TypeError: BlobBuilder is not a constructor

var BlobBuilder = window.BlobBuilder || window.MozBlobBuilder || window.WebKitBlobBuilder || window.MSBlobBuilder;
var bb = new BlobBuilder();
bb.append(arr.stString());

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Got it working with your code. I only had to change some little detail:

if(e.name == ‘TypeError‘ && window.BlobBuilder){
        var bb = new BlobBuilder();
        bb.append(data);
        out = bb.getBlob(datatype);
        console.debug("case 2");
    }
bb.append(data); // data must be with no brackets
My function(constructor) that works now for all browsers:

var NewBlob = function(data, datatype)
{
    var out;

    try {
        out = new Blob([data], {type: datatype});
        console.debug("case 1");
    }
    catch (e) {
        window.BlobBuilder = window.BlobBuilder ||
                window.WebKitBlobBuilder ||
                window.MozBlobBuilder ||
                window.MSBlobBuilder;

        if (e.name == ‘TypeError‘ && window.BlobBuilder) {
            var bb = new BlobBuilder();
            bb.append(data);
            out = bb.getBlob(datatype);
            console.debug("case 2");
        }
        else if (e.name == "InvalidStateError") {
            // InvalidStateError (tested on FF13 WinXP)
            out = new Blob([data], {type: datatype});
            console.debug("case 3");
        }
        else {
            // We‘re screwed, blob constructor unsupported entirely   
            console.debug("Errore");
        }
    }
    return out;
}