reencode non-fragmented mp4 into fragmented mp4

I'm using MediaSource extension to play videos. However,MediaSource only supports fragmented MP4 videos.

How can I convert a non-fragmented MP4 video into a fragmented MP4 video in JavaScript so that I can use MediaSource to play the video?

MP4Box's isFragmented boolean returns false on non-fragmented videos:

blob.arrayBuffer().then((videoBuffer => {
    mp4boxfile.onReady = function (info) {
        const isFragmented = info.isFragmented;
        if (!isFragmented) {
            //Need to reencode video into a fragmented video here
        }
    }

//...

    const chunkSize = Math.ceil(videoBuffer.byteLength / 80000);
    let start = 0;
    let currentEnd = 80000;
    let videoBufferChunk;

    for (let i = 0; i < chunkSize; i++) {
        videoBufferChunk = videoBuffer.slice(start, currentEnd);

        videoBufferChunk.fileStart = start;
        start = mp4boxfile.appendBuffer(videoBufferChunk);
        currentEnd = start + 80000;
        mp4boxfile.flush();
    }
//...
}

How can I reencode a MP4 video into a fragmented one with JavaScript?

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