Javascript by it's nature treats everything as a string, certainly all HTML controls that suport a value it will always be a string. Well more accurately everything is an object, and whenever you read that object in your code it tends to be implicitly converted or cast as a string. You have to explicitly cast your varables to a number when you require that behaviour, using the Number(string value) function e.g.



you can also consider parseFloat() and parseInt()
http://www.javascripter.net/faq/convert2.htm

Another usefull link:
http://www.w3schools.com/jsref/jsref_Number.asp

Thanku very much for ur reply.I'm trying like this but no result its resulted true for every input.can u please tell me wts wrong with this?
if(f.age.value!=""&&f.id.value!=""&&f.name.value!="")
{
var myNumber;
myNumber = Number(f.age.value);
if(myNumber == NaN)
{
alert("You must enter a number");
return false;
}else{return true;}

}

else{
if(f.age.value=="")
{
alert("you have to fill your age");
return false;
}
if(f.id.value=="")
{alert("you have to fill your id");
return false;
}

if(f.name.value=="");
{
alert("you have to enter name");
return false;
}

}
}
</script>

Sorry my mistake try this instead:


see here:
http://www.w3schools.com/jsref/jsref_Number.asp