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

This event is not cancelable.

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

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

ontransitionrun = (event) => {};

A TransitionEvent. Inherits from Event.

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: ''.

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",
  );
};

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", () => {
  message.textContent = "transitionrun fired";
});

el.addEventListener("transitionstart", () => {
  message.textContent = "transitionstart fired";
});

el.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).

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.

• The TransitionEvent interface
• CSS properties: transition, transition-delay, transition-duration, transition-property, transition-timing-function
• Related events: transitionend, transitionstart, transitioncancel