Try this:
<!DOCTYPE html>
<html>
<body>
<div id="result">Good Luck!</div>
<script>
var placeHolder = document.getElementById('result');
var listener = window.document.addEventListener('keydown', function(e){
if (e.keyCode == 38) flip();
}, false);
function flip(){
var choice = Math.random();
if(choice < 0.51){
placeHolder.innerText = 'Heads';
} else {
placeHolder.innerText = 'Tails';
}
}
</script>
</body>
</html>