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.

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

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

ontransitionstart = (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 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");
};

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", () => {
  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).

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