---
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: jQuery
source: https://surveyjs.io/form-library/examples/lazy-loading-dropdown/jquery
index: https://surveyjs.io/form-library/examples/overview.md
---

# Lazy Loading (jQuery)

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

### `src/index.css`

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

### `src/index.js`

```js
import $ from "jquery";
import { Model } from "survey-core";
import "survey-js-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();
}

$("#surveyElement").Survey({ model: survey });
```

### `src/json.js`

```js
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/theme.js`

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

### `package.json`

```json
{
  "dependencies": {
    "jquery": "latest",
    "survey-core": "latest",
    "survey-js-ui": "latest"
  }
}
```

## 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)
- [Vue 3](https://surveyjs.io/form-library/examples/lazy-loading-dropdown/vue3js.md)
- [Vanilla JS](https://surveyjs.io/form-library/examples/lazy-loading-dropdown/vanillajs.md)