So we know that if c is not between 48 and 57, the phone number has characters other than digits. If so, we return false in line 15. You can also pass in a value if desired. To test if the check box is checked, test Checkbox.checked property. It returns true if the checkbox is checked.
Checkbox.value property returns the value that was passed in from the input.
Radio buttons are used when you want an exclusive selection. Usually you have several buttons in a group, and when one is checked, the other becomes unchecked. The syntax for retrieving checked state and value is the same as check boxes. Check for the five-digit format of the zip code and alert the user appropriately. The cursor should be returned to the first text field where the error occured.