---
title: Custom Form Validation Using an Event
product: Form Library
description: Learn how to add custom validation to any input field of a form to ensure submitted data matches the requirements set out. View this free demo for JavaScript.
framework: React
source: https://surveyjs.io/form-library/examples/add-custom-input-validation/reactjs
index: https://surveyjs.io/form-library/examples/overview.md
---

# Custom Form Validation Using an Event (React)

Custom JavaScript validation for forms allows you to create more specific input validation logic than [built-in validators](https://surveyjs.io/form-library/examples/javascript-form-validation/). In SurveyJS Form Library, you can add custom validation in two ways—with the [`onValidateQuestion`](https://surveyjs.io/form-library/documentation/surveymodel#onValidateQuestion) event or expressions. This example demonstrates how to handle the event. Switch between React, Vue, jQuery, Angular, and Vanilla JS to view the example for your JavaScript framework/library.

## Event Handler vs Expressions

### Event Handler

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.

### Expressions

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](https://surveyjs.io/form-library/examples/add-custom-survey-data-validation/).

## Handle the `onValidateQuestion` Event

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`](https://surveyjs.io/form-library/documentation/surveymodel#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.

## Files

### `public/index.html`

```html
<div id="surveyElement" style="position: absolute; top: 0; left: 0; right: 0; bottom: 0; min-height: 100%; height:100%"></div>
```

### `src/SurveyComponent.jsx`

```js
import React from "react";
import { Model } from "survey-core";
import { Survey } from "survey-react-ui";
import "survey-core/survey-core.min.css";
import "./index.css";
import { json } from "./json";

function validateDomain(_, options) {
  if (options.name === "email") {
    const email = options.value;

    if (!email)
      return;

    const domain = email.trim().substring(email.indexOf("@") + 1);
    if (domain !== "surveyjs.io") {
      options.error = "Please enter an e-mail address hosted at surveyjs.io.";
    }
  }
}
function SurveyComponent() {
    const survey = new Model(json);
    survey.onComplete.add((sender, options) => {
        console.log(JSON.stringify(sender.data, null, 3));
    });
    survey.onValidateQuestion.add(validateDomain);
    return (<Survey model={survey} />);
}

export default SurveyComponent;
```

### `src/index.css`

```css
/* You can add your custom CSS here. */
```

### `src/index.js`

```js
import React from "react";
import { createRoot } from "react-dom/client";
import SurveyComponent from "./SurveyComponent";

const root = createRoot(document.getElementById("surveyElement"));
root.render(<SurveyComponent />);
```

### `src/json.js`

```js
export const json = {
  "checkErrorsMode": "onValueChanged",
  "elements": [
    {
      "type": "text",
      "name": "email",
      "isRequired": true,
      "title": "Enter your work e-mail address"
    }
  ]
};
```

### `src/theme.js`

```js
export const themeJson = {};
```

### `package.json`

```json
{
  "dependencies": {
    "react": "latest",
    "react-dom": "latest",
    "survey-core": "latest",
    "survey-react-ui": "latest"
  },
  "devDependencies": {
    "react-scripts": "latest"
  }
}
```

## Other Frameworks

- [Angular](https://surveyjs.io/form-library/examples/add-custom-input-validation/angular.md)
- [Vue 3](https://surveyjs.io/form-library/examples/add-custom-input-validation/vue3js.md)
- [jQuery](https://surveyjs.io/form-library/examples/add-custom-input-validation/jquery.md)
- [Vanilla JS](https://surveyjs.io/form-library/examples/add-custom-input-validation/vanillajs.md)