---
title: ExpressionValidator
product: Form Library
api-type: class
description: A class that implements validation using expressions.
source: https://surveyjs.io/form-library/documentation/api-reference/expressionvalidator
---

# `ExpressionValidator`

A class that implements validation using [expressions](https://surveyjs.io/form-library/documentation/design-survey/conditional-logic#expressions).

[View Demo](https://surveyjs.io/form-library/examples/javascript-form-validation/ (linkStyle))

## Inheritance

[`Base`](https://surveyjs.io/form-library/documentation/api-reference/base.md) → [`SurveyValidator`](https://surveyjs.io/form-library/documentation/api-reference/surveyvalidator.md) → `ExpressionValidator`

## Properties

### `expression`

**Type**: `string`

A Boolean [expression](https://surveyjs.io/form-library/documentation/design-survey/conditional-logic#expressions). If it evaluates to `false`, validation fails.

[View Demo](https://surveyjs.io/form-library/examples/javascript-form-validation/ (linkStyle))