JSFiddle - React, Tailwind, and code Playground

JavaScript

function occurs(letters) {
    var countletters = {}, index = 0;

    return function counter() {
        if (letters.length <= index) {
            return countletters;
        }

        return function incrument() {
            var letter = letters[index++];
            countletters[letter] ? countletters[letter] += 1 : countletters[letter] = 1;
            return counter();
        }();
    }();
}

var str = "AAABBABC";
var x = occurs(str);
alert(
    "In the string: " + str +
    " the number of A's = " + x["A"] + ", B's = " + x["B"] + " and C's = " + x["C"]);