slider.js

(function () {
  var mrg = 0;
  var id = 'slider';
  var klass = 'slider';
  var interval = 2000;

  var mainEl = document.getElementById(id)
  if (!mainEl) {
    console.error('Failed to find El#' + id);
    return;
  }
  var insideEl = mainEl.getElementsByClassName(klass)[0]
  if (!insideEl) {
    console.error('Failed to find El.' + klass + ' inside El#' + id);
    return;
  }

  var hdl = setInterval(function() {
    mrg++;
    var marginPx = (mrg % 5) * 1000
    insideEl.style = 'margin-left: -' + marginPx + 'px'
  }, interval);
})()