Convert LoverCase to UpperCase and converse (JavaScript)

Question

Have a string for example = 'fGvB233k;' need result = 'FgVb233K;'

<script>
        var str ='fGvb233k;';
        var res;
        var i;
        for (i=0; i < str.length; i++) {
            if (str[i] === str[i].toUpperCase())
            {
            str[i] = str[i].toLowerCase() ; 
            console.log(str[i]);
            }
            else {
             str[i] = str[i].toUpperCase() ;     

            }
        };
        console.log(str);
    </script>

my script doesnt work.... result of script: G B 2 3 3 ;

Answer 1

Example: JSBin You cannot mutate a string you need to create a new one:

var swapCase = function(letters){
    var str = "";
    for(var i = 0; i<letters.length; i++){
        if(letters[i] === letters[i].toLowerCase()){
            str += letters[i].toUpperCase();
        }else {
            str += letters[i].toLowerCase();
        }
    }
    console.log(str);
    return str;
};