JSFiddle - React, Tailwind, and code Playground

HTML

Click in this frame, then try holding down some keys :)
<br /><br />
Number of keys held down:
<div id="out">0</div>

JavaScript

var keys_count = 0;
var keys = {};

$(document).keydown(function (e) {
    if(keys[e.which]) return;
    
    keys_count = keys_count + 1;
    keys[e.which] = true;
    
    printKeys();
});

$(document).keyup(function (e) {
    keys_count = keys_count - 1;
    delete keys[e.which];
    
    printKeys();
});

function printKeys() {
    $('#out').html(keys_count);
}