I am writing this code blindly as you didn't provide any code. You will need to pass the value of this into javascript (as an example) and check if it is a numeric or text.
Add this to your HTML file:
<script type="text/javascript">
var number = funcVPC(strVPC:=Eval("VPC"));
if(isNaN(number)){
document.write(num1 + " is not a number <br/>");
}else{
document.write(num1 + " is a number <br/>");
}
</script>
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If this helps please don't forget to mark as correct answer.