This entry must be a number. The purpose of this script is to perform data validation when user clicks on the submit button. If there is no value in the text box then it returns an alert message. Script checks if you do not choose a type of credit card. If don't select a credit card of choice, then it returns an alert message.
Function definition begin.
Begin loop over each character in the user's input. Nrl.length is the length of the input. Begin the block of code executed within the loop. Begin the block of code executed within the loop. Those are only characters we allow to input in the form.