Let us assume that only digits are permitted for the telephone number field (with no spaces, or dashes or parentheses for now). Some examples of valid telephone numbers are:

    44181362500
    002356487
    56478303

Any entry that contains non-numeric values should be considered invalid; the user should be informed of this, and the form should not be submitted.

We can create a useful function notNumeric() that returns true if an argument passed to it is not a number. To write the function we can make use of the special value returned by JavaScript when the result of an attempt to perform a numeric expression is not a number: JavaScript evaluates such expressions to the String "NaN". Our function can be written as follows:

      function notNumeric( fieldString )
      {
      if ( String(fieldString * 1) == "NaN")
      return true;
      else
      return false;
      }
    

We can use this function to extend the validate() function to now only return true if the telephone number consists of digits:

      function validate()
      {
      // at this point no invalid fields have been encountered
      var fieldsValid = true;
      // test field firstName
      if ( isEmpty( orderform.firstName.value ) )
      {
      alert( "First name must have a value - form not submitted" );
      fieldsValid = false;
      }
      // test field familyName
      if ( isEmpty( orderform.familyName.value ) )
      {
      alert( "Family name must have a value - form not submitted" );
      fieldsValid = false;
      }
      // test field telephone
      if ( notNumeric( orderform.telephone.value ) )
      {
      alert( "Telephone number must consist of nothing but digits -
      form not submitted" );
      fieldsValid = false;
      }
      // return Boolean valid fields status
      return fieldsValid;
      }
    

So if a telephone number of "abc" is entered:

the browser will display the following alert window (and not post the form):