How do I append a blob file using @azure/storage-blob sdk ? (NodeJS)

My file “mainfile.json” in blob storage has the following content :

[
  { "name": "abc", "id": "01", "location": "random" },
  { "month": "Jan", "project": "50%", "training": "50%" },
]

The data I’m trying to add to it is this :

{"month": "Feb", "project":"60%", "training":"40%"}

I want it to be something like this :

[
  { "name": "abc", "id": "01", "location": "random" },
  { "month": "Jan", "project": "50%", "training": "50%" },
  {"month": "Feb", "project":"60%", "training":"40%"}
]

I’m using @azure/storage-blob sdk to carry out the same and here’s my code below :

const blobServiceClient = require("./getCred");
const fs = require("fs");

async function appendBlob() {
  const containerClient =
    blobServiceClient.getContainerClient("containername");
  //gets the main content from a blob
  const blobClient = containerClient.getBlobClient("mainfile.json");
  //the new appended content gets written into this blob
  const blockBlobClient = containerClient.getBlockBlobClient("data.json");
  // the data that needs to be appended
  const data = fs.readFileSync("new-data.json", "utf-8", (err) => {
    if (err) {
      console.log("File not read");
    }
  });

  // Get blob content from position 0 to the end
  // In Node.js, get downloaded data by accessing downloadBlockBlobResponse.readableStreamBody
  const downloadBlockBlobResponse = await blobClient.download();
  const downloaded = (
    await streamToBuffer(downloadBlockBlobResponse.readableStreamBody)
  ).toString();
  const append = await appendingFile(downloaded, data);

  const uploadBlobResponse = await blockBlobClient.upload(
    append,
    append.length
  );
  console.log(
    `Uploaded block blob to testing.json successfully`,
    uploadBlobResponse.requestId
  );

  // [Node.js only] A helper method used to read a Node.js readable stream into a Buffer
  async function streamToBuffer(readableStream) {
    return new Promise((resolve, reject) => {
      const chunks = [];
      readableStream.on("data", (data) => {
        chunks.push(data instanceof Buffer ? data : Buffer.from(data));
      });
      readableStream.on("end", () => {
        resolve(Buffer.concat(chunks));
      });
      readableStream.on("error", reject);
    });
  }

  async function appendingFile(content, toBeAdded) {
    return new Promise((resolve, reject) => {
      let temp = content.concat(toBeAdded);
      console.log(temp);
      resolve(temp);
      reject(new Error("Error occurred"));
    });
  }
}

But I get the following as the output :

[
  {
    "name": "KK",
    "id": "01",
    "location": "chennai"
  },
  {
    "month": "December",
    "project": "50%",
    "training": "50%"
  }
]
{
  "month": "January",
  "adaptive-cards": "50%",
  "azure-func-app": "50%"
}

My entire approach could be wrong as I’m new to coding. Please help me with this. Thanks in advance.