Windows® 8.1 Apps with HTML5 and JavaScript Unleashed

Performing Custom Validation

If you need to add custom validation rules to a form element then you can take advantage of the JavaScript setCustomValidity() method. You can use this method to associate a custom validation error message with a form field.

Imagine, for example, that you have a complex set of rules for validating a username in a user registration form. For example, you want to ensure that the username is a certain length, unique in the database, and does not contain special characters. This is a good candidate for a custom validation.

The following HTML form includes a userName field:

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<form>    <div>        <label>            User Name:            <input id="userName" required />        </label>     ...