Problem with a[download]

downloadlink.js

var bb = new BlobBuilder();        
var ctx = canvas.getContext('2d');
var d = ctx.getImageData(0, 0, canvas.width, canvas.height).data;
        bb.append(d);
        downloadLink.href = window.URL.createObjectURL(bb.getBlob(MIME_TYPE));
        downloadLink.dataset.downloadurl = [MIME_TYPE, FILE_NAME, downloadLink.href].join(':');