track elements on screen

gistfile1.txt

/* global ga */
function throttle(callback, limit) {
  let wait = false;

  return function() {
    if (!wait) {
      callback.call();
      wait = true;

      setTimeout(function() {
        wait = false;
      }, limit);
    }
  }
}

function isElementOnScreen(elementId) {
  const windowHeight = window.innerHeight;
  const windowPos = document.body.scrollTop;
  const el = document.getElementById(elementId);

  return el.offsetTop < windowHeight + windowPos;
}

let trackedOffers = false;
let trackedForm = false;

const trackElements = throttle(function() {
  if (isElementOnScreen('offers') && !trackedOffers) {
    trackedOffers = true;
    ga('send', 'event', 'Scroll', 'Page viewed', 'Ofertas');
  }

  if (isElementOnScreen('contact-form') && !trackedForm) {
    trackedForm = true;
    ga('send', 'event', 'Scroll', 'Page viewed', 'Form');
  }

  if (trackedOffers && trackedForm) {
    window.removeEventListener('scroll', trackElements);
  }
}, 100);

window.addEventListener('scroll', trackElements);