HTML transitionrun DOM Event: Reference and Examples

When someone interacts with a webpage, the browser sends events to the relevant JavaScript code. The transitionrun event happens when a CSS transition is created, including before a delay ends.

When transitionrun happens

A CSS transition is created, including before a delay ends.

You can listen for it using addEventListener(). That keeps event handling separate from HTML attributes such as onclick and works well with modern code organization.

A small working example

<style>
    .animated-box { opacity: 1; transition: opacity 1s; }
    .animated-box:hover { opacity: .3; }
</style>
<div id="demo-target" class="animated-box">Animate this box with CSS</div>
<p id="event-status" role="status">Waiting for transitionrun...</p>
<script>
    const eventTarget = document.getElementById("demo-target");
    const eventStatus = document.getElementById("event-status");

    if (eventTarget) {
        eventTarget.addEventListener("transitionrun", (eventDetails) => {
            eventStatus.textContent = "transitionrun event received";
            console.log(eventDetails.type, eventDetails.target);
        });
    }
</script>

Reading the code

The browser listens for transitionrun on the relevant target. When the event occurs, the callback receives eventDetails, updates the status message and logs the event name and target in Developer Tools.

Trigger a transition and inspect its starting phase.

The CSS rule above makes the animation or transition possible. Without a real animated property, a listener alone cannot make these events fire.

Try it in your browser

Open the page, show the Console panel, and follow this test: Trigger a transition and inspect its starting phase. Then change one thing in the example and observe whether the event still fires.

Common Beginner Mistakes

• Expecting transitionrun to fire without its required user action or browser condition.
• Trigger a transition and inspect its starting phase.
• Adding repeated event listeners without considering cleanup or unwanted side effects.

Key Takeaways

• transitionrun reports a CSS transition is created, including before a delay ends.
• addEventListener("transitionrun", ...) listens for the event on the appropriate target.
• Trigger a transition and inspect its starting phase.