[解決済み] ドキュメントの高さ変更の検出

// create an Observer instance
const resizeObserver = new ResizeObserver(entries => 
  console.log('Body height changed:', entries[0].target.clientHeight)
)

// start observing a DOM node
resizeObserver.observe(document.body)

// click anywhere to rnadomize height
window.addEventListener('click', () =>
  document.body.style.height = Math.floor((Math.random() * 5000) + 1) + 'px'
)

click anywhere to change the height

function onElementHeightChange(elm, callback) {
  var lastHeight = elm.clientHeight, newHeight;

  (function run() {
    newHeight = elm.clientHeight;
    if (lastHeight != newHeight)
      callback(newHeight)
    lastHeight = newHeight

    if (elm.onElementHeightChangeTimer)
      clearTimeout(elm.onElementHeightChangeTimer)

    elm.onElementHeightChangeTimer = setTimeout(run, 200)
  })()
}

// to clear the timer use:
// clearTimeout(document.body.onElementHeightChangeTimer);

// DEMO:
document.write("click anywhere to change the height")

onElementHeightChange(document.body, function(h) {
  console.log('Body height changed:', h)
})

window.addEventListener('click', function() {
  document.body.style.height = Math.floor((Math.random() * 5000) + 1) + 'px'
})