Converting base64 to blob in javascript

I tried to convert a JPEG's base64 string to a blob on a Cordova/hybrid app running on iOS 8 using the following function b64toBlob.

b64toBlob = function(b64, onsuccess, onerror) {
    var img = new Image();

    img.onerror = onerror;

    img.onload = function onload() {
        var canvas = document.createElement('canvas');
        canvas.width = img.width;
        canvas.height = img.height;

        var ctx = canvas.getContext('2d');
        ctx.drawImage(img, 0, 0, canvas.width, canvas.height);

        canvas.toBlob(onsuccess);
    };

    img.src = b64;
}

However it's giving an error

Uncaught Error: TypeError: undefined is not a function (evaluating 'canvas.toBlob(onsuccess)')

when we do

var imageData = "data:image/jpeg;base64," + imageData

b64toBlob(imageData,
    function(imageBlob) {
       uploadBlob(imageBlob)
    }, function(error) {
        console.log(error)
    });

How can be work around this error?