HTMLElement: transitionstart event

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

The transitionstart event is fired when a CSS transition has actually started, i.e., after any transition-delay has ended.

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

Examples

This code adds a listener to the transitionstart event:

element.addEventListener('transitionstart', () => {
  console.log('Started transitioning');
});

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

element.ontransitionstart = () => {
  console.log('Started transitioning');
};

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

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

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

transition.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).

Specifications

Specification
CSS Transitions Level 2 # transitionstart

Browser compatibility

BCD tables only load in the browser