MediaSource.isTypeSupported()

The MediaSource.isTypeSupported() static method returns a boolean value which is true if the given MIME type is likely to be supported by the current user agent.

That is, if it can successfully create SourceBuffer objects for that MIME type. If the returned value is false, then the user agent is certain that it cannot access media of the specified format.

mimeType

The MIME media type that you want to test support for in the current browser. This may include the codecs parameter to provide added details about the codecs used within the file.

A boolean value which is true if the browser feels that it can probably play media of the specified type. This is not a guarantee, however, and your code must be prepared for the possibility that the media will not play correctly if at all. A value of false is a guarantee that media of the given type will not play, however.

All web APIs that work with media files use a "no/maybe/probably" approach (or, in this case, "no or probably") when determining if a media type can be used. This is because media files are complex, intricate constructs with far too many subtle variations to be absolutely certain of anything until you actually use the contents of the media.

The following snippet is from an example written by Nick Desaulniers (view the full demo live, or download the source for further investigation.)

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

if ('MediaSource' in window && MediaSource.isTypeSupported(mimeCodec)) {
  var mediaSource = new MediaSource;
  //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
  var mediaSource = this;
  var sourceBuffer = mediaSource.addSourceBuffer(mimeCodec);
  fetchAB(assetURL, function (buf) {
    sourceBuffer.addEventListener('updateend', function (_) {
      mediaSource.endOfStream();
      video.play();
      //console.log(mediaSource.readyState); // ended
    });
    sourceBuffer.appendBuffer(buf);
  });
};

• Media Source Extensions API
• Guide to media types and formats on the web
• The "codecs" parameter in common media types
• SourceBuffer
• SourceBufferList