Create base64 out of image in Javascript

base64.js

function base64Image(src) {
  return new Promise(function(resolve, reject) {
    var img = new Image();

    img.setAttribute('crossOrigin', 'anonymous');

    img.onload = function() {
      var canvas = document.createElement("canvas");
      canvas.width = this.width;
      canvas.height = this.height;

      var ctx = canvas.getContext("2d");
      ctx.drawImage(this, 0, 0);

      var dataURL = canvas.toDataURL("image/png");

      resolve(dataURL);
    };

    img.onerror = function(err) {
      reject(err);
    }

    img.src = src;
  });
}