JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" id="1">
<button type="button" id="2">Click!</button>

JavaScript

$(document).ready(function() {

    $('#2').click(function() {
        var counter = {};
        var strLength = $('#1').val().length;
        for(i=0;i<=strLength;i++) {
            var currentChar = $('#1').val().substr(i, 1);
            if(counter.hasOwnProperty(currentChar)) {
                counter[currentChar].count += 1;
            } else {
                counter[currentChar] = {char: currentChar, count: 1};
            }
        }
        var highest = {char: null, count: null};
        $.each(counter, function(index, value) {
            if(value.count > highest.count) {
                highest = value;
            }
        });
        alert(highest.char + '\n' + highest.count);
    });
    
});