Element: transitionstart event

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

This event is not cancelable.

Syntax

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

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

ontransitionstart = (event) => {};

Event type

A TransitionEvent. Inherits from Event.

Event properties

Also inherits properties from its parent Event.

TransitionEvent.propertyName Read only: A string containing the name CSS property associated with the transition.
TransitionEvent.elapsedTime Read only: A float giving the amount of time the transition has been running, in seconds, when this event fired. This value is not affected by the transition-delay property.
TransitionEvent.pseudoElement Read only: A string, starting with ::, containing the name of the pseudo-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 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:

html

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

css

.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", () => {
  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

Specification
CSS Transitions # transitionstart

Browser compatibility

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