Algorithm - Find Largest Upper Case Character when a same character in lower Case & Upper Case occurs in a string

findBiggestAlphabet = function(S){
    console.log(`Processing..`, S)
    const char_arr = [...S];
    var max_char = "";
    for(var i=0; i<S.length-1;i++){
        var char = S[i];
        var char_c = char.toUpperCase()
        //console.log(`Processing ${char}`);
        var new_arr = char_arr.slice(i+1, char_arr.length);

        var exist_cap = new_arr.find(x=>x===char_c)? true: false;
        if (exist_cap){
            if (char_c>max_char){
                max_char = char_c;
            }
        }
    }
    if (max_char) return max_char;
    return "NO";
}

var A_1 = "aabcDBA";
var A_2 = "dabcDBA";
var A_3 = "abcde";
console.log(findBiggestAlphabet(A_1));
console.log(findBiggestAlphabet(A_2));
console.log(findBiggestAlphabet(A_3));