Using base conversion to compress number values

baseCompression.js

function compress(z) {
        z = z.toString();
        var a = [];
        z.split("").forEach((e,i)=>{a[i] = parseInt(e);});
        var u = "0123456789qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM!?";
        var x = u.substring(0,Math.max(...a)+1);
        var s = x.length;
        var d = u.length;
        var v = 0;
        var n = z.length;
        for (var i = 0; i < n; i++) {
            v = v * s + x.indexOf(z.charAt(i));
        }
        if (v < 0) return 0;
        var r = v % d;
        var c = u.charAt(r);
        var q = Math.floor(v / d);
        while (q) {
            r = q % d; q = Math.floor(q / d); c = u.charAt(r) + c;
        }
        return c;
    };