get an image file with XMLHttpRequest and encode with base64

var xhr = new XMLHttpRequest();
var imgurl = "http://lh5.ggpht.com/-ZpKKJYkqobU/ULk2lb2yh5I/AAAAAAAAAAA/z74Pv1Zbfjc/photo.jpg";

xhr.open('GET', imgurl, true);
xhr.responseType = 'arraybuffer';//watch out this line!
xhr.onload = function(){
var blob = new Uint8Array(this.response);
//encode with base64
var uri = "data:image/jpeg;base64," + Base64.encode(blob);
document.getElementById("myImage").src = uri;
};
xhr.send();

var Base64 = {
_keyStr : "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=",

// public method for encoding
encode : function (input) {
var output = "";
var chr1, chr2, chr3, enc1, enc2, enc3, enc4;
var i = 0;
//input = Base64._utf8_encode(input); //comment out to encode binary file(like image)

while (i < input.length) {

chr1 = input[i++];
chr2 = input[i++];
chr3 = input[i++];

enc1 = chr1 >> 2;
enc2 = ((chr1 & 3) << 4) | (chr2 >> 4);
enc3 = ((chr2 & 15) << 2) | (chr3 >> 6);
enc4 = chr3 & 63;

if (isNaN(chr2)) {
enc3 = enc4 = 64;
} else if (isNaN(chr3)) {
enc4 = 64;
}
output = output +
Base64._keyStr.charAt(enc1) + Base64._keyStr.charAt(enc2) +
Base64._keyStr.charAt(enc3) + Base64._keyStr.charAt(enc4);
}
return output;
}
}