index.js

var tracking = [];
const throttle = require('lodash.throttle');
const defaults = require('lodash.defaults');
const defaultOptions = {top: 0, bottom: 0, left: 0, right: 0};

const _onScroll = throttle(() => window.requestAnimationFrame(checkForVisibleElements), 100, {leading: false});

function getTracking() {
  return tracking;
}

function isVisible(elm, options) {
  let rect = elm.getBoundingClientRect();

  return rect.bottom - options.bottom > 0 &&
    rect.right - options.right > 0 &&
    rect.left + options.left < (window.innerWidth || document.documentElement.clientWidth) &&
    rect.top + options.top < (window.innerHeight || document.documentElement.clientHeight);
}

function _handleVisible(elm, fn, options) {
  untrack(elm);
  fn(elm);
}

function _trackNewElement(elm, fn, options) {
  if (isVisible(elm, options)) {
    return _handleVisible(elm, fn, options);
  }
  tracking.push({elm: elm, fn: fn, options: options});
}

function checkForVisibleElements() {
  Array.from(tracking).forEach((v) => {
    if (isVisible(v.elm, v.options)) {
      _handleVisible(v.elm, v.fn, v.options);
    }
  });

  if (tracking.length === 0) {
    untrackAll();
  }
}

function track(elm, fn, options) {
  if (elm.length) {
    elm = Array.from(elm);
  }

  let elements = [].concat(elm);
  if (typeof fn !== 'function') {
    throw new Error('You must pass a callback function');
  }

  options = defaults(options, defaultOptions);

  window.requestAnimationFrame(() => {
    elements.forEach(element => {
      _trackNewElement(element, fn, options);

      if (tracking.length === 1) {
        window.addEventListener('scroll', _onScroll);
      }
    });
  });

  return elements;
}

function untrackAll() {
  tracking = [];
  window.removeEventListener('scroll', _onScroll);
}

function untrack(elm) {
  let elmIndex = -1;

  tracking.some((v, i) => {
    if (v.elm == elm) {
      elmIndex = i;
      return true;
    }
  });

  if (elmIndex !== -1) {
    tracking.splice(elmIndex, 1);
  }
}

export default {track, untrackAll, untrack, checkForVisibleElements, getTracking};