The ontransitioncancel property of the GlobalEventHandlers mixin is an event handler that processes transitioncancel events.

The transitioncancel event is sent when a CSS transition is cancelled. The transition is cancelled when:

• The value of the transition-property property that applies to the target is changed
• The display property is set to "none".
• The transition is stopped before it has run to completion, e.g. by moving the mouse off a hover-transitioning element.

A Function to be called when a transitioncancel event occurs indicating that a CSS transition has been cancelled on the target, where the target object is an HTML element (HTMLElement), document (Document), or window (Window). The function receives as input a single parameter: a TransitionEvent object describing the event which occurred; the event's TransitionEvent.elapsedTime property's value should be the same as the value of transition-duration.

In this example, we use the transitionrun and transitionend events to detect when the transition begins and ends, to cause a text update to occur during the transition. This could also be used to trigger animations or other effects, to allow chaining of reactions.

In addition, we also use a click event to make the box disappear (display: none;), showing how it triggers the transitioncancel event to fire.

This creates a <div> which we'll style with CSS below to make into a box that resizes and changes color and such.

<div class="box"></div>

The CSS below styles the box and applies a transition effect which makes the box's color and size change, and causes the box to rotate, while the mouse cursor hovers over it.

.box {
  margin-left: 70px;
  margin-top: 30px;
  border-style: solid;
  border-width: 1px;
  display: block;
  width: 100px;
  height: 100px;
  background-color: #0000FF;
  color: #FFFFFF;
  padding: 20px;
  font: bold 1.6em "Helvetica", "Arial", sans-serif;
  -webkit-transition: width 2s, height 2s, background-color 2s, -webkit-transform 2s, color 2s;
  transition: width 2s, height 2s, background-color 2s, transform 2s, color 2s;
}

.box:hover {
  background-color: #FFCCCC;
  color: #000000;
  width: 200px;
  height: 200px;
  -webkit-transform: rotate(180deg);
  transform: rotate(180deg);
}

Next, we need to establish our event handlers to change the text content of the box when the transition begins and ends.

let box = document.querySelector(".box");
box.ontransitionrun = function(event) {
  box.textContent = "Zooming...";
}
box.ontransitionend = function(event) {
  box.textContent = "Done!";
}

box.onclick = function() {
  box.style.display = 'none';
  timeout = window.setTimeout(appear, 2000);
  function appear() {
    box.style.display = 'block';
  }
}

box.ontransitioncancel = function(event) {
  console.log('transitioncancel fired after ' + event.elapsedTime + ' seconds.');
}

The resulting content looks like this:

Notice what happens when you hover your mouse cursor over the box, then move it away.

Also note the log that appears in the JavaScript console when you click the box, or move the cursor away before the transition has run to completion.

• The transitioncancel event this event handler is triggered by
• TransitionEvent
• The transitionrun event, which occurs when the transition begins