The MediaSource interface of the Media Source Extensions API represents a source of media data for an HTMLMediaElement object. A MediaSource object can be attached to a HTMLMediaElement to be played in the user agent.

MediaSource()

Constructs and returns a new MediaSource object with no associated source buffers.

MediaSource.activeSourceBuffers Read only

Returns a SourceBufferList object containing a subset of the SourceBuffer objects contained within MediaSource.sourceBuffers — the list of objects providing the selected video track, enabled audio tracks, and shown/hidden text tracks.

MediaSource.duration

Gets and sets the duration of the current media being presented.

MediaSource.handle Read only Experimental

Inside a dedicated worker, returns a MediaSourceHandle object, a proxy for the MediaSource that can be transferred from the worker back to the main thread and attached to a media element via its HTMLMediaElement.srcObject property.

MediaSource.readyState Read only

Returns an enum representing the state of the current MediaSource, whether it is not currently attached to a media element (closed), attached and ready to receive SourceBuffer objects (open), or attached but the stream has been ended via MediaSource.endOfStream() (ended.)

MediaSource.sourceBuffers Read only

Returns a SourceBufferList object containing the list of SourceBuffer objects associated with this MediaSource.

MediaSource.canConstructInDedicatedWorker Read only Experimental

A boolean; returns true if MediaSource worker support is implemented, providing a low-latency feature detection mechanism.

Inherits methods from its parent interface, EventTarget.

MediaSource.addSourceBuffer()

Creates a new SourceBuffer of the given MIME type and adds it to the MediaSource.sourceBuffers list.

MediaSource.clearLiveSeekableRange()

Clears a seekable range previously set with a call to setLiveSeekableRange().

MediaSource.endOfStream()

Signals the end of the stream.

MediaSource.removeSourceBuffer()

Removes the given SourceBuffer from the MediaSource.sourceBuffers list.

MediaSource.setLiveSeekableRange()

Sets the range that the user can seek to in the media element.

MediaSource.isTypeSupported()

Returns a boolean value indicating if the given MIME type is supported by the current user agent — this is, if it can successfully create SourceBuffer objects for that MIME type.

sourceclose

Fired when the MediaSource instance is not attached to a media element anymore.

sourceended

Fired when the MediaSource instance is still attached to a media element, but endOfStream() has been called.

sourceopen

Fired when the MediaSource instance has been opened by a media element and is ready for data to be appended to the SourceBuffer objects in sourceBuffers.

The following simple example loads a video with XMLHttpRequest, playing it as soon as it can. This example was written by Nick Desaulniers and can be viewed live here (you can also download the source for further investigation). The function getMediaSource(), which is not defined here, returns a MediaSource.

const video = document.querySelector("video");

const assetURL = "frag_bunny.mp4";
// Need to be specific for Blink regarding codecs
// ./mp4info frag_bunny.mp4 | grep Codec
const mimeCodec = 'video/mp4; codecs="avc1.42E01E, mp4a.40.2"';
let mediaSource;

if ("MediaSource" in window && MediaSource.isTypeSupported(mimeCodec)) {
  mediaSource = getMediaSource();
  console.log(mediaSource.readyState); // closed
  video.src = URL.createObjectURL(mediaSource);
  mediaSource.addEventListener("sourceopen", sourceOpen);
} else {
  console.error("Unsupported MIME type or codec: ", mimeCodec);
}

function sourceOpen() {
  console.log(this.readyState); // open
  const sourceBuffer = mediaSource.addSourceBuffer(mimeCodec);
  fetchAB(assetURL, (buf) => {
    sourceBuffer.addEventListener("updateend", () => {
      mediaSource.endOfStream();
      video.play();
      console.log(mediaSource.readyState); // ended
    });
    sourceBuffer.appendBuffer(buf);
  });
}

function fetchAB(url, cb) {
  console.log(url);
  const xhr = new XMLHttpRequest();
  xhr.open("get", url);
  xhr.responseType = "arraybuffer";
  xhr.onload = () => {
    cb(xhr.response);
  };
  xhr.send();
}

The handle property can be accessed inside a dedicated worker and the resulting MediaSourceHandle object is then transferred over to the thread that created the worker (in this case the main thread) via a postMessage() call:

// Inside dedicated worker
let mediaSource = new MediaSource();
let handle = mediaSource.handle;
// Transfer the handle to the context that created the worker
postMessage({ arg: handle }, [handle]);

mediaSource.addEventListener("sourceopen", () => {
  // Await sourceopen on MediaSource before creating SourceBuffers
  // and populating them with fetched media — MediaSource won't
  // accept creation of SourceBuffers until it is attached to the
  // HTMLMediaElement and its readyState is "open"
});

Over in the main thread, we receive the handle via a message event handler, attach it to a <video> via its HTMLMediaElement.srcObject property, and play the video:

worker.addEventListener("message", (msg) => {
  let mediaSourceHandle = msg.data.arg;
  video.srcObject = mediaSourceHandle;
  video.play();
});

• SourceBuffer
• SourceBufferList