function incrementa(a) {
        for (let x in a) {
            a[x]++;
        }

    }

    function stampa(a) {
        for (let x in a) {
            document.write("<li>"+a[x]+"</li>");
        }

    }

    a = [10,11,12,13,14,15];
    document.write("<ul>");
    stampa(a);
    document.write("<li>---</li>");
    incrementa(a);
    stampa(a);
    document.write("</ul>");