Unveil() function rewritten without jQuery

unveil-func-vanilla.js

function unveil() {
  // create an array of images that are in view
  // by filtering the intial array
  var inview = images.filter(function(img) {
    // if the image is set to display: none
    if (img.style.display === "none") return;

    var rect = img.getBoundingClientRect(), 
        wt = window.scrollY, // window vertical scroll distance
        wb = wt + w.innerHeight, // last point of document visible in browser window
        et = wt + rect.top, // distance from document top to top of element
        eb = wt + rect.bottom; // distance from top of document to bottom of element

    // the bottom of the element is below the top of the browser (- threshold)
    // && the top of the element is above the bottom of the browser (+ threshold)
    return eb >= wt - th && et <= wb + th;
  });

  if (w.CustomEvent) {
    var unveilEvent = new CustomEvent('unveil');
  } else {
    var unveilEvent = document.createEvent('CustomEvent');
    unveilEvent.initCustomEvent('unveil', true, true);
  }

  inview.forEach(function(inviewImage){
    inviewImage.dispatchEvent(unveilEvent);
  });
  // alternative -- two arrays: https://stackoverflow.com/questions/11731072/dividing-an-array-by-filter-function
  // another possibility -- use getElementsByClassName which returns _live_ HTML Collection
  // https://developer.mozilla.org/en-US/docs/Web/API/Element/getElementsByClassName
  images = [].slice.call(document.querySelectorAll("img.unveil"));
}