Question

I’m trying to detect when my document height changes. Once it does, I need to run a few functions to help organize my page layout.

I’m not looking for window.onresize. I need the entire document, which is larger than the window.

How do I observe this change?

Answer 1

Update (Oct 2020):

resizeObserver is a wonderful API (support table)

// 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

---

Old answer:

Although a "hack", this simple function continuously "listens" (through setTimeout) to changes in an element’s height and fire a callback when a change was detected.

It’s important to take into account an element’s height might change regardless of any action taken by a user (resize, click, etc.) and so, since it is impossible to know what can cause a height change, all that can be done to absolutely guarantee 100% detection is to place an interval height checker :

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'
})

LIVE DEMO