A CodePen by Miguel. Which keycode is? - Discover which keycode is the key pressed. A simple script with pure JS to now the keyCode of the keyDown.

## index.html
<body>
  <a href="//miduga.es" title="Desarrollado por miduga para desarrolladores">Developed by miduga</a>
  <div id="i">
    Press a key<br/><span>And discover his keyCode to use it in your JavaScript projects.</span></div>
  <div id="k"></div>

## script.js
var d = document,i=d.getElementById('i'),
    k = d.getElementById('k');

d.onkeydown = d.body.onkeydown = function(e)
{
  e = e || window.event;
  i.innerHTML = '';
  k.innerHTML = e.keyCode || e.which;
  return false;
}

## style.css
body {
  font-weight: 300;
  font-family: "Roboto Condensed", "Helvetica Neue", "Open Sans Condensed", sans-serif;
  margin: 0;padding: 0;}

body:before {
  box-shadow: 0 0 10px rgba(168, 168, 168, 0.8);
  content: "";
  height: 10px;
  left: 0;
  position: fixed;
  top: -10px;
  width: 100%;
  z-index: 100;
}

a {
  text-decoration: none;
  color: #008ccd;
  transition: all 0.12s ease-out;
  text-align: center;
  width: 100%;
  display: block;
  bottom: 15px;
  position: absolute;
}

#i, #k {
  position: fixed;
  font-size: 200px;
  text-align: center;
  width: 100%;
  left: 0;
  height: 200px;
  line-height: 30px;
  top: 50%;
  margin-top: -50px;
  color: #777;
  text-shadow: -1px -1px 0px #fff;}

span {
  font-size: 20px;
  color: #999
}

#i {
  font-size: 80px
}
	<body>
	<a href="//miduga.es" title="Desarrollado por miduga para desarrolladores">Developed by miduga</a>
	<div id="i">
	Press a key<br/><span>And discover his keyCode to use it in your JavaScript projects.</span></div>
	<div id="k"></div>
	var d = document,i=d.getElementById('i'),
	k = d.getElementById('k');

	d.onkeydown = d.body.onkeydown = function(e)
	{
	e = e \|\| window.event;
	i.innerHTML = '';
	k.innerHTML = e.keyCode \|\| e.which;
	return false;
	}
	body {
	font-weight: 300;
	font-family: "Roboto Condensed", "Helvetica Neue", "Open Sans Condensed", sans-serif;
	margin: 0;padding: 0;}

	body:before {
	box-shadow: 0 0 10px rgba(168, 168, 168, 0.8);
	content: "";
	height: 10px;
	left: 0;
	position: fixed;
	top: -10px;
	width: 100%;
	z-index: 100;
	}

	a {
	text-decoration: none;
	color: #008ccd;
	transition: all 0.12s ease-out;
	text-align: center;
	width: 100%;
	display: block;
	bottom: 15px;
	position: absolute;
	}

	#i, #k {
	position: fixed;
	font-size: 200px;
	text-align: center;
	width: 100%;
	left: 0;
	height: 200px;
	line-height: 30px;
	top: 50%;
	margin-top: -50px;
	color: #777;
	text-shadow: -1px -1px 0px #fff;}

	span {
	font-size: 20px;
	color: #999
	}

	#i {
	font-size: 80px
	}