NodeJS video streaming transcode on the fly

I'm building a nodejs app that streams a video from the disk to the user. Thanks to this question, i have a working video feed for a "static" video file that is stored on disk, so far so good.

My problem is that i need to transcode the video on the fly, to do this i used fluent-ffmpeg and i was successful at implementing the transcoding, but the HTML5 player only shows the first 3-4s of the video, and then stops. I'm guessing the problem is the filesize, but even when i harcode it nothing changes.

Any idea ? Thanks a lot :)

var file = 'Big_Buck_Bunny_1080p_surround_FrostWire.com.mp4';
    fs.stat(file, function(err, stats) {
        var range = req.headers.range
        if (!range) { // 416 Wrong range
            return res.sendStatus(416)
        }
        var positions = range.replace(/bytes=/, "").split("-");
        var start = parseInt(positions[0], 10);
        var total = stats.size;
        var end = positions[1] ? parseInt(positions[1], 10) : total - 1;
        var chunksize = (end - start) + 1;

        res.writeHead(206, {
            "Content-Range": "bytes " + start + "-" + end + "/" + total,
            "Accept-Ranges": "bytes",
            "Content-Length": chunksize,
            "Content-Type": "video/mp4"
        })

        var stream = fs.createReadStream(file, { start: start, end: end, autoclose: true })
        .on("open", function() {
            const ffmpegCommand = ffmpeg()
                .input(stream)
                .outputFormat('mp4')
                .outputOptions([ '-movflags faststart', '-frag_size 4096', '-cpu-used 2', '-deadline realtime', '-threads 4' ])
                .videoBitrate(640, true)
                .audioBitrate(128)
                .audioCodec('aac')
                .videoCodec('libx264')
                .output(res)
                .run()
        }).on("error", function(err) {
            res.end(err)
        })
    })

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