microtask.js

const microtask = () => {
  let node = document.createTextNode(''),
    queue = [],
    i = 0

  new window.MutationObserver(() => {
      while (queue.length) queue.shift()()
  }).observe(node, { characterData: true })

  return fn => {
    queue.push(fn)
    node.data = (i = 1 - i)
  }
}