Load an image and convert it to a string.

xhr_base_64.js

var xhr = new XMLHttpRequest();
xhr.responseType = 'arraybuffer';
xhr.onload = function() {
  if (this.status == 200) {
    var buffer = new Uint8Array(this.response);
    var i = buffer.length;
    var binary = new Array(i);
    while (i--) {
      binary[i] = String.fromCharCode(buffer[i]);
    }
    var base64 = window.btoa(binary.join(''));
  }
};
xhr.open('GET', URL, true);
xhr.send();