Detect scroll down

listener.js

const listeners = { key: 0 }

const handler = (callback) => {
  // Init
  let y = 0;
  let toBottom = false;
  return () => {
    const scrollY = window.scrollY
    if (y < scrollY && !toBottom) {
        toBottom = true;
        callback(toBottom);
    }
    if (y > scrollY && toBottom) {
        toBottom = false;
        callback(toBottom);
    }
    y = window.scrollY;
  }
}

const addListener = callback => {
  const fn = handler(callback);
  const key = listeners.key;
  listeners[key] = fn;
  listeners.key++;
  window.addEventListener('scroll', fn);
  return key;
}

const removeListener = key => {
  const fn = listeners[key];
  window.removeEventListener('scroll', fn);
  delete listeners[key];
}