slider.js

function onHold(el, cb) {
    let interval;
    
    el.addEventListener('mousedown', () => interval = setInterval(() => requestAnimationFrame(cb), 10));
    
    el.addEventListener('mouseup', () => clearInterval(interval));
    
    el.addEventListener('mouseleave', () => clearInterval(interval));
}

function slider({
  target,
  rightButton,
  leftButton,
}) {
  function scrollToRight() {
     container.scrollLeft += 10; 
  }

  function scrollToLeft() {
    container.scrollLeft -= 10;
  }
  
  onHold(buttonRight, scrollToRight);

  onHold(buttonLeft, scrollToLeft);
}