Thetransitionstart event is fired when aCSS transition has actually started, i.e., after anytransition-delay has ended.

This event is not cancelable.

Syntax

Use the event name in methods likeaddEventListener(), or set an event handler property.

addEventListener("transitionstart", (event) => { })ontransitionstart = (event) => { }

Event type

ATransitionEvent. Inherits fromEvent.

Event properties

Also inherits properties from its parentEvent.

TransitionEvent.propertyNameRead only

A string containing the name CSS property associated with the transition.

TransitionEvent.elapsedTimeRead only

Afloat giving the amount of time the transition has been running, in seconds, when this event fired. This value is not affected by thetransition-delay property.

TransitionEvent.pseudoElementRead only

A string, starting with::, containing the name of thepseudo-element the animation runs on. If the transition doesn't run on a pseudo-element but on the element, an empty string:''.

Examples

This code adds a listener to thetransitionstart event:

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

The same, but using theontransitionstart property instead ofaddEventListener():

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>Hover over me</div><div></div>

.transition {  width: 100px;  height: 100px;  background: red;  transition-property: transform, background;  transition-duration: 2s;  transition-delay: 1s;}.transition:hover {  transform: rotate(90deg);  background: transparent;}

To this, we'll add some JavaScript to indicate where thetransitionstart andtransitionrun events fire.

const transition = document.querySelector(".transition");const message = document.querySelector(".message");transition.addEventListener("transitionrun", () => {  message.textContent = "transitionrun fired";});transition.addEventListener("transitionstart", () => {  message.textContent = "transitionstart fired";});transition.addEventListener("transitionend", () => {  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

Browser compatibility

See also

• TheTransitionEvent interface
• CSS properties:transition,transition-delay,transition-duration,transition-property,transition-timing-function
• Related events:transitionend,transitionrun,transitioncancel