[рони]

Allegro75,
чуть покороче ...

const invertCase = (str) => {
    let index = 0;
    let result = '';
    let lowerStr = str.toLowerCase();
    let upperStr = str.toUpperCase();
    for (; index < str.length; index++){
        if (str[index] === lowerStr[index]) {
            result += upperStr[index];
        }
        else result += lowerStr[index];
    }
    return result;
}
alert(invertCase('Hello, World!'));