The basic method is to attach to the form's onsubmit event, read out all the form elements' classes and perform validation if required. If a field fails validation, reveal field validation advice and prevent the form from submitting.

Include the javascript libraries:

<script src="prototype.js" type="text/javascript"></script> <script src="validation.js" type="text/javascript"></script>

You write elements like this:

<input class="required validate-number" id="field1" name="field1" />

passing the validation requirements in the class attribute.

You then activate validation by passing the form or form's id attribute like this:

<script type="text/javascript"> new Validation('form-id'); // OR new Validation(document.forms[0]); </script>

It has a number of tests built-in but is extensible to include your custom validation checks.

The validator also avoids validating fields that are hidden or children of elements hidden by the CSS property display:none. This way you can give a field the class of 'required' but it's only validated if it is visible on the form. The demo illustrates what I am talking about

Here's the list of classes available to add to your field elements:

• required (not blank)
• validate-number (a valid number)
• validate-digits (digits only)
• validate-alpha (letters only)
• validate-alphanum (only letters and numbers)
• validate-date (a valid date value)
• validate-email (a valid email address)
• validate-url (a valid URL)
• validate-date-au (a date formatted as; dd/mm/yyyy)
• validate-currency-dollar (a valid dollar value)
• validate-selection (first option e.g. 'Select one...' is not selected option)
• validate-one-required (At least one textbox/radio element must be selected in a group - see below*)

*To use the validate-one-required validator you must first add the class name to only one checkbox/radio button in the group (last one is probably best) and then place all the input elements within a parent element, for example a div element. That way the library can find all the checkboxes/radio buttons to check and place the validation advice element at the bottom of the parent element to make it appear after the group of checkboxes/radio buttons.

When the validation object is initialised you can pass the option {stopOnFirst : true} to enable the stop on first validation failure behaiour. The demo above has this set to false which is the default. If set to true only the first validation failure advice will be displayed when the form is submitted instead of all at once.

<script type="text/javascript"> new Validation('form-id',{stopOnFirst:true}); </script>

You can also pass the option {immediate : true} to enable field valiation when leaving each field. That is on the onblur event for all the form elements.

By default the library will add an event listener to the form's onsubmit event and stop the event if the validation fails. If you pass the option {onSubmit : false} it wont do that. This way you can call the validate function manually within your own javascript.

By default the library will focus on the first field that contains an error. If you pass the option {focusOnError : false} it wont do that.

You can also pass the option {useTitles : true} to make the field validators use the form elements' title attribute value as the error advice message.

You can set callbacks by using the options {onFormValidate : yourFunction, onElementValidate : yourFunction}.

onFormValidate is called after form validation takes place and takes two arguments: the validation result (true or false) and a reference to the form. OnElementValidate is called after each form element is validated and also takes 2 arguments: the validation result (true or false) and a reference to the form element.

Instead of using the error message in the validator you can create your own validation advice page element. Now when the script is creating the advice element it first looks for an element with an id matching 'advice-' + validation-class-name + '-' + element.id and if not found then one matching 'advice-' + element.id . If your form element does not have an id attribute then match the name attribute. If it finds an element it will make that one appear. See the 'Donation' field in the demo for an example. If you make a custom validation advice element make sure you set the style to display : none .

Also if you reference the effects.js file from Scriptaculous in your page head, it'll use a fade-in effect for the validation advice.

<script src="effects.js" type="text/javascript"></script>

As well as the validation css classes above, the validation library uses CSS classes to indicate validation status:

validation-failed and validation-passed

The validation advice element has a class of validation-advice and an id matching the following pattern

'advice-' + validation-class-name + '-' + element.id

so if the field ' birthdate' uses the ' validate-date' validation class then the validation advice element will have an id of ' advice-validate-date-birthdate'.

By default the class attaches an event observer to the form's onsubmit event. If you prefer to do the form submit via javascript yourself you can still validate the form like this:

<script type="text/javascript"> var valid = new Validation('form-id', {onSubmit:false}); var result = valid.validate(); </script>

The instance method, validate(), will return true or false.

The class has an instance function which resets all the field validation:

<script type="text/javascript"> var valid = new Validation('form-id'); valid.reset(); </script>

Note that it doesn't reset the form, just the validation.

The Validation class also has some static methods that can be used independantly.

Validation.validate([element OR element id] [, options])

This validates the field (or field with that id), using all validation classes present. You can also pass the option {useTitle : true} to make the field validator use the form element's title attribute value as the error advice message.

You can run a specific validation test on a field or field value by doing this:

Validation.get('validator-name').test(value [, element]);

To add your own validator do this:

Validation.add('class-name', 'Error message text', function(value [, element]) { return /* do validation here */ }, options);

or this:

Validation.add('class-name', 'Error message text', options);

The first example above includes a function as the third argument. The function enables you to write your own custom validation. The options argument is optional. The second example the third argument has become the options argument. Validator options can be used to perform common validation options without the need to write them into a function. Multiple options can be combined to create a complex validator and they can also enhance your custom validation function. Here are the available options and example usage below:

Validation.add('class-name', 'Error message text', { pattern : new RegExp("^[a-zA-Z]+$","gi"), // only letter allowed minLength : 6, // value must be at least 6 characters maxLength : 13, // value must be no longer than 13 characters min : 5, // value is not less than this number max : 100, // value is not more than this number notOneOf : ['password', 'PASSWORD'], // value does not equal anything in this array oneOf : ['fish','chicken','beef'], // value must equal one of the values in this array is : '5', // value is equal to this string isNot : 'turnip', //value is not equal to this string equalToField : 'password', // value is equal to the form element with this ID notEqualToField : 'username', // value is not equal to the form element with this ID include : ['validate-alphanum'] // also tests each validator included in this array of validator keys (there are no sanity checks so beware infinite loops!) });

For example here's one of the in-built ones:

Validation.add('validate-alpha', 'Please use letters only (a-z) in this field.', function (v) { return Validation.get('IsEmpty').test(v) || /^[a-zA-Z]+$/.test(v) });

And here's a custom one using options:

Validation.addAllThese('validate-password', 'Your password must be more than 6 characters and not be 'password' or the same as your name', { minLength : 7, notOneOf : ['password','PASSWORD','1234567','0123456'], notEqualToField : 'username' });

If you supply a custom function and a combination of options they are all tested and if all are true the field validates.

When you add a new validator it is added to a static group of validation methods with the class name as key. You then must use the class in the form elements to use your custom validation function.

To make adding mupltiple custom validators easier you can use Validation.addAllThese() like this:

Validation.addAllThese([ ['required', 'This is a required field.', function(v) { return !Validation.get('IsEmpty').test(v); }], ['validate-number', 'Please use numbers only in this field.', function(v) { return Validation.get('IsEmpty').test(v) || !isNaN(v); }], ['validate-digits', 'Please use numbers only in this field.', function(v) { return Validation.get('IsEmpty').test(v) || !/[^d]/.test(v); }] ]);

You pass an array, where each element of the array is an array with 3 or 4 elements: [className, error, function, options] or [className, error, options]

Original article : http://tetlaw.id.au/view/javascript/really-easy-field-validation