slider.js

const items = document.querySelectorAll('.img');
const itemCount = items.length;
const nextItem = document.querySelector('.next');
const previousItem = document.querySelector('.previous');
let count = 0;

function showNextItem() {
  items[count].classList.remove('active');

  if(count < itemCount - 1) {
    count++;
  } else {
    count = 0;
  }

  items[count].classList.add('active');
  console.log(count);
  document.getElementById('output').innerHTML = count;
}

function showPreviousItem() {
  items[count].classList.remove('active');

  if(count > 0) {
    count--;
  } else {
    count = itemCount - 1;
  }

  items[count].classList.add('active');
  console.log(count);
  document.getElementById('output').innerHTML = count;
}
function keyPress(e) {
  e = e || window.event;
  
  if (e.keyCode == '37') {
    showPreviousItem();
    
  } else if (e.keyCode == '39') {
    showNextItem();
  }
}

nextItem.addEventListener('click', showNextItem);
previousItem.addEventListener('click', showPreviousItem);
document.addEventListener('keydown', keyPress);