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HTML

              
                <h1>Detect Transition Start</h1>
<p>There is no method for detecting a css transition start in JavaScript. This demo uses the transitionEnd event in a sneaky way to detect transition start.</p>
<p>It requires wrapping the transitionEnd event with a custom event 'transition-end'. This alone is useful for dealing with browser prefixes.</p>
<p>But this solution is not yet ideal. It also requires some specific css with an invisible 0.00001s transition to detect the start. It doesn't matter what property you transition but it must change. The search for a better solution continues...<p>
<a id="hover-me">Hover me</a>
<div id="transition-status"></div>
<p>This could be useful in cases similar to how the window resize event is used. If the height or width of a container is changing with css transitions, you may wish to update something during the transition. Not just when it ends.<p>
              
            
!

CSS

              
                #hover-me{
    transition: opacity 0.00001s linear, color 3s linear; /* must be 0.00001s. 0s did not work :( */
    background: #CCC;
    padding: 10px;
    border-radius: 10px;
    border: solid 1px;
    display: inline-block;    
}
#hover-me:hover{
  opacity: .999999; /* Doesn't matter what property but it must change */
  color: red;
  cursor: pointer;
}

#transition-status{
  margin: 1em 0;
}
              
            
!

JS

              
                
// Detect which transition event (from modernizer)
var whichTransitionEvent = function(){
  var t;
  var el = document.createElement('fakeelement');
  var transitions = {
	'transition':'transitionend',
	'OTransition':'oTransitionEnd',
	'MozTransition':'transitionend',
	'WebkitTransition':'webkitTransitionEnd'
  };

  for(t in transitions){
	if( el.style[t] !== undefined ){
	  return transitions[t];
	}
  }
};

var transitionEvent = whichTransitionEvent();

if(transitionEvent){
	// Set-up transition-start custom event
	if (window.CustomEvent) {
	  var transitionStartEvent = new CustomEvent('transition-start',{'bubbles': true, 'cancelable': true});
	  var transitionEndEvent = new CustomEvent('transition-end',{'bubbles': true, 'cancelable': true});
	} else {
	  var transitionStartEvent = document.createEvent('CustomEvent');
	  var transitionEndEvent = document.createEvent('CustomEvent');
	  transitionStartEvent.initCustomEvent('transition-start', true, true);
	  transitionEndEvent.initCustomEvent('transition-end', true, true);
	}
	document.body.addEventListener(transitionEvent, function(evnt) {
		// Guard detect and trigger transition start here
    var target = evnt.target || evnt.srcElement;
    console.log(evnt.elapsedTime);
    // FF does not report the exact time here.
		if(evnt.elapsedTime <= 0.00001){
			evnt.target.dispatchEvent(transitionStartEvent);
		} else {
			evnt.target.dispatchEvent(transitionEndEvent);
		}
	});
}

var statusElm = document.getElementById("transition-status");
var hoverElm = document.getElementById("hover-me");

hoverElm.addEventListener('transition-start', function() {
	var d = new Date();
	statusElm.innerHTML = d.getHours() +':'+ d.getMinutes() +':'+ d.getSeconds() +':'+ d.getMilliseconds() +' Transition has started.';
});

hoverElm.addEventListener('transition-end', function() {
	var d = new Date();
    statusElm.innerHTML = d.getHours() +':'+ d.getMinutes() +':'+ d.getSeconds() +':'+ d.getMilliseconds() +' Transition has ended.';
});
              
            
!
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