using global variables in a sweet.js case

I'm trying to create a macro that expands an incrementing value into the output every time it is called. So far I've got this, but it re-initializes the value to 0 every time:

macro test1 {
    case { _ $x
    } => {
        var n = n ? n+1 : 0;
        letstx $n = [makeValue(n,#{here})];
        return #{
            $n;
        }
    }
}

test1 ()
test1 ()

yields:

0;
0;

when what I want is:

0;
1;

How do I define n to be a global variable, so that I can increment and retain its value outside the macro?

Update:

I can get it to work by changing the assignment to n to an eval, but this really feels like cheating:

var n = eval("if (typeof n != 'undefined') n++; else n=0; n")

At the moment (there are plans make this better) you can just use the global object:

macro test1 {
    case { _ $x
    } => {
        window.n = typeof window.n !== 'undefined' ? window.n+1 : 0;
        letstx $n = [makeValue(n,#{here})];
        return #{
            $n;
        }
    }
}

test1 ()
test1 ()