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.

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: 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 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 Module Level 1 # transitionstart

Browser compatibility

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This page was last modified on Jul 28, 2026 by MDN contributors.