References

Beginner-friendly references for web development, with live, editable examples.

The HTML onloadstart event

Event All modern browsers Updated
Quick answer

The HTML onloadstart attribute runs JavaScript when the browser starts loading a resource. It is an inline handler for the loadstart event; in modern code prefer addEventListener('loadstart', …).

Overview

The onloadstart event attribute runs JavaScript when resource loading starts. In JavaScript the event itself is named loadstart — drop the on prefix when you call addEventListener.

It is one of the media events, fired by <audio> and <video> elements as their loading and playback state changes. These events drive custom players — progress bars, buffering spinners, play/pause UI.

You can wire this up with the inline onloadstart HTML attribute, but the modern, recommended approach is element.addEventListener('loadstart', handler) in JavaScript. That keeps behavior out of your markup, lets you attach several handlers to the same event, and makes them easy to remove. The inline attribute is fine for quick demos.

Syntax

<element onloadstart="handler()">…</element>

element.addEventListener('loadstart', handler);

Best practices

  • Prefer element.addEventListener('loadstart', handler) over the inline onloadstart attribute — it separates behavior from markup and allows multiple handlers.
  • Attach media events to the <video>/<audio> element to build custom player UI.
  • Read the element's state (currentTime, duration, buffered) inside the handler.
  • Still provide native controls and captions for accessibility.

Frequently asked questions

What is the onloadstart event?
It runs JavaScript when resource loading starts. In JavaScript the event is named loadstart.
Which elements fire this event?
Media elements — <audio> and <video> — as their playback or loading state changes.
How do I build a custom video player?
Listen to the media events and read/set properties like currentTime and paused on the element to drive your own controls.
Should I use the onloadstart attribute or addEventListener?
Prefer addEventListener('loadstart', …) in JavaScript. The inline onloadstart attribute works but mixes behavior into the markup and allows only one handler per element.
onloadedmetadata onlostpointercapture