Detect Document Height Change

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

You can use an absolute positioned iframe with zero width inside the element you want to monitor for height changes, and listen to resize events on its contentWindow. For example:

HTML

<body>
  Your content...
  <iframe class="height-change-listener" tabindex="-1"></iframe>
</body>

CSS

body {
  position: relative;
}
.height-change-listener {
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  height: 100%;
  width: 0;
  border: 0;
  background-color: transparent;
}

JavaScript (using jQuery but could be adapted to pure JS)

$('.height-change-listener').each(function() {
  $(this.contentWindow).resize(function() {
    // Do something more useful
    console.log('doc height is ' + $(document).height());
  });
});

If for whatever reason you have height:100% set on body you'll need find (or add) another container element to implement this on. If you want to add the iframe dynamically you'll probably need to use the <iframe>.load event to attach the contentWindow.resize listener. If you want this to work in IE7 as well as browsers, you'll need to add the *zoom:1 hack to the container element and also listen to the 'proprietary' resize event on the <iframe> element itself (which will duplicate contentWindow.resize in IE8-10).

Here's a fiddle...

Just my two cents. If by any chance you're using angular then this would do the job:

$scope.$watch(function(){ 
 return document.height();
},function onHeightChange(newValue, oldValue){
 ...
});

Update: 2020

There is now a way to accomplish this using the new ResizeObserver. This allows you to listen to a whole list of elements for when their element changes size. The basic usage is fairly simple:

const observer = new ResizeObserver(entries => {
  for (const entry of entries) {
    // each entry is an instance of ResizeObserverEntry
    console.log(entry.contentRect.height)
  }
})
observer.observe(document.querySelector('body'))

The one downside is that currently there is only support for Chrome/Firefox, but you can find some solid polyfills out there. Here's a codepen example I wrote up:

https://codepen.io/justin-schroeder/pen/poJjGJQ?editors=1111

Detect Document Height Change

Update (Oct 2020):

Old answer:

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