Node.js - assign writable stream to a variable

As in below code, I can write base64 encoded stream into a file but I want to assign the base64 encoded string to a variable rather than creating a file so that I can use it for the purpose I want. How can I assign response.pipe(base64.encode()); to a variable as base64 encoded string ? I am using base64-stream to encode incoming stream.

const request = xero.call('GET', `/Invoices/${invoiceId}`);
  request.setHeader('Accept', 'application/pdf');

  request.on('response', async (response) => {
    const data = response.pipe(base64.encode());
    const file = fs.createWriteStream(`${invoiceId}.txt`);
    data.pipe(file);
  });
  request.end();

via Shaolin