Custom Form Validation Using Expressions

Custom JavaScript validation for forms allows you to create more specific input validation logic than built-in validators. SurveyJS Form Library supports two ways to add custom validation—with an event or expressions. This example demonstrates how to configure expressions. 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. Refer to the following demo for more information: Custom Form Validation Using an Event.

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.

Follow the steps below to implement a custom validator:

1. Create a JavaScript function that validates data.
   This function should return a Boolean value that indicates a validation result (true if the value is valid, false otherwise).

2. Register the function in FunctionFactory.
   The following code registers the myFunc function under the name foo:

   import { FunctionFactory } from "survey-core";
   FunctionFactory.Instance.register("foo", myFunc);
   

3. Use the validator within expressions.
   To reference the validator within an expression, use curly brackets: {foo}.

In this custom form validation example, the form field accepts only e-mail addresses hosted at surveyjs.io. The isDomainValid custom validator checks whether the entered e-mail address is valid. If the validation fails, the form field displays an error message. Validation is triggered immediately after you leave the input field because the Survey's checkErrorsMode property 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.