encrypt alphabet to unicode does not work

Question

This is project from freecodecamp where a string is encrypt by the values of the letters are shifted by 13 places All letters will be uppercase. Do not transform any non-alphabetic character (i.e. spaces, punctuation), but do pass them on. i test numadd by regex numadd is either equal to num+13 or numadd-26. some letter went wrong and do not return an uppercase letter please someone explain whats wrong

function rot13(str) {
    var str2 = '';
    var arr1 = []
    var arr2 = []
    var reg = /[^A-Z]/
    // var char
    if (str.includes(' ') == false) {
        for (let i = 0; i < str.length; i++) {
            var char = '';
            var numres
            let num = str.charCodeAt(i)
            // console.log(num)
            // var numadd = num + 13;
            // if num+13 not uppercase
            if (reg.test(num + 13)) {
                numres = num - 13
            }
            // if num+13 is uppercase
            else {
                numres = num + 13
            }
            char = String.fromCharCode(numres)
            str2 += char
        }
        console.log(str2);
    }
}
rot13("SERRPBQRPNZC");

Answer 1

reg.test(num + 13) checks the textual representation of the number num + 13 (applying implicit type conversion) so it always says True (do not match an uppercase letter) and always is computed numres = num - 13.

Use reg.test(String.fromCharCode(num + 13)) instead as follows:

function rot13(str) {
    var str2 = '';
    var arr1 = []
    var arr2 = []
    var reg = /[^A-Z]/
    // var char
    if (str.includes(' ') == false) {
        for (let i = 0; i < str.length; i++) {
            var char = '';
            var numres
            let num = str.charCodeAt(i)
            // console.log(num)
            // var numadd = num + 13;
            // if num+13 not uppercase
            //           ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
            if (reg.test(String.fromCharCode(num + 13))) {
                numres = num - 13
            }
            // if num+13 is uppercase
            else {
                numres = num + 13
            }
            char = String.fromCharCode(numres)
            str2 += char
        }
        console.log(str2);
    }
}
rot13("SERRPBQRPNZC");