test.js

const isElementInViewport = function (el) {
  let rect = el.getBoundingClientRect()
  return (
    rect.top >= 0 &&
    rect.left >= 0 &&
    rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
    rect.right <= (window.innerWidth || document.documentElement.clientWidth)
  )
}

const getLogoElement = function () {
  return document.getElementsByClassName('js-logo')[0]
}

const handleScroll = function () {
  const rocketLogo = getLogoElement()

  if (!isElementInViewport(rocketLogo)) {
    jQuery('.header').addClass('header--scrolled')
  } else {
    jQuery('.header').removeClass('header--scrolled')
  }
}