Comparing scroll position to element in scroll handler

scroll-into-view-scroll-handler.js

import throttle from 'lodash.throttle'

function handleScroll () {
  // get the updated scroll position
  const yOffset = window.pageYOffset
  // compare to the cached pixel value when we want to reveal the element
  if (yOffset > this.scrollIntoViewThreshold) {
    // now reveal the element!
  }
}

// ensure we don't fire this handler too often
// for a good intro into throttling and debouncing, see:
// https://css-tricks.com/debouncing-throttling-explained-examples/
const throttledScrollHandler = throttle(handleScroll, 100)
// now re-check on scroll
window.addEventListener('scroll', throttledScrollHandler)