Custom Form Validation Using an Event

Custom JavaScript validation for forms allows you to create more specific input validation logic than built-in validators. In SurveyJS Form Library, you can add custom validation in two ways—with the onValidateQuestion event or expressions. This example demonstrates how to handle the event. Switch between React, Vue, Knockout, jQuery, and Angular to view the example for your JavaScript framework.

To create custom form validation in JavaScript with the onValidateQuestion event, you need to implement a corresponding event handler. Users have no access to this handler and therefore cannot disable custom validation.

For custom validation with expressions, you need to implement and register a custom validator in your JavaScript code to make the validator available within expressions. Users cannot modify the validator, but they can decide whether or not to use it in their expressions when they design their surveys and thus enable or disable custom validation. Refer to the following demo for more information: Custom Form Validation Using Expressions.

An onValidateQuestion event handler accepts the survey as the first argument and an object with the following fields as the second argument:

• question - The question being validated.
• name - The question's name.
• value - A question value being validated.
• error - An error message that you should specify if validation fails.

In this custom form validation example, the form field accepts only e-mail addresses hosted at surveyjs.io. A callback attached to the onValidateQuestion event handler validates the entered e-mail address and assigns an error message to the options.error field if the address is invalid.

The onValidateQuestion event may occur and run validation at different points in your application, depending on the checkErrorsMode property. In this demo, the event occurs immediately after you change the form field value because checkErrorsMode is set to "onValueChanged". If you do not set this property, validation activates before a user proceeds to the next page or completes the survey.