var reader = new FileReader()
reader.onload = function(){
    var buffer = new Buffer(reader.result)
    //temp資料夾應已存在
    fs.writeFile(`temp/${Date.now()}.mp4`, buffer, {}, (err, res) => {
        if(err){
            console.error(err)
            return
        }
        console.log('video saved')
    })
}
reader.readAsArrayBuffer(blob)//假設blob已定義,且為MP4視訊