问MediaRecorder不生成有效的WebM文件。

EN

const wantMimeType = 'video/webm;codecs=h264,opus';
if (MediaRecorder.isTypeSupported(wantMimeType)) {
  let mediaRecorder = new MediaRecorder(stream, {
    mimeType: wantMimeType,
  });
  // ...drive the recorder
  mediaRecorder.onstop = (event) => {
    try {
      // returns 'video/x-matroska;codecs=avc1,opus' in Chrome,
      // and the binary file content also says Matroska instead of WebM
      let mimetype = mediaRecorder.mimeType;
      if (!mimeType.startsWith('video/webm')) {
        throw new Error(`We requested "${wantMimeType},
          but the browser gave us "${mediaRecorder.mimeType}"`);
      }
      let blob = new Blob(chunks, { type: mimeType });
      // ...convert to data: URL
      // ...play it in <video src="data:..."> // won't work, if Matroska
    } catch (ex) {
      alert(ex);
    }
  };
}