Very useful for resizing images before upload using html5 canvas.

dataURItoBlob.js

/**
 * Convert base64 encode image into Blob
 * 
 * Usage:
 *  var dataURL = canvas.toDataURL('image/jpeg', 0.5);
 *  var blob = dataURItoBlob(dataURL);
 *  var fd = new FormData(document.forms[0]);
 *  fd.append("canvasImage", blob);
 * 
 * See: http://stackoverflow.com/a/5100158/1063090
 */
 
function dataURItoBlob(dataURI, isURLEncoded) {
    var byteString;

    if(isURLEncoded) {
      // convert base64/URLEncoded data component to raw binary data held in a string
      if (dataURI.split(',')[0].indexOf('base64') >= 0)
        byteString = atob(dataURI.split(',')[1]);
      else
        byteString = unescape(dataURI.split(',')[1]);
    } else {
        // convert base64 to raw binary data held in a string
        // doesn't handle URLEncoded DataURIs
        byteString = atob(dataURI.split(',')[1]);
    }

    // separate out the mime component
    var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0]

    // write the bytes of the string to an ArrayBuffer
    var ab = new ArrayBuffer(byteString.length);
    var ia = new Uint8Array(ab);
    for (var i = 0; i < byteString.length; i++) {
        ia[i] = byteString.charCodeAt(i);
    }

    // write the ArrayBuffer to a blob, and you're done
    var bb = new BlobBuilder();
    bb.append(ab);
    return bb.getBlob(mimeString);
}