How to concat chunks of buffer into video file node js?

I am trying to upload chunks of base64 to node js server and save those chunks into one file

let chunks = [];

app.post('/api', (req, res) => {
    let {blob} = req.body;
    //converting chunks of base64 to buffer
    chunks.push(Buffer.from(blob, 'base64'));
    res.json({gotit:true})

});

app.post('/finish', (req, res) => {
    let buf = Buffer.concat(chunks);
    fs.writeFile('finalvideo.webm', buf, (err) => {
        console.log('Ahh....', err)
    });
    console.log('SAVED')
    res.json({save:true})
});

Problem with the above code is video is not playable I don't why Am I really doing something wrong and I've also tried writable streams it is not working either

UPDATE - I

Instead of sending blobs I've implemented to send binary but even though I am facing a problem like TypeError: First argument must be a string, Buffer, ArrayBuffer, Array, or array-like object.

client.js

 postBlob = async blob => {
       let arrayBuffer = await new Response(blob).arrayBuffer();
        let binary = new Uint8Array(arrayBuffer)
        console.log(binary) // logging typed Uint8Array
        axios.post('/api',{binary})
            .then(res => {
                console.log(res)
            })

    };

server.js

 let chunks = [];

    app.post('/api', (req, res) => {
        let {binary} = req.body;



        let chunkBuff = Buffer.from(binary) // This code throwing Error
        chunks.push(chunkBuff);

        console.log(chunkBuff)

         res.json({gotit:true})

    });

//Somehow combine those chunks into one file
app.post('/finish', (req, res) => {
    console.log('Combinig the files',chunks.length);

     let buf = Buffer.concat(chunks);

    console.log(buf) //empty buff
    fs.writeFile('save.webm', buf, (err) => {
        console.log('Ahh....', err)
    });

    res.json({save:true})
});

UPDATE - II

I am able to receive the binary chunk and append to a stream but in the final video only first chunk is playing I don't know what happened to other chunks and the video ends.

code

const writeMyStream = fs.createWriteStream(__dirname+'/APPENDED.webm', {flags:'a', encoding:null});

app.post('/api', (req, res) => {
    let {binary} = req.body;
 let chunkBuff = Buffer.from(new Uint8Array(binary));
    writeMyStream.write(chunkBuff);
res.json({gotit:true})

});

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