JSFiddle - React, Tailwind, and code Playground

HTML

<div id="out"></div>
<input type="text" id="counts">

CSS

#counts {
    padding: 4px 6px;
    width: 400px;
    border: 1px #DDD solid;
}

JavaScript

var out = document.getElementById('out');

for (var i = 0, letter; i < 10; i++) {
    (function(i) {
        setTimeout(function() {
            letter = String.fromCharCode(97 + Math.floor(Math.random() * 26));
            out.appendChild(document.createTextNode(letter));
            if (i == 9) {
                calculate(out.innerHTML);
            }
        }, 100 * i); // 2000
    })(i);
}

function calculate(str) {
    for (var l = str.length, counts = {}; l--;) {
        counts[str[l]] = (counts[str[l]] || 0) + 1;
    }
    
    var res = [], el;
    for (el in counts) {
        res.push(el + '=' + counts[el]);
    }
    document.getElementById('counts').value = res.join(', ');
}