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

Question

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

Answer 1

The image you're using has CORS headers

Accept-Ranges : bytes
Access-Control-Allow-Origin : * // this one
Age : 69702
Connection : keep-alive

which is why it can be modified in a canvas like that.
Other images that don't have CORS headers are subject to the same-origin policy, and can't be loaded and modified in a canvas in that way.

Answer 2

I need to convert Image URL to Base64

See

Understanding the HTML5 Canvas image security rules

Browser Canvas CORS Support for Cross Domain Loaded Image Manipulation

Why does canvas.toDataURL() throw a security exception?

---

One possible workaround would be to utilize FileReader to convert img , or other html element to a data URI of html , instead of only img src; i.e.g., try opening console at jsfiddle http://jsfiddle.net/guest271314/9mg5sf7o/ , "right-click" on data URI at console, select "Open link in new tab"

var elem = $("div")[0].outerHTML;
var blob = new Blob([elem], {
    "type": "text/html"
});
var reader = new FileReader();
reader.onload = function (evt) {
    if (evt.target.readyState === 2) {
        console.log(
                     // full data-uri
                     evt.target.result
                     // base64 portion of data-uri
                   , evt.target.result.slice(22, evt.target.result.length));
        // window.open(evt.target.result)
    };
};
reader.readAsDataURL(blob);

---

Another possible workaround approach would be to open image in new tab , window ; alternatively an embed , or iframe element ; utilize XMLHttpRequest to return a Blob , convert Blob to data URI

var xhr = new XMLHttpRequest();
// load `document` from `cache`
xhr.open("GET", "", true); 
xhr.responseType = "blob";
xhr.onload = function (e) {
    if (this.status === 200) {
        // `blob` response
        console.log(this.response);
        var reader = new FileReader();
        reader.onload = function(e) {
            // `data-uri`
            console.log(e.target.result);
        };
        reader.readAsDataURL(this.response);        
    };
};
xhr.send();