Edit in JSFiddle

document.getElementById("number").addEventListener('input', function(e){
		document.getElementById("output").innerHTML = prime_test(this.value);
});

function prime_test(x) {
	if (!x) {
  	return "NaN";
  }
  if (x < 2) {
  	return false;
  }
	for (var y = 2; y * y <= x; ++y) {
  	if (x % y == 0) {
    	return false;
    }
  }
  return true;
}

Input Number: <input id="number" type="number"><br/>
<br/>
Is prime: 
<div id="output">
</div>