---
title: Custom Form Validation Using Expressions
product: Form Library
description: Validate input fields of a form to ensure that entered values satisfy the given validation rules. Refer to this free demo for JavaScript to learn more.
framework: Angular
source: https://surveyjs.io/form-library/examples/add-custom-survey-data-validation/angular
index: https://surveyjs.io/form-library/examples/overview.md
---
# Custom Form Validation Using Expressions (Angular)
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/). SurveyJS Form Library supports two ways to add custom validation—with an event or [expressions](https://surveyjs.io/form-library/documentation/design-survey-conditional-logic#expressions). This example demonstrates how to configure expressions. 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. Refer to the following demo for more information: [Custom Form Validation Using an Event](https://surveyjs.io/form-library/examples/add-custom-input-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.
## Implement a Custom Validator
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).
1. Register the function.
The following code registers the `myFunc` function under the name `foo`:
```js
import { registerFunction } from "survey-core";
registerFunction({ name: "foo", func: myFunc });
```
1. 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`](https://surveyjs.io/form-library/documentation/surveymodel#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.
## Files
### `src/app/components/json.ts`
```ts
export const json = {
"checkErrorsMode": "onValueChanged",
"elements": [
{
"type": "text",
"name": "email",
"isRequired": true,
"title": "Enter your work e-mail address",
"validators": [{
"type": "expression",
"expression": "isDomainValid({email})",
"text": "Please enter an e-mail address hosted at surveyjs.io."
}]
}
]
};
```
### `src/app/components/survey.component.css`
```css
/* You can add your custom CSS here. */
```
### `src/app/components/survey.component.html`
```html
```
### `src/app/components/survey.component.ts`
```ts
import { Component, OnInit } from "@angular/core";
import { Model } from "survey-core";
import { json } from "./json";
import "./survey.component.css";
import "survey-core/survey-core.min.css";
import { registerFunction } from "survey-core";
function isDomainValid([email]) {
if (typeof email !== 'string' || !email.includes('@')) {
return false;
}
const domain = email.trim().substring(email.indexOf("@") + 1);
return domain === "surveyjs.io";
}
registerFunction({ name: "isDomainValid", func: isDomainValid });
@Component({
// tslint:disable-next-line:component-selector
selector: "component-survey",
templateUrl: "./survey.component.html",
styleUrls: ["./survey.component.css"]
})
export class SurveyComponent implements OnInit {
model: Model;
ngOnInit() {
const survey = new Model(json);
survey.onComplete.add((sender, options) => {
console.log(JSON.stringify(sender.data, null, 3));
});
this.model = survey;
}
}
```
### `src/app/components/theme.ts`
```ts
export const themeJson = {};
```
### `src/app/app.component.css`
```css
```
### `src/app/app.component.html`
```html
```
### `src/app/app.component.ts`
```ts
import { Component } from "@angular/core";
@Component({
selector: "app-root",
templateUrl: "./app.component.html",
styleUrls: ["./app.component.css"]
})
export class AppComponent {
title = "CodeSandbox";
}
```
### `src/app/app.module.ts`
```ts
import { BrowserModule } from "@angular/platform-browser";
import { NgModule } from "@angular/core";
import { AppComponent } from "./app.component";
import { SurveyModule } from "survey-angular-ui";
import { SurveyComponent } from "./components/survey.component";
@NgModule({
declarations: [AppComponent, SurveyComponent],
imports: [BrowserModule, SurveyModule],
providers: [],
bootstrap: [AppComponent]
})
export class AppModule { }
```
### `src/environments/environment.prod.ts`
```ts
export const environment = {
production: true
};
```
### `src/environments/environment.ts`
```ts
// The file contents for the current environment will overwrite these during build.
// The build system defaults to the dev environment which uses `environment.ts`, but if you do
// `ng build --env=prod` then `environment.prod.ts` will be used instead.
// The list of which env maps to which file can be found in `.angular-cli.json`.
export const environment = {
production: false
};
```
### `src/index.html`
```html
```
### `src/main.ts`
```ts
import { enableProdMode } from "@angular/core";
import { platformBrowserDynamic } from "@angular/platform-browser-dynamic";
import { AppModule } from "./app/app.module";
import { environment } from "./environments/environment";
if (environment.production) {
enableProdMode();
}
platformBrowserDynamic()
.bootstrapModule(AppModule)
.catch(err => console.log(err));
```
### `src/polyfills.ts`
```ts
/**
* This file includes polyfills needed by Angular and is loaded before the app.
* You can add your own extra polyfills to this file.
*
* This file is divided into 2 sections:
* 1. Browser polyfills. These are applied before loading ZoneJS and are sorted by browsers.
* 2. Application imports. Files imported after ZoneJS that should be loaded before your main
* file.
*
* The current setup is for so-called "evergreen" browsers; the last versions of browsers that
* automatically update themselves. This includes Safari >= 10, Chrome >= 55 (including Opera),
* Edge >= 13 on the desktop, and iOS 10 and Chrome on mobile.
*
* Learn more in https://angular.io/docs/ts/latest/guide/browser-support.html
*/
/***************************************************************************************************
* BROWSER POLYFILLS
*/
/** IE9, IE10 and IE11 requires all of the following polyfills. **/
// import 'core-js/es6/symbol';
// import 'core-js/es6/object';
// import 'core-js/es6/function';
// import 'core-js/es6/parse-int';
// import 'core-js/es6/parse-float';
// import 'core-js/es6/number';
// import 'core-js/es6/math';
// import 'core-js/es6/string';
// import 'core-js/es6/date';
// import 'core-js/es6/array';
// import 'core-js/es6/regexp';
// import 'core-js/es6/map';
// import 'core-js/es6/weak-map';
// import 'core-js/es6/set';
/** IE10 and IE11 requires the following for NgClass support on SVG elements */
// import 'classlist.js'; // Run `npm install classlist.js`.
/** IE10 and IE11 requires the following for the Reflect API. */
// import 'core-js/es6/reflect';
/** Evergreen browsers require these. **/
// Used for reflect-metadata in JIT. If you use AOT (and only Angular decorators), you can remove.
import "core-js/proposals/reflect-metadata";
/**
* Required to support Web Animations `@angular/platform-browser/animations`.
* Needed for: All but Chrome, Firefox and Opera. http://caniuse.com/#feat=web-animation
**/
// import 'web-animations-js'; // Run `npm install web-animations-js`.
/***************************************************************************************************
* Zone JS is required by default for Angular itself.
*/
import "zone.js/dist/zone"; // Included with Angular CLI.
/***************************************************************************************************
* APPLICATION IMPORTS
*/
```
### `src/styles.css`
```css
/* You can add your custom CSS here. */
```
### `src/typings.d.ts`
```ts
/* SystemJS module definition */
declare var module: NodeModule;
interface NodeModule {
id: string;
}
```
### `.angular-cli.json`
```json
{
"apps": [
{
"root": "src",
"outDir": "dist",
"assets": [ "assets", "favicon.ico" ],
"index": "index.html",
"main": "main.ts",
"polyfills": "polyfills.ts",
"prefix": "app",
"styles": [ "styles.css" ],
"scripts": [ ],
"environmentSource": "environments/environment.ts",
"environments": {
"dev": "environments/environment.ts",
"prod": "environments/environment.prod.ts"
}
}
]
}
```
### `_tsconfig.json`
```json
{
"compileOnSave": false,
"compilerOptions": {
"baseUrl": "./",
"outDir": "./dist/out-tsc",
"sourceMap": true,
"declaration": false,
"allowSyntheticDefaultImports": true,
"downlevelIteration": true,
"experimentalDecorators": true,
"moduleResolution": "node",
"importHelpers": true,
"resolveJsonModule": true,
"target": "es2015",
"module": "es2020",
"lib": [
"es2018",
"dom"
]
}
}
```
### `package.json`
```json
{
"name": "surveyjs-angular",
"scripts": {
"ng": "ng",
"start": "ng serve",
"build": "ng build --prod",
"test": "ng test",
"lint": "ng lint",
"e2e": "ng e2e"
},
"private": true,
"dependencies": {
"@angular/animations": "14.1.1",
"@angular/cdk": "14.1.1",
"@angular/common": "14.1.1",
"@angular/compiler": "14.1.1",
"@angular/core": "14.1.1",
"@angular/forms": "14.1.1",
"@angular/platform-browser": "14.1.1",
"@angular/platform-browser-dynamic": "14.1.1",
"@angular/router": "14.1.1",
"core-js": "3.6.4",
"rxjs": "6.5.4",
"survey-core": "latest",
"survey-angular-ui": "latest",
"tslib": "1.13.0",
"zone.js": "0.11.7"
},
"devDependencies": {
"@angular/cli": "1.6.6",
"@angular/compiler-cli": "^5.2.0",
"@angular/language-service": "^5.2.0",
"@types/core-js": "0.9.46",
"@types/jasmine": "~2.8.3",
"@types/jasminewd2": "~2.0.2",
"@types/node": "~6.0.60",
"codelyzer": "^4.0.1",
"jasmine-core": "~2.8.0",
"jasmine-spec-reporter": "~4.2.1",
"karma": "~2.0.0",
"karma-chrome-launcher": "~2.2.0",
"karma-coverage-istanbul-reporter": "^1.2.1",
"karma-jasmine": "~1.1.0",
"karma-jasmine-html-reporter": "^0.2.2",
"protractor": "~5.1.2",
"ts-node": "~4.1.0",
"tslint": "~5.9.1",
"typescript": "3.9.7"
},
"keywords": [ "angular", "surveyjs" ],
"description": "SurveyJS-Angular example project"
}
```
## Other Frameworks
- [React](https://surveyjs.io/form-library/examples/add-custom-survey-data-validation/reactjs.md)
- [Vue 3](https://surveyjs.io/form-library/examples/add-custom-survey-data-validation/vue3js.md)
- [jQuery](https://surveyjs.io/form-library/examples/add-custom-survey-data-validation/jquery.md)
- [Vanilla JS](https://surveyjs.io/form-library/examples/add-custom-survey-data-validation/vanillajs.md)