JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id='input'>11AAJj</textarea>
<pre id='report'></pre>

JavaScript

var r = document.getElementById('report'),
    t = document.getElementById('input');

(t.onkeyup = function(e) {
    r.innerHTML = getReport(t.value);
})();

function repeatedChars(s) {
    var charsFound = {}, repeatedChars = '';
    for (var i = 0; i < s.length; i++) {
            if (charsFound[s.charAt(i)]) repeatedChars += s.charAt(i);
            else charsFound[s.charAt(i)] = true;
        }
    return repeatedChars;
}


function repeatedAll(s) { return repeatedChars(s); }


function item(txt) { return '- ' + txt + '\n'; }
function getReport(s) {
    var score = s.length,
        alnum = repeatedAll(s);
    return (
        item(score + ' characters long') +
        item('repeated characters:       -' + (score -= alnum.length*1, alnum.length*1) + ' (' + alnum + ')') +
        '\nTotal score: ' + score
    );
}