<html>
<body>
<script type="text/javascript">  
  function getcube(){  
    var number=document.getElementById("number").value;  
    alert("Cube is : " + number*number*number);  
  }  
</script>  
<form>  
Enter No:<input type="text" id="number" name = "number"/><br/>  
<input type="button" value="Cube" onclick="getcube()"/>  
</form>
</body>
</html>  