File chunk upload to azure storage blob, file seems broken

I'm trying to upload excel file to azure storage blob in chunks, using the stage block and commitblock from BlobBlockClient Class. File upload seems to success but when i try to download and open the file, there it seems to be broken.

I'm using react and node js to do this. Code follows below

In UI

const chunkSize = (1024 * 1024) * 25;  // file chunk size

// here slicing the file and sending it to api method

const fileReader = new FileReader();
const from = currentChunkIndexRef.current * chunkSize;
const to = from + chunkSize;
const blob = file.slice(from, to);

fileReader.onload = ((e: any) => uploadChunksToBlob(e, file, obj));
fileReader.readAsDataURL(blob);

// api method

const uploadChunksToBlob = async (event: any, file: File, obj: any) => {
try {
  const totalChunks = Math.ceil(file.size / chunkSize);
  const uploadChunkURL = `/upload?currentChunk=${currentChunkIndexRef.current}&totalChunks=${totalChunks}&file=${file.name}&type=${file.type}`;
  console.log(event.target.result)
  const fileUpload = await fetch(uploadChunkURL, {
    method: "POST",
    headers: { "Content-Type": "application/octet-stream" },
    body: JSON.stringify(event.target.result),
  });
  const fileUploadJson = await fileUpload.json();
  const isLastChunk = (totalChunks - 1) === currentChunkIndexRef.current;
  if(!isLastChunk) {
    console.log({ Chunk: currentChunkIndexRef.current });
    currentChunkIndexRef.current = currentChunkIndexRef.current + 1;
    // eslint-disable-next-line @typescript-eslint/no-use-before-define
    uploadFileToAzureBlob(file, obj);
  } else {
    console.log("File Uploaded")
  }
  // 
} catch (error) {
  console.log("uploadFileToAzureBlob Catch Error" + error);
}

}

// In Node

const sharedKeyCredential = new StorageSharedKeyCredential(
config.StorageAccountName,
config.StorageAccountAccessKey
);
const pipeline = newPipeline(sharedKeyCredential);
const blobServiceClient = new BlobServiceClient(
`https://${config.StorageAccountName}.blob.core.windows.net`,
 pipeline
 );
const containerName = getContainerName(req.headers.key, req.headers.clientcode);
const identifier = uuid.v4();
const blobName = getBlobName(identifier, file);

const containerClient = blobServiceClient.getContainerClient(containerName);
const blockBlobClient = containerClient.getBlockBlobClient(blobName);


try {

   let bufferObj = Buffer.from(`${file}_${Number(currentChunk)}`, "utf8"); // Create buffer object, specifying utf8 as encoding

   let base64String = bufferObj.toString("base64"); // Encode the Buffer as a base64 string

   blockIds = [...blockIds, base64String];
   const bufferedData = Buffer.from(req.body);

   let resultOfUnitArray = new Uint8Array(bufferedData.length);
   for (let j = 0; j < bufferedData.length; j++) {
     resultOfUnitArray[j] = bufferedData.toString().charCodeAt(j);
   } // Converting string to bytes
   const stageBlockResponse = await blockBlobClient.stageBlock(base64String, resultOfUnitArray, resultOfUnitArray.length, {
  onProgress: (e) => {
    console.log("bytes sent: " + e.loadedBytes);
  }
});
   if ((Number(totalChunks) - 1) === (Number(currentChunk))) {
     const commitblockResponse = await blockBlobClient.commitBlockList(blockIds, {blobHTTPHeaders: req.headers});
     res.json({ uuid: identifier, message: 'File uploaded to Azure Blob storage.' });
   } else {
     res.json({ message: `Current Chunks ${currentChunk} is Successfully Uploaded` });
   }
} catch (err) {
   console.log({ err })
   res.json({ message: err.message });
}

I don't know, what i'm doing wrong here.

Any help would be appreciated Thank you