fs.readFile(attachment, function(err, data) {
   var base64data = new Buffer(data).toString('base64');
   [your API call here]
});

It takes some time until the results are there, so by the time you've got the data, the outer scopes execution is already over.

From the format you posted, it seems like the image buffer is located in store.storeImage.buffer. storeImage.data must be a getter that is converting it to something else - from the looks of it, probably an array?

Try using store.storeImage.buffer.toString('base64') - passing 'base64' to toString is a feature of buffers specifically, other types don't handle it the same.

Edit: Apparently store.storeImage itself is the buffer. So you should use simply store.storeImage.toString('base64')

Edit: Apparently it describes itself as a buffer despite not actually offering a node.js Buffer object. I suggest converting the array to a buffer and then encoding as base64: Buffer.from(store.storeImage.data).toString('base64')

Since you’re receiving a Buffer back as output, Buffer.toString('base64') will convert the raw binary data in the buffer to a base64 representation of the data.

Buffer.toString() can also take other encodings, you can read more about the other supported encodings in the docs.

So for your above code you would use this to get your image as base64

const dataImagePrefix = `data:image/png;base64,`
const query = 'SELECT image FROM table WHERE id=(SELECT max(id) FROM table)'

connection.query(query, (err, result, fields) => {
  if (err) {
    console.log(err)
    
    // Exit after handling error
    return 
  }

  // Return all results and for each result row
  // convert to a base64 string with the dataImagePrefix prepended to each
  return results.map(result => `${dataImagePrefix}${result.image.toString('base64')}`)
})