---
title: Asynchronous Functions in Expressions
product: Form Library
description: Explore this interactive demo for JavaScript showing how to use asynchronous functions in expression questions for dynamic survey content. Follow our step-by-step instructions to implement custom asynchronous functions in your SurveyJS form.
framework: Angular
source: https://surveyjs.io/form-library/examples/asynchronous-functions-in-expression-questions/angular
index: https://surveyjs.io/form-library/examples/overview.md
---

# Asynchronous Functions in Expressions (Angular)

Expression questions support custom asynchronous functions that you can use for time-consuming operations, such as requests to a server. In this example, you can select a country from a drop-down list. Two Expression questions use asynchronous functions to load and display the selected country's region and official name.

To implement an asynchronous function for use in expressions, follow the steps below:

1. Declare an asynchronous JavaScript function.      
This function accepts all arguments in one array-like object. You can pass as many arguments as needed when you call the function from an expression. To return the function's result, pass it to the `this.returnResult` function.

    ```js
    function asyncFunc(params) {
        const arg1 = params[0];
        const arg2 = params[1];
        // ...
        setTimeout(() => {
            // Return the function result via the callback
            this.returnResult(yourValue);
        }, 100);
    }
    ```

1. Register your function.     
Call a static `registerFunction` method. It accepts a function name that you want to use in expressions, the function itself, and the `isAsync` flag that indicates an asynchronous function.

    ```js
    import { registerFunction } from "survey-core";

    registerFunction({
        name: "asyncFunc",
        func: asyncFunc,
        isAsync: true
    });
    ```

1. Use your function in an Expression question.     
Specify the question's [`expression`](https://surveyjs.io/form-library/documentation/api-reference/expression-model#expression) property.

    ```js
    const surveyJson = {
        "elements": [{
            "type": "expression",
            "name": "asyncExpression",
            // ...
            "expression": "asyncFunc({question1}, {question2})"
        }, {
            // ...
        }]
    };
    ```

1. *(Optional)* Hide an Expression question until it has a value.       
You may want to hide your Expression question until its asynchronous function has executed. To do it, set the [`visibleIf`](https://surveyjs.io/form-library/documentation/api-reference/expression-model#visibleIf) property to a Boolean expression that uses the `notempty` function. Refer to the following help topic for more information: [Conditional Visibility](https://surveyjs.io/form-library/documentation/design-survey/conditional-logic#conditional-visibility).

    ```js
    const surveyJson = {
        "elements": [{
            "type": "expression",
            "name": "asyncExpression",
            // ...
            "expression": "asyncFunc({question1}, {question2})",
            "visibleIf": "{asyncExpression} notempty"
        }, {
            // ...
        }]
    };
    ```

## Files

### `src/app/components/json.ts`

```ts
export const json = {
  "checkErrorsMode": "onValueChanged",
  "elements": [
    {
      "type": "dropdown",
      "name": "country",
      "title": "Select a country",
      "choicesByUrl": {
        "url": "https://surveyjs.io/api/CountriesExample",
        "valueName": "name"
      }
    },
    {
      "type": "expression",
      "name": "officialname",
      "title": "Official name of {country} is:",
      "expression": "getOfficialCountryName({country})",
      "visibleIf": "{officialname} notempty"
    },
    {
      "type": "expression",
      "name": "region",
      "title": "{country} is located in:",
      "expression": "getCountryRegion({country})",
      "visibleIf": "{region} notempty"
    }
  ]
};
```

### `src/app/components/survey.component.css`

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

### `src/app/components/survey.component.html`

```html
<survey [model]="model" style="position: absolute; top: 0; left: 0; right: 0; bottom: 0; height:100%"></survey>
```

### `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 getCountryInfo(country, returnResultCallback, property) {
    if (!country) {
        returnResultCallback();
        return;
    }
    fetch("https://surveyjs.io/api/CountriesExample?name=" + country)
        .then(response => response.json())
        .then(data => {
            const countryInfo = data[0];
            returnResultCallback(countryInfo[property]);
        })
        .catch(error => {
            console.error("Error:", error);
        });
}

function getOfficialCountryName([country]) {
    return getCountryInfo(country, this.returnResult, "officialName");
}

function getCountryRegion([country]) {
    return getCountryInfo(country, this.returnResult, "region");
}
  
registerFunction({
    name: "getOfficialCountryName",
    func: getOfficialCountryName,
    isAsync: true
});

registerFunction({
    name: "getCountryRegion",
    func: getCountryRegion,
    isAsync: true
});
@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
<component-survey></component-survey>
```

### `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
<app-root></app-root>
```

### `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/asynchronous-functions-in-expression-questions/reactjs.md)
- [Vue 3](https://surveyjs.io/form-library/examples/asynchronous-functions-in-expression-questions/vue3js.md)
- [jQuery](https://surveyjs.io/form-library/examples/asynchronous-functions-in-expression-questions/jquery.md)
- [Vanilla JS](https://surveyjs.io/form-library/examples/asynchronous-functions-in-expression-questions/vanillajs.md)