JavaScript: The Definitive Guide, Fourth Edition

Form Verification Example

We’ll close our discussion of forms with an extended example that demonstrates several of the concepts we’ve been discussing. Example 15-2 shows how you might use the onsubmit event handler of the Form object to perform input validation so that you can notify the user and prevent the form from being submitted when it contains missing or invalid data. After studying this example, you may want to turn back to Example 1-3, the form-programming example we began this book with. The code of that example probably makes more sense now that you are a JavaScript expert!

Example 15-2 defines a verify( ) function suitable for use as a generic form validator. It checks for required fields that are empty. In addition, it can check that a numeric value is in fact numeric and also falls within a specified numeric range. This verify( ) function relies on the type property of a form element to determine which kind of element it is. The function also relies on additional user-defined properties to distinguish optional fields from required fields and to specify the allowed ranges for numeric fields. Note how the function reads the value property of an input field and uses the name property of a field when reporting errors.

Figure 15-2 shows an example form that uses this verification scheme, with the error message that is displayed when the user attempts to submit the form before correctly filling it in.

Figure 15-2. A form that failed validation

Example 15-2. Performing ...