HTML beforetoggle DOM Event: Reference and Examples

When someone interacts with a webpage, the browser sends events to the relevant JavaScript code. The beforetoggle event happens when a popover or details state is about to change.

When beforetoggle happens

A popover or details state is about to change.

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

<button type="button" popovertarget="demo-target">Toggle note</button>
<div id="demo-target" popover>Helpful information.</div>
<p id="event-status" role="status">Waiting for beforetoggle...</p>
<script>
    const eventTarget = document.getElementById("demo-target");
    const eventStatus = document.getElementById("event-status");

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

Reading the code

The browser listens for beforetoggle 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.

Click a popover trigger; this event exposes oldState and newState in supporting browsers.

eventDetails.oldState and eventDetails.newState describe the transition in supporting browsers.

Try it in your browser

Open the page, show the Console panel, and follow this test: Click a popover trigger; this event exposes oldState and newState in supporting browsers. Then change one thing in the example and observe whether the event still fires.

Common Beginner Mistakes

• Expecting beforetoggle to fire without its required user action or browser condition.
• Click a popover trigger; this event exposes oldState and newState in supporting browsers.
• Adding repeated event listeners without considering cleanup or unwanted side effects.

Key Takeaways

• beforetoggle reports a popover or details state is about to change.
• addEventListener("beforetoggle", ...) listens for the event on the appropriate target.
• Click a popover trigger; this event exposes oldState and newState in supporting browsers.