onload.js

const tracking = new WeakMap()
const clz = 'onload-' + Math.random().toString(36).slice(2)

const observer = new MutationObserver(function (mutations) {
  for (let j = 0; j < mutations.length; j++) {
    const { addedNodes, removedNodes, target } = mutations[j]

    for (let i = 0; i < addedNodes.length; i++) {
      if (!addedNodes[i].classList) continue
      if (addedNodes[i].classList.contains(clz)) tracking.get(addedNodes[i]).onload(target)
      const els = addedNodes[i].getElementsByClassName(clz)
      for (let k = 0; k < els.length; k++) tracking.get(els[k]).onload(target)
    }

    for (let i = 0; i < removedNodes.length; i++) {
      if (!removedNodes[i].classList) continue
      if (removedNodes[i].classList.contains(clz)) tracking.get(removedNodes[i]).offload(target)
      const els = removedNodes[i].getElementsByClassName(clz)
      for (let k = 0; k < els.length; k++) tracking.get(els[k]).offload(target)
    }
  }
})

observer.observe(document, {
  childList: true,
  subtree: true,
})

module.exports = function (node, onload, offload) {
  node.classList.add(clz)
  tracking.set(node, { onload, offload })
}