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validating a text box using javascript

I have a javascript that reads the value of the firstname
textbox on a form.

If a number was entered I want it to return an alert and
say that you can only enter a string

var vFirstName = document.getElementById("<%= txtFirstName.ClientID %>")

I tried to use the "typeof" something like the code below but its not working

 var vFirstName = document.getElementById("<%= txtFirstName.ClientID %>")
            var getValueFirstName = vFirstName.value
            var result = (typeof getValueFirstName);
            if (result != 'string') {
                alert("First name should be string or letters");
                return false;

please assist
1 Solution
If isNaN(document.getElementById("<%= txtFirstName.ClientID %>")
will be true if the field is a string, or empty.

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