[рони]

Tri,

function getCharCount(str, char) {
            let counter = 0;
            str = str.toLowerCase();
            char = char.toLowerCase();

            for (let letter of str) counter += letter == char
            return counter
        }
        alert(getCharCount("ААААаааа", "а"));
        alert(getCharCount("строка", "с"));
        alert(getCharCount("строка", "ы"));