(Actual) smooth scrolling with Zepto.js - with easing

zepto.smoothScroll.js

function easeInOutQuart( t ) {
  const t1 = t - 1
  return t < 0.5 ? 8 * t * t * t * t : 1 - 8 * t1 * t1 * t1 * t1
}
function smoothScroll(el, to, duration) {
  var initial = $(window).scrollTop()
  var dest = to - initial
  var start = null
  function step (timestamp) {
    if (!start) start = timestamp
    var progress = timestamp - start
    var percentage = progress / duration
    var tick = (easeInOutQuart(percentage) * dest) + initial
    window.scrollTo(0, tick)
    if (progress < duration) { window.requestAnimationFrame(step) }
    else { return }
  }
  window.requestAnimationFrame(step)
}

$('.scrollTo').on('click', function(e) {
  e.preventDefault()
  smoothScroll($(window), $($(e.currentTarget).attr('href')).offset().top, 600)
})