use-is-in-viewport


A react hook to use the
IntersectionObserver
declaratively in your React app for the purposes of finding out if an element is in a given
viewport.


- Motivation
- Guiding principles
- Installation
- API docs
- Return value
- Input options
- options.threshold
- options.target
- options.viewport
- options.{modTop, modRight, modBottom, modLeft}
- Example usage

Motivation

I wrote isInViewport for the jQuery world back in the
day and while how we build interfaces has changed massively since, the problem
isInViewport solves still remains. Since then, the web
platform has grown massively and now gives us better primitives to solve this problem in the shape
of the
Intersection Observer API.

I was looking for a simple way to use the
Intersection Observer API
in my React app for the purposes of checking if a component is in some viewport. I started off by
writing a wrapper around the Intersection
Observer API but that still didn't _feel_ right. Its dev ergonomics felt off due to the callback
based core interface it shares with the Intersection Observer API.

Then, React Hooks happened! I started
playing around attempting to write a generic useIntersectionObserver hook. I gave up on that idea
in favour of a more directed hook. One which solves a problem that I and many other devs have.

The following guiding principles in combination with the
community helped shape the api and
serve as a north star for this hook.

$3

1. Solve a real problem
- Test an element in a viewport for visibility
2. Make it straightforward to consume the package
- Simple, to the point name
- Easy installation
- Correct peer dependencies to prevent foot-guns
- Tons of docs and examples (in progress)
3. Make it easy to address the most likely use-cases
- e.g., "Tell me when an element is visible in the current window viewport", "Tell me when 75% of
an element is visible in current window viewport"
4. Make it possible to address other use-cases without unnecessary noise
- e.g., "Tell me when an element is visible in my custom viewport", "Let me customize the
viewport I want to pass down", "Let me use this with a component that uses React.forwardRef",
etc
5. Provide as much developer feedback as possible
- made possible by types that ship with the project
6. Only solve the actual problem
- e.g., providing a polyfill for Intersection Observer is not this package's job
7. Be trustworthy
- All use cases have corresponding integration tests using Cypress (in progress)

> With all
> all major browsers having added support for Intersection Observer API,
> this package is even more easier to drop into your application!

Installation

npm install use-is-in-viewport

yarn or any other package manager will also work.

You can also directly import the module from unpkg either in your html or in
your javascript.

Please note that this hook declares react and as _peer dependency_. Therefore, you must have
react installed to use this package.

Note: This hook relies on
Intersection Observer API
and hence if you want to use it in a browser that doesn't support it, the onus of shipping the
polyfill is on the developer.

Please open an issue if these defaults
cause a problem in your application.

API

$3

> The nomenclature (target, viewport, threshold, etc) are borrowed from that of the
> Intersection Observer API

- Return value
- Input options

#### Return value

It returns an array that contains the following in order:

1. a flag that is one of null, true, false based on the visibility of target element in
provided viewport
- null -> first call to the hook when nothing is initialized
- true -> target element is visible in the given viewport
- false -> target element is visible in the given viewport
2. A callback ref to pass to the
element you want to know the visibility of.
3. A callback ref to pass to the
element you want to use as the viewport.

#### Options

The hook accepts an optional options object. When not provided, "sane" defaults are used. They are
described in the options section below.

#### _options.threshold_

The threshold describes what percent of the target element should intersect with the given
viewport for it to be considered as visible in the viewport.

It can be a number or an array of numbers. The number is interpreted as a percent of the target
element's dimensions.

Passing an array of numbers is likely to be useless for most use cases. It only exists as an
artefact of the library this hook is built on and hence will most likely will be deprecated and
removed based on feedback from the community.

Default: 0% -> as soon as even 1px of your target element is visible in viewport it'll be
reported as visible in viewport.

Example:

``jsx
// this would report an element as visible in its parent document viewport when
// at least 50% of the target intersects with the viewport
const [isInViewport, targetRef] = useIsInViewport({ threshold: 50 })
...


The target accepts a ref for the element you want
track the visibility of in a viewport. This is useful when you have a ref that is created outside
of this hook (for e.g., passed in via ref prop from another component, a forwarded ref, etc).

This ref is wrapped and a new ref is returned at index 1 in the returned array. The returned ref is
what you must pass to the ref property of the element you want to track the visibility of.

Default: undefined

Example:

`jsx
const targetRef = useCallback(node => console.log(node)) // can come from anywhere
// or
const targetRef = useRef(null) // can come from anywhere

const [isInViewport, wrappedTargetRef] = useIsInViewport({ target: targetRef })
...


The viewport accepts a ref for the element you
want to use as the viewport. This _must_ be a parent of the element you want to track the visibility
of. This options is useful when you have a ref that is created outside of this hook (for e.g.,
passed in via ref prop from another component, a forwarded ref, etc).

This ref is wrapped and a new ref is returned at index 2 in the returned array. The returned ref is
what you must pass to the ref property of the element you want to use as the viewport.

Also, if you want plan to use the same viewport for multiple child elements, then the viewport ref
must be _chained_ as shown in the example below. This might feel a bit weird at first but this
chaining is necessary so that we can -

1. Preserve whatever behaviour the incoming viewport ref has (it could be a fn or a object ref)
2. Have only one viewport ref that can then be passed to the element you want to use as viewport

Default: undefined

Example:

`jsx
const MyElement = React.forwardRef(function MyElement(props, parentRef) {
const [isFirstDivInViewport, firstDiv, wrappedViewportRef] = useIsInViewport({
viewport: parentRef
})
const [isSecondDivInViewport, secondDiv, finalViewportRef] = useIsInViewport({
viewport: wrappedViewportRef, // viewport ref is chained
threshold: 20
})

return (


`

#### _options.{modTop, modRight, modBottom, modLeft}_

These values map directly to
rootMargin
in Intersection Observer API. The can have values similar to the CSS
margin property.

> The values in rootMargin define offsets added to each side of the intersection root's bounding box
> to create the final intersection root bounds.

Defaults:

- modTop: '0px'
- modRight: '0px'
- modBottom: '0px'
- modLeft: '0px'

Example:

`jsx
...
const [isInViewport, targetRef] = useIsInViewport({
modTop: '10px',
modRight: '1em',
modBottom: '2.5rem',
modLeft: '10%'
})
...
`

Example usage

A CRA based example app (which is also used in the test suite) can be found under
examples/cra. Inline examples showcasing some of the use-cases are below.

$3

As soon as at least 1px of the target div is visible in the parent document viewport,
isInViewport evaluates to true.

`jsx
import React from 'react'
import useIsInViewport from 'use-is-in-viewport'

export default function MyElement() {
const [isInViewport, targetRef] = useIsInViewport()

return (


As soon as at least 1px of the target div is visible in its parent div (chosen as the parent by
passing viewportRef to it), isInViewport evaluates to true.

`jsx
import React from 'react'
import useIsInViewport from 'use-is-in-viewport'

export default function MyElement() {
const [isInViewport, targetRef, viewportRef] = useIsInViewport()

return (


In some cases, the parent element might want to control the ref of some element inside your
component. It is obviously up to your component to decide what element to assign the incoming ref.

In this example, we assign the incoming ref to the target element that we are watching for in the
document viewport. To do so, we thread it through useIsInViewport hook by using the target
option and assining the incoming ref to it. The hook takes care of updating the incoming ref
such that it is completely transparent to the parent element that passed the ref in.

`jsx
import React from 'react'
import useIsInViewport from 'use-is-in-viewport'

export const RefForwardingElement = React.forwardRef(function RefForwardingElement(
props,
incomingTargetRef
) {
const [isInViewport, targetRef] = useIsInViewport({
target: incomingTargetRef // thread the incoming target ref through the hook
})

return (


Same as the previous example, but here we thread the incoming ref into the viewport element
instead of the target element.

`jsx
import React from 'react'
import useIsInViewport from 'use-is-in-viewport'

export const RefForwardingElement = React.forwardRef(function RefForwardingElement(
props,
incomingViewportRef
) {
const [isInViewport, targetRef, viewportRef] = useIsInViewport({
viewport: incomingViewportRef // thread the incoming viewport ref through the hook
})

return (


In this example, we have a custom viewport element and we want to track whether its child divs are
in viewport or not. We have also chosen to use different thresholds for the two child target
divs.

As you might notice, we thread the viewport ref from the first call to useIsInViewport into the
second call to useIsInViewport. This is because an element can only be assigned one ref. And
here that element is the viewport element. Therefore, we use the transparent ref update capability
of useIsInViewport (as shown in the previous examples) to wrap the viewport ref from the first
call of the hook with a new one that is then passed to the viewport div.

`jsx
import React from 'react'
import useIsInViewport from 'use-is-in-viewport'

export default function MyElement() {
const [isDivOneInViewport, divOneTargetRef, viewportRefToChain] = useIsInViewport({
threshold: 50
})
const [isDivTwoInViewport, divTwoTargetRef, viewportRef] = useIsInViewport({
viewport: viewportRefToChain,
threshold: 75
})

return (


`js
import { useState } from 'react'
import useIsInViewport from 'use-is-in-viewport'

export function useClampedIsInViewport(options) {
const [isInViewport, ...etc] = useIsInViewport(options)
const [wasInViewportAtleastOnce, setWasInViewportAtleastOnce] = useState(isInViewport)

setWasInViewportAtleastOnce(prev => {
// this will clamp it to the first true
// received from useIsInViewport
if (!prev) {
return isInViewport
}
return prev
})

return [wasInViewportAtleastOnce, ...etc]
}
``