartnext); surname.setAttribute('required', ''); surname.classList.toggle('need'); console.log(surname); for (let i = 0; i < cartnext.length; i++) { cartnext[i].addEventListener('click', () => { if(surname.value === '') { surname.classList.add('error'); } }); }