ArrayBuffer to base64 encoded string

Question

I need an efficient (read native) way to convert an ArrayBuffer to a base64 string which needs to be used on a multipart post.

Answer 1

For those who like it short, here's an other one using Array.reduce which will not cause stack overflow:

var base64 = btoa(
  new Uint8Array(arrayBuffer)
    .reduce((data, byte) => data + String.fromCharCode(byte), '')
);