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    Associates an event emitter with an element to broadcast its position in the viewport with respect to the scroll position of a container.


    Borrowing heavily from the scrollMonitor package, ScrollTrack implements the standard Node event emitter in place of the custom implementation used by ScrollMonitor. ScrollTrack also (subjectively) simplifies the code base, uses transpiled ES6 classes, has full test coverage, supports vh-based offsets, and addresses a few (subjective) bugs.

    The module consists of a ScrollElementContainer class and a ScrollElement class. By default, when required, the module instantiates a singleton instance of ScrollElementContainer and exports it. Therefore, when creating a new ScrollElement (via ScrollElementContainer.create()), the ScrollElement is added to the singleton and available globally (assuming you have a reference to the module on a given module). This approach will avoid adding additional scroll and resize listeners to the window even if you require the module from multiple places in your project.

    To create a new container, do not instantiate a new ScrollElementContainer. Instead, call the singleton's createContainer method. This will ensure that your new container still uses the singleton for capturing scroll and resize events (rather than piling on more listeners unneccesarily).

    Capturing scroll and resize events on the window can quickly cause performance issues. The module is designed to avoid such issues by implementing the architecture defined above, while also using passive event listening when available. Passive event listening provides performance-conscientious scroll and resize event tracking on the window (or any ScrollElementContainer item).

    This module's only dependencies are the events module, which provides access to the native node EventEmitter in a browser environment, and the debounce module to debounce the resize event listener on the window.


    Install via npm:

    $ npm i scroll-track

    Basic Usage

    // Exports the ScrollElementContainer singleton
    const scrollTrack = require('scroll-track');
    const el = document.querySelector('.some-selector');
    // NOTE: the create method will throw an error if passed an invalid selector string
    const tracker = scrollTrack.create(el);
    // or, const tracker = scrollTrack.create('.some-selector');
    // or, const tracker = scrollTrack.create(10);
    // or, const tracker = scrollTrack.create({top: 10, bottom: 20});
    tracker.once('enter-viewport', () => console.log('entered viewport'));
    tracker.on('exit-viewport', () => console.log('exited viewport'));


    Create watcher objects with scrollMonitor.create(item). An optional second argument lets you receive events before or after this element enters the viewport. See "Offsets".

    item can be one of the following:

    • Element - the ScrollElement will watch the area contained by the DOM element.
    • Object - top and bottom properties will be used to set the ScrollElement's top and bottom properties.
    • Number - the ScrollElement will watch a 1px area this many pixels from the top. Negative numbers will watch from the bottom.
    • String - it will use the string as a CSS selector and watch the first match.
    • NodeList / Array - it will use the first DOM element.

    ScrollElement are automatically recalculated on the first scroll event after the height of the document changes. They will also fire events immediately after an event listener is attached if appropriate.

    ScrollElement Properties

    • ScrollElement.isInViewport - true if any part of the element is visible, false if not.
    • ScrollElement.isFullyInViewport - true if the entire element is visible ^1.
    • ScrollElement.isAboveViewport - true if any part of the element is above the viewport.
    • ScrollElement.isBelowViewport - true if any part of the element is below the viewport.
    • - distance from the top of the document to the top of this watcher.
    • ScrollElement.bottom - distance from the top of the document to the bottom of this watcher.
    • ScrollElement.height - top - bottom.
    • ScrollElement.watchItem - the element, number, or object that this watcher is watching.

    ScrollElement Methods

    • ScrollElement.recalculateLocation - recalculates the location of the element in relation to the document.
    • ScrollElement.getOffsets - returns an object that defines the top and bottom offsets of this watcher.
    • ScrollElement.destroy - removes this ScrollElement and its event listeners.
    • ScrollElement.lock - locks the ScrollElement at its current location.
    • ScrollElement.unlock - unlocks the ScrollElement.

    ScrollElement is an instance of EventEmitter, so it also has familiar methods such as on, once, removeListener, etc. Review all inherited methods from the Node events documentation.

    These methods are automatically called by the ScrollElementContainer, but they can be called manually if necessary:

    • ScrollElement.update - updates the boolean properties in relation to the viewport. Does not trigger events.
    • ScrollElement.triggerCallbacks - triggers any callbacks that need to be called.

    ScrollElement Events

    • visibility-change - when the element enters or exits the viewport.
    • state-change - similar to visibility-change but is also called if the element goes from below the viewport to above it in one scroll event or when the element goes from partially to fully visible or vice versa.
    • enter-viewport - when the element enters the viewport.
    • fully-enter-viewport - when the element is completely in the viewport ^2.
    • exit-viewport - when the element completely leaves the viewport.
    • partially-exit-viewport - when the element goes from being fully in the viewport to only partially ^3.

    ScrollElement Locking

    Sometimes you want to change the element you're watching, but want to continue watching the original area. One common use case is setting position: fixed on an element when it exits the viewport, then removing positioning when it when it reenters.

    const scrollTrack = require('scroll-track');
    const tracker = scrollTrack.create(element);
    tracker.on('exit-viewport', () => {
    tracker.on('enter-viewport', () => {

    Because the watcher was locked, the scroll monitor will not recalculate its location until unlocked.

    ScrollElement Offsets

    If you want to trigger an event when the edge of an element is near the edge of the viewport, you can use offsets. The offset is the second argument to ScrollElementContainer.create(). You can also create viewport-height-relative offsets by passing a string suffixed with vh.

    This will trigger events when an element gets within 200px of the viewport:

    const scrollTrack = require('scroll-track');
    scrollTrack.create(element, 200);

    This will trigger when the element is 200px inside the viewport:

    const scrollTrack = require('scroll-track');
    scrollTrack.create(element, -200);

    If you only want it to affect the top and bottom differently:

    const scrollTrack = require('scroll-track');
    scrollTrack.create(element, {top: 200, bottom: 50});

    If you only want it to affect the top and not the bottom:

    const scrollTrack = require('scroll-track');
    scrollTrack.create(element, {top: 200});

    This will trigger when the element when it is 1 viewport height from entering the view:

    const scrollTrack = require('scroll-track');
    scrollTrack.create(element, {top: '100vh'});


    ScrollElementContainer Properties

    • ScrollElementContainer.viewportTop - distance from the top of the document to the top of the viewport.
    • ScrollElementContainer.viewportBottom - distance from the top of the document to the bottom of the viewport.
    • ScrollElementContainer.viewportHeight - height of the viewport.
    • ScrollElementContainer.contentHeight - height of the document.

    ScrollElementContainer Methods

    • ScrollElementContainer.createContainer(containerElement) - returns a new ScrollElementContainer that can be used just like the scrollTrack module.
    • ScrollElementContainer.create(item, offsets) - returns a new watcher. item is a DOM element, NodeList, CSS selector, an object with .top and .bottom properties, or a number.
    • ScrollElementContainer.update() - update all ScrollElements within the ScrollElementContainer and trigger their events.
    • ScrollElementContainer.recalculateLocations() - recalculate the location of all unlocked ScrollElements and trigger events if needed.

    Creating a ScrollElementContainer

    By default, the container for any ScrollElement created via the ScrollElementContainer.create() method will be the window. If you want to create a ScrollElement within a container element, first create a container, and then call that container's create method to create a ScrollElement within it.

    const containerEl = document.querySelector('.container');
    const container = scrollMonitor.createContainer(containerEl);
    const scrollEl = document.querySelector('.some-element');
    const scrollElement = container.create(scrollEl);
    scrollElement.on('enter-viewport', () => console.log('.some-element has entered the viewport'));
    scrollElement.on('exit-viewport', () => console.log('.some-element has left the viewport'));

    Tests, Coverage, Docs, Demos and Building for Release

    To run the tests:

    $ npm test

    To generate a coverage report:

    $ npm run coverage

    To generate documentation:

    $ npm run docs

    To generate the demos:

    $ npm run demo

    To generate a bundle for distribution:

    $ npm run dist

    To run a watch when authoring new examples:

    $ npm run watch


    npm i scroll-track

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