A query about NAN() method of javascript

I am using NAN() method to test whether a user is entering a numeric value of not.It is working fine when a user is not entering a numeric value.Suppose he/she enter "abc" instead of a numeric value then it is showing a error message but he/she change the value to a numeric number still it is giving a same error message even after refreshing a page.
What could be the error?
How to fix it?

Below is the code.


<script type="text/javascript" >
function validate()
{
var tos=document.getElementById('tos').value;
var df=document.getElementById('df').value;
if(df=="")
{
alert("Please enter the value of Df");
return false;
}
if (isNaN(df.value))
{
alert('Please enter a numberic value of Df');
return false;
}
}
</script>


Thanks in advance