How to convert an image into base64 string

html-content.html

<img id="image" src="http://www.gravatar.com/avatar/90348ht093409g7wetdgf?s=128&d=identicon&r=PG">
<canvas id="canvasElement"/>
<div id="base64string" />

js-converter.js

var canvasEl = document.getElementById("canvasElement");
var ctx = canvasEl.getContext("2d");
var img = document.getElementById("image");
ctx.drawImage(img, 15, 15);
alert(canvasEl.toDataURL());
document.getElementById('base64string').innerHTML = img;