element-inview

Get notified when a DOM element enters or exits the viewport.

Largely basd on in-view

Installation

npm install --save element-inview

Basic Usage

With element-inview, you can register handlers that are called when an element enters or exits the viewport. Each handler receives one element, the one entering or exiting the viewport, as its only argument.

inView('.someSelector')
    .on('enter', doSomething)
    .on('exit', el => {
        el.style.opacity = 0.5;
    });

API

element-inview maintains a separate handler registry for each set of elements captured with inView(<selector>). Each registry exposes the same four methods. element-inview also exposes four top-level methods. (is, offset, threshold, test).

inView(<selector>).on(<event>, <handler>)

Register a handler to the elements selected by selector for event. The only events element-inview emits are 'enter' and 'exit'.

inView('.someSelector').on('enter', doSomething);

inView(<selector>).once(<event>, <handler>)

Register a handler to the elements selected by selector for event. Handlers registered with once will only be called once.

inView('.someSelector').once('enter', doSomething);

inView.is(<element>)

Check if element is in the viewport.

inView.is(document.querySelector('.someSelector'));
// => true

inView.offset(<offset>)

By default, element-inview considers something in viewport if it breaks any edge of the viewport. This can be used to set an offset from that edge. For example, an offset of 100 will consider elements in viewport if they break any edge of the viewport by at least 100 pixels. offset can be a positive or negative integer.

inView.offset(100);
inView.offset(-50);

Offset can also be set per-direction by passing an object.

inView.offset({
    top: 100,
    right: 75,
    bottom: 50,
    left: 25
});

inView.threshold(<threshold>)

Set the ratio of an element's height and width that needs to be visible for it to be considered in viewport. This defaults to 0, meaning any amount. A threshold of 0.5 or 1 will require that half or all, respectively, of an element's height and width need to be visible. threshold must be a number between 0 and 1.

inView.threshold(0);
inView.threshold(0.5);
inView.threshold(1);

inView.test(<test>)

Override element-inview's default visibility criteria with a custom function. This function will receive the element and the options object as its only two arguments. Return true when an element should be considered visible and false otherwise.

inView.test((el, options) => {
    // ...
});

inView(<selector>).check()

Manually check the status of the elements selected by selector. By default, all registries are checked on window's scroll, resize, and load events.

inView('.someSelector').check();

inView(<selector>).emit(<event>, <element>)

Manually emit event for any single element.

inView('.someSelector').emit('exit', document.querySelectorAll('.someSelector')[0]);

Browser Support

element-inview supports all modern browsers and IE9+.

As a small caveat, element-inview utilizes MutationObserver to check the visibility of registered elements after a DOM mutation. If that's functionality you need in IE9-10, consider using a polyfill.

Performance

Any library that watches scroll events runs the risk of degrading page performance. To mitigate this, currently, element-inview only registers a single, throttled (maximum once every 100ms) event listener on each of window's load, resize, and scroll events and uses those to run a check on each registry.

Utilizing IntersectionObserver

There's an emerging browser API, IntersectionObserver, that aims to provide developers with a performant way to check the visibility of DOM elements. Going forward, element-inview will aim to delegate to IntersectionObserver when it's supported, falling back to polling only when necessary.

License MIT