HTMLElement: transitionrun event

Experimental: This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The transitionrun event is fired when a CSS transition is first created, i.e. before any transition-delay has begun.

Bubbles	Yes
Cancelable	No
Interface	`TransitionEvent`
Event handler property	`ontransitionrun`

Examples

This code adds a listener to the transitionrun event:

el.addEventListener('transitionrun', () => {
  console.log('Transition is running but hasn\'t necessarily started transitioning yet');
});

The same, but using the ontransitionrun property instead of addEventListener():

el.ontransitionrun = () => {
  console.log('Transition started running, and will start transitioning when the transition delay has expired');
};

Live example

In the following example, we have a simple <div> element, styled with a transition that includes a delay:

<div class="transition">Hover over me</div>
<div class="message"></div>

.transition {
  width: 100px;
  height: 100px;
  background: rgba(255,0,0,1);
  transition-property: transform, background;
  transition-duration: 2s;
  transition-delay: 1s;
}

.transition:hover {
  transform: rotate(90deg);
  background: rgba(255,0,0,0);
}

To this, we'll add some JavaScript to indicate where the transitionstart and transitionrun events fire.

const el = document.querySelector('.transition');
const message = document.querySelector('.message');

el.addEventListener('transitionrun', function() {
  message.textContent = 'transitionrun fired';
});

el.addEventListener('transitionstart', function() {
  message.textContent = 'transitionstart fired';
});

el.addEventListener('transitionend', function() {
  message.textContent = 'transitionend fired';
});

The difference is that:

transitionrun fires when the transition is created (i.e. at the start of any delay).
transitionstart fires when the actual animation has begun (i.e. at the end of any delay).

The transitionrun will occur even if the transition is canceled before the delay expires. If there is no transition delay or if transition-delay is negative, both transitionrun and transitionstart are fired.

Specifications

Specification
CSS Transitions Level 2 # transitionrun

Browser compatibility

BCD tables only load in the browser