The easy way to convert an URL Image to Base64 or Blob in Javascript/Jquery

I'm working on an offline mode for a simple application, and I'm using Indexeddb (PounchDB as a library), I need to convert an Image to Base64 or BLOB to be able to save it.

I have tried this code which works for only one Image (the Image provided) I don't know why it doesn't work for any other image:

function convertImgToBase64URL(url, callback, outputFormat){
    var img = new Image();
    img.crossOrigin = 'Anonymous';
    img.onload = function(){
        var canvas = document.createElement('CANVAS'),
        ctx = canvas.getContext('2d'), dataURL;
        canvas.height = img.height;
        canvas.width = img.width;
        ctx.drawImage(img, 0, 0);
        dataURL = canvas.toDataURL(outputFormat);
        callback(dataURL);
        canvas = null; 
    };
    img.src = url;
}
convertImgToBase64URL('http://upload.wikimedia.org/wikipedia/commons/4/4a/Logo_2013_Google.png', function(base64Img){
alert('it works');
      $('.output').find('img').attr('src', base64Img);  
   
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<div class="output"> <img> </div>

It works fine, but only with one Image, If I try any other Image it doesn't work