---
title: Lazy Loading
product: Form Library
description: Reduce form load time in your JavaScript app with automatic lazy loading. Server-side filtering allows easy search by an item being typed even if only a part of a drop-down list has been loaded.
framework: Vue 3
source: https://surveyjs.io/form-library/examples/lazy-loading-dropdown/vue3js
index: https://surveyjs.io/form-library/examples/overview.md
---

# Lazy Loading (Vue 3)

Lazy loading (also "on-demand loading" and "pagination") is a technique that delays loading non-critical data until it is required. This technique improves performance and reduces form load time. SurveyJS Form Library allows you to paginate choices in Single- and Multi-Select Dropdown questions. This feature is configured in the same manner for both types. In this example the first Dropdown lazy-loads the entire list of countries, while the other two are cascading Dropdowns: you first select a region, and then lazy-load only the countries within that region. You can also type in a search string to filter the country list on the server. Switch between available JavaScript frameworks to view a dedicated demo version for jQuery, React, Angular, Vue.js, or Vanilla JavaScript.

## Configure On-Demand Loading

To configure lazy loading, follow the steps below:

1. Handle the [`onChoicesLazyLoad`](https://surveyjs.io/form-library/documentation/surveymodel#onChoicesLazyLoad) event.         
This event occurs when a question requests the next portion of choice items. Within the event handler, you need to send a request for the next portion to the server. Refer to the event description for information on the handler's parameters.

1. Enable a question's [`choicesLazyLoadEnabled`](https://surveyjs.io/form-library/documentation/questiondropdownmodel#choicesLazyLoadEnabled) property.            
This property specifies that choices for the given question should be loaded on demand.

## Preload the Default Value

When you load choices from a server, choice display texts become available only after data is loaded, which does not happen until a user opens the Dropdown or Tag Box. However, a display text for a default choice item is required before that. In this case, you can individually load data for the default item within the [`onGetChoiceDisplayValue`](https://surveyjs.io/form-library/documentation/surveymodel#onGetChoiceDisplayValue) event handler. Refer to the event description for information on the handler's parameters.

In this demo, the `onGetChoiceDisplayValue` event handler is used to get the display text "France" for the default value "FRA".

## Implement Lazy Loading in Cascading Dropdowns

In cascading dropdowns, the options of a dependent dropdown are determined by the selected value in a parent dropdown. When implementing lazy loading in this scenario, pay attention to the following details:

- In the `onChoicesLazyLoad` event handler, you need to include the parent dropdown's value in the query string of your API request, along with the `skip`, `take`, and `filter` parameters.
- Use the [`resetValueIf`](https://surveyjs.io/form-library/documentation/api-reference/dropdown-menu-model#resetValueIf) expression to automatically clear the dependent dropdown's value when the parent dropdown changes. In the following code, `region` is the name of the parent dropdown:

    ```js
    "resetValueIf": "{region} empty || {region} notempty"
    ```

The rest of the setup is the same as for a regular lazy-loaded dropdown.

## See Also

[Lazy Loading for Long Forms](/form-library/examples/survey-lazy/ (linkStyle))

## Files

### `public/index.html`

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

### `src/App.vue`

```html
<template>
    <SurveyComponent :model="survey" />
</template>
<script setup lang="ts">
    import { Model } from "survey-core";
    import { SurveyComponent } from "survey-vue3-ui";
    import "survey-core/survey-core.min.css";
    import "./index.css";
    import { json } from "./json";

    const survey = new Model(json);
    survey.onComplete.add((sender, options) => {
        console.log(JSON.stringify(sender.data, null, 3));
    });
    survey.onChoicesLazyLoad.add((_, options) => {
      if (options.question.getType() !== "dropdown") return;
    
      // Form query strings for different questions
      let url;
      if (options.question.name === "country") {
        url = `https://surveyjs.io/api/CountriesExamplePagination?skip=${options.skip}&take=${options.take}`;
      }
      if (options.question.name === "reg_country") {
        url = `https://surveyjs.io/api/GetCountriesPaginationRegionFilter?skip=${options.skip}&take=${options.take}`
        const region = survey.getValue("region"); // Obtain a value of the "Region" field
        if (region) {
          url += `&region=${encodeURIComponent(region)}`;
        }
      }
      if (options.filter) {
        url += `&filter=${encodeURIComponent(options.filter)}`;
      }
      sendRequest(url, (data) => { options.setItems(data.countries, data.total); });
    });
    
    survey.onGetChoiceDisplayValue.add((_, options) => {
      if (options.question.getType() !== "dropdown") return;
    
      if (options.question.name === "country" || options.question.name === "reg_country") {
        const valuesStr = options.values.map(value => "values=" + value).join("&");
        const url = 'https://surveyjs.io/api/GetCountryNames?' + valuesStr;
        sendRequest(url, (data) => { options.setItems(data.countryNames); });
      }
    });
    
    function sendRequest(url, onloadSuccessCallback) {
      const xhr = new XMLHttpRequest();
      xhr.open("GET", url);
      xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
      xhr.onload = () => {
        if (xhr.status === 200) {
          onloadSuccessCallback(JSON.parse(xhr.response));
        }
      };
      xhr.send();
    }
</script>
```

### `src/index.css`

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

### `src/json.ts`

```ts
export const json = {
  "elements": [
    {
      "type": "dropdown",
      "name": "country",
      "title": "Select a country",
      "isRequired": true,
      "defaultValue": "FRA",
      "choicesLazyLoadEnabled": true,
      "choicesLazyLoadPageSize": 40
    },
    {
      "type": "panel",
      "name": "countriesByRegion",
      "title": "Filter countries by selected region",
      "description": "Only countries from the selected region are queried.",
      "elements": [{
        "type": "dropdown",
        "name": "region",
        "title": "Select a region",
        "choices": ["Africa", "Americas", "Asia", "Europe", "Oceania"]
      }, {
        "type": "dropdown",
        "name": "reg_country",
        "title": "Select a country",
        "choicesLazyLoadEnabled": true,
        "resetValueIf": "{region} empty || {region} notempty"
      }]
    }
  ]
};
```

### `src/main.ts`

```ts
import { createApp } from "vue";
import App from "./App.vue";

const app = createApp(App);
app.mount("#app");
```

### `src/shims-vue.d.ts`

```ts
/* eslint-disable */
declare module "*.vue" {
    import type { DefineComponent } from "vue"
    const component: DefineComponent<{}, {}, any>
    export default component
}
```

### `src/theme.ts`

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

### `.eslintrc.js`

```js
module.exports = {
    root: true,
    env: {
        node: true
    },
    extends: [
        "plugin:vue/vue3-essential",
        "eslint:recommended",
        "@vue/typescript/recommended",
        "@vue/prettier",
        "@vue/prettier/@typescript-eslint"
    ],
    parserOptions: {
        ecmaVersion: 2020
    },
    rules: {
        "no-console": process.env.NODE_ENV === "production" ? "warn" : "off",
        "no-debugger": process.env.NODE_ENV === "production" ? "warn" : "off"
    },
    overrides: [
        {
            files: [
                "**/__tests__/*.{j,t}s?(x)",
                "**/tests/unit/**/*.spec.{j,t}s?(x)"
            ],
            env: {
                jest: true
            }
        }
    ]
};
```

### `babel.config.js`

```js
module.exports = {
  presets: ["@vue/cli-plugin-babel/preset"]
};
```

### `package.json`

```json
{
  "name": "surveyjs-library-vue3",
  "version": "0.1.0",
  "scripts": {
    "serve": "vue-cli-service serve",
    "build": "vue-cli-service build",
    "lint": "vue-cli-service lint"
  },
  "dependencies": {
    "core-js": "^3.6.5",
    "tslib": "2.6.1",
    "vue": "^3.4.1",
    "survey-core": "latest",
    "survey-vue3-ui": "latest",
    "vue-router": "^4.0.0-0",
    "vuex": "^4.0.0-0"
  },
  "devDependencies": {
    "@typescript-eslint/eslint-plugin": "^2.33.0",
    "@typescript-eslint/parser": "^2.33.0",
    "@vue/cli-plugin-babel": "~4.5.0",
    "@vue/cli-plugin-eslint": "~4.5.0",
    "@vue/cli-plugin-pwa": "~4.5.0",
    "@vue/cli-plugin-router": "~4.5.0",
    "@vue/cli-plugin-typescript": "~4.5.0",
    "@vue/cli-plugin-vuex": "~4.5.0",
    "@vue/cli-service": "~4.5.0",
    "@vue/compiler-sfc": "^3.0.0",
    "@vue/eslint-config-prettier": "^6.0.0",
    "@vue/eslint-config-typescript": "^5.0.2",
    "@vue/test-utils": "^2.0.0-0",
    "eslint": "^6.7.2",
    "eslint-plugin-prettier": "^3.1.3",
    "eslint-plugin-vue": "^7.0.0-0",
    "node-sass": "^4.12.0",
    "prettier": "^1.19.1",
    "sass-loader": "^8.0.2",
    "typescript": "~3.9.3"
  }
}
```

### `tsconfig.json`

```json
{
  "compilerOptions": {
    "target": "esnext",
    "module": "esnext",
    "strict": true,
    "jsx": "preserve",
    "importHelpers": true,
    "moduleResolution": "node",
    "skipLibCheck": true,
    "esModuleInterop": true,
    "allowSyntheticDefaultImports": true,
    "sourceMap": true,
    "baseUrl": ".",
    "types": [
      "webpack-env",
      "jest"
    ],
    "paths": {
      "@/*": [
        "src/*"
      ]
    },
    "lib": [
      "esnext",
      "dom",
      "dom.iterable",
      "scripthost"
    ]
  },
  "include": [
    "src/**/*.ts",
    "src/**/*.tsx",
    "src/**/*.vue",
    "tests/**/*.ts",
    "tests/**/*.tsx"
  ],
  "exclude": [
    "node_modules"
  ]
}
```

## Other Frameworks

- [Angular](https://surveyjs.io/form-library/examples/lazy-loading-dropdown/angular.md)
- [React](https://surveyjs.io/form-library/examples/lazy-loading-dropdown/reactjs.md)
- [jQuery](https://surveyjs.io/form-library/examples/lazy-loading-dropdown/jquery.md)
- [Vanilla JS](https://surveyjs.io/form-library/examples/lazy-loading-dropdown/vanillajs.md)