Demo
<iframe height='265' scrolling='no' title='Factorial program' src='//codepen.io/fastfoodcoding/embed/LjLBvr/?height=265&theme-id=light&default-tab=result&embed-version=2' frameborder='no' allowtransparency='true' allowfullscreen='true' style='width: 100%;'>See the Pen <a href='https://codepen.io/fastfoodcoding/pen/LjLBvr/'>Factorial program</a> by fastfoodcoding (<a href='https://codepen.io/fastfoodcoding'>@fastfoodcoding</a>) on <a href='https://codepen.io'>CodePen</a>.
</iframe>
Code
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
$("#btn").click(function(){
var number = $("#num").val();
var output = 1;
if(number <= 0){
output = 1;
}else{
for(i=1; i<=number; i++){
output = output*i;
}
}
$("#result").html(output);
});
});
</script>
</head>
<body>
Enter a number:
<input type = "text" name = "value" id = "num"></input>
<button id= "btn"> Submit </button>
<p id = "result"> </p>
</body>
</html>