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    VScroll is a JavaScript library providing virtual scroll engine. Can be seen as a core for platform-specific solutions designed to represent unlimited datasets using virtualization technique. Below is the diagram of how the VScroll engine is being distributed to the end user.

    Basically, the consumer layer can be omitted and the end Application developers can use VScroll directly. Currently there exist two consumer implementations built on top of VScroll:

    Getting started


    <script src="https://cdn.jsdelivr.net/npm/vscroll"></script>
      const workflow = new VScroll.Workflow({...});


    npm install vscroll
    import { Workflow } from 'vscroll';
    const workflow = new Workflow({...});


    The main entity distributed via vscroll is the Workflow class. Its instantiating runs the virtual scroll engine. The constructor of the Workflow class requires an argument of the following type:

    interface WorkflowParams<ItemData> {
      consumer: IPackage;
      element: HTMLElement;
      datasource: IDatasource<ItemData>;
      run: OnDataChanged<ItemData>;

    This is a TypeScript definition, but speaking of JavaScript, an argument object must contain 4 fields described below.

    1. Consumer

    A simple data object that provides information about a consumer. It is not critical to omit this, but if the result solution is going to be published as a separate 3d-party library ("consumer"), the name and the version of the result package should be passed as follows:

    consumer: {
      name: 'my-vscroll-consumer',
      version: 'v1.0.0-alpha.1'

    2. Element

    An HTML element the Workflow should use as a scrollable part of the viewport. It should be present in DOM before instantiating the Workflow.

    element: document.getElementById('vscroll'),

    This element should be wrapped with another container with constrained height and overflow scroll/auto. And it also must have two special padding elements marked with special attributes for the virtualization purpose.

    <div id="viewport">
      <div id="vscroll">
        <div data-padding-backward></div>
        <div data-padding-forward></div>
    #viewport {
      height: 300px;
      overflow-y: scroll;

    3. Datasource

    This is a special object, providing dataset items in runtime. Basically, a consumer app should expose a Datasource factory to be used by the end App, but in the simplest case it can be defined as follows:

    datasource: {
      get: (index, count, success) => {
        const data = [];
        for (let i = index; i <= index + count - 1; i++) {
          data.push({ id: i, text: 'item #' + i });

    The Workflow will request data via get method. This particular Datasource sample implements an unlimited synchronous stream of data generated in runtime, but depends on the end App needs it can be limited or partially limited, asynchronous, index-inverted, powered with cache, Promise- or even Observable-based (instead of Callback-based) etc.

    Along with the Workflow, VScroll exposes method makeDatasource which can be used for creating Datasource factory, so the end Datasource might be instantiated via operator new:

    const Datasource = VScroll.makeDatasource();
    datasource: new Datasource({
      get: (index, count, success) => ...

    This option also makes the Adapter API available via datasource.adapter property after the Datasource is instantiated. It provides massive functionality to assess and manipulate Scroller's data at runtime.

    Using TypeScript, the above example should be written as follows:

    interface Data {
      id: number;
      text: string;
    const Datasource = VScroll.makeDatasource();
    const datasource = new Datasource<Data>(...);

    This obliges the Datasource.get method to deal with Data items and also provides strong typing for Adapter API props and methods.

    4. Run

    A callback that is called every time the Workflow decides that the UI needs to be changed. Its argument is a list of items to be present in the UI. This is a consumer responsibility to detect changes and display them in the UI.

    run: items => {
      // assume currentItems contains a list of items currently presented in the UI
      if (!items.length && !currentItems.length) {
      displayNewItemsInsteadCurrentOnes(items, currentItems);
      currentItems = items;

    Each item is an instance of the Item class implementing the Item interface, whose props can be used for proper implementation of the run callback:

    Name Type Description
    element HTMLElement HTML element associated with the item
    $index number Integer index of the item in the Datasource. Correlates with the first argument of the Datasource.get method
    data Data Data (contents) of the item. This is what the Datasource.get passes to the Scroller via success-callback as an array of data-items typed as Data[]
    invisible boolean A flag that determines whether the item should be hidden (if true) or visible (if false) when the run method is called
    get () => ItemAdapter<Data> A shortcut method returning { element, $index, data } object

    Run callback is the most complex and environment-specific part of the vscroll API, which is fully depends on the environment for which the consumer is being created. Framework specific consumer should rely on internal mechanism of the framework to provide runtime DOM modifications.

    There are some requirements on how the items should be processed by run(items) call:

    • after the run(items) callback is completed, there must be items.length elements in the DOM between backward and forward padding elements;
    • old items that are not in the list should be removed from DOM; use currentItems[].element reference for this purpose;
    • old items that are in the list should not be removed and recreated, as it may lead to an unwanted shift of the scroll position; just don't touch them;
    • new items elements should be rendered in accordance with items[].$index comparable to $index of elements that remain: $index must increase continuously and the directions of increase must persist across the run calls; Scroller maintains $index internally, so you only need to properly inject the items[].element into the DOM;
    • new elements should be rendered but not visible, and this should be achieved by "fixed" positioning and "left"/"top" coordinates placing the item element out of view; the Workflow will take care of visibility after calculations; an additional attribute items[].invisible can be used to determine if a given element should be hidden;
    • new items elements should have "data-sid" attribute, which value should reflect items[].$index;


    This repository has a minimal demonstration of the App-consumer implementation considering all of the requirements listed above: https://dhilt.github.io/vscroll/. This is all-in-one HTML demo with vscroll taken from CDN. The source code of the demo is here. The approach is rough and non-optimized, if you are seeking for more general solution for native JavaScript applications, please take a look at vscroll-native project. It is relatively new and has no good documentation, but its source code and demo may shed light on vscroll usage in no-framework environment.

    Another example is ngx-ui-scroll. Before 2021 vscroll was part of ngx-ui-scroll, and its demo page contains well-documented samples that can be used to get an idea on the API and functionality offered by vscroll. The code of the UiScrollComponent clearly demonstrates the Workflow instantiation in the context of Angular. Also, since ngx-ui-scroll is the intermediate layer between vscroll and the end Application, the Datasource is being provided from the outside. Method makeDatasource is used to provide Datasource class to the end Application.

    Adapter API

    Adapter API is a powerful feature of the vscroll engine. Please refer to ngx-ui-scroll Adapter API doc as it can be applied to vscroll case with only one important difference: vscroll does not have RxJs entities, it has Reactive ones instead. It means, for example, eof$ has no "subscribe" method, but "on":

    // ngx-ui-scroll
    myDatasource.adapter.bof$.subscribe(value =>
      value && console.log('Begin of file is reached')
    // vscroll
    myDatasource.adapter.bof$.on(value =>
      value && console.log('Begin of file is reached')

    Adapter API becomes available as Datasource.adapter property after the Datasource is instantiated via operator "new". In terms of "vscroll" you need to get a Datasource class by calling makeDatasource method, then you can instantiate it. makeDatasource accepts 1 argument, which is an Adapter custom configuration. Currently this config can only be used to redefine the just mentioned Adapter reactive props. Here's an example of how simple Reactive props can be overridden with RxJs Subject and BehaviorSubject entities: ui-scroll.datasource.ts.

    An important note is that the Adapter getting ready breaks onto 2 parts: instantiation (which is synchronous with the Datasource instantiation) and initialization (which occurs during the Workflow instantiating). Adapter gets all necessary props and methods during the first phase, but this starts work only when the second phase is done. Practically this means

    • you may arrange any Adapter reactive subscriptions in your app/consumer right after the Datasource is instantiated,
    • some of the initial (default) values can be unusable, like Adapter.bufferInfo.minIndex = NaN (because Scroller's Buffer is empty before the very first Datasource.get call),
    • Adapter methods do nothing when called before phase 2, they immediately resolve some default "good" value ({ immediate: true, success: true, ... }).

    If there is some logic that could potentially run before the Adapter initialization and you don't want this to happen, the following approach can be applied:

    myDatasource = new VScroll.makeDatasource()({...});
    myDatasource.adapter.init$.once(() => {
      console.log('The Adapter is initialized'); // 2nd output
    workflow = new VScroll.Workflow({...});
    console.log('The Workflow runs'); // 1st output

    VScroll will receive its own Adapter API documentation later, but for now please refer to ngx-ui-scroll.


    - to Mike Feingold as he started all this story in far 2013,

    - to Joshua Toenyes as he transferred ownership to the "vscroll" npm repository which he owned but did not use,

    - to all contributors of related repositories (link, link).

    2021 © Denis Hilt


    npm i vscroll

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