let u = "I should have known that you would have a perfect answer for me!!!";

function movingShift(s, shift) {
        let arr = [];

        s.split("").forEach((x, idx) => {
            let source = x.toUpperCase().charCodeAt(0);
            if (source < 65 || source > 90) {
                arr.push(x);
                shift++;
                return;
            }
            let index = (source - 65 + (shift)) % 26 + 65;
            let letter = String.fromCharCode(index);
            x === x.toLowerCase() ? arr.push(letter.toLowerCase()) : arr.push(letter);
            shift++;
        })

        let cipher = arr.join("");
        return cipher;
    }

    let v = movingShift(u, 1);

    function demovingShift(v, shift) {
        shift = -1;
        let arr = [];
        v.split("").forEach((x, idx) => {
            let source = x.toUpperCase().charCodeAt(0);
            if (source < 65 || source > 90) {
                arr.push(x);
                shift--;
                return;
            }
            let index = (source - 65 + (shift)) % 26 + 65;
            let letter = String.fromCharCode(index);
            x === x.toLowerCase() ? arr.push(letter.toLowerCase()) : arr.push(letter);
            shift--;
        })
        return arr.join("");
    }

    console.log(movingShift(u, 1));
    console.log(demovingShift(v, 1));

J vltasl rlhr zdfog odxr ypw atasl rlhr p gwkzzyq zntyhv lvz wp!!!

I sho;ld ha<e k45=4 :.a: ?5; =5;2* .a<+ a 6+8,+): a49=+8 ,58 3+!!!