HTML loadstart DOM Event: Reference and Examples

When someone interacts with a webpage, the browser sends events to the relevant JavaScript code. The loadstart event happens when the browser starts looking for media data.

When loadstart happens

The browser starts looking for media data.

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

<video id="demo-target" controls preload="metadata" width="320">
    <source src="sample-video.mp4" type="video/mp4">
</video>
<p id="event-status" role="status">Waiting for loadstart...</p>
<script>
    const eventTarget = document.getElementById("demo-target");
    const eventStatus = document.getElementById("event-status");

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

Reading the code

The browser listens for loadstart 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.

Assign a media source and let the browser start fetching it.

To test this example, put a real supported media file at the shown path. Events tied to buffering or download behavior may not fire in the same order on every connection. An unavailable sample file is not proof that your event listener is broken.

Try it in your browser

Open the page, show the Console panel, and follow this test: Assign a media source and let the browser start fetching it. Then change one thing in the example and observe whether the event still fires.

Common Beginner Mistakes

• Expecting loadstart to fire without its required user action or browser condition.
• Assign a media source and let the browser start fetching it.
• Adding repeated event listeners without considering cleanup or unwanted side effects.

Key Takeaways

• loadstart reports the browser starts looking for media data.
• addEventListener("loadstart", ...) listens for the event on the appropriate target.
• Assign a media source and let the browser start fetching it.