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.

Guiding principles

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

useIsInViewport([options]) => [boolean | null, TargetCbRef, ViewportCbRef]

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:

// 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 })
...
<div ref={wrappedTargetRef}>{ isInViewport ? 'Visible' : 'Nope' }</div>

options.target

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:

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 })
...
<div ref={wrappedTargetRef}>{ isInViewport ? 'Visible' : 'Nope' }</div>

options.viewport

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:

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 (
    <section ref={finalViewportRef}>
      <div ref={firstDiv}>{isFirstDivInViewport ? 'Visible' : 'Nope'}</div>
      <div ref={secondDiv}>{isSecondDivInViewport ? 'Visible' : 'Nope'}</div>
    </section>
  )
})


function App() {
  const ref = // however you want to create a ref (useRef, raw fn, useCallback, useMemo, React.createRef, etc)

  return <MyElement ref={ref} />
}

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:

...
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.

Example 1: Element with its parent document as viewport

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

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

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

  return (
    <div ref={targetRef}>
      <p>{isInViewport ? 'In viewport' : 'Out of viewport'}</p>
    </div>
  )
}

Example 2: Element with a custom viewport

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.

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

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

  return (
    <div ref={viewportRef}>
      <div ref={targetRef}>
        <p>{isInViewport ? 'In viewport' : 'Out of viewport'}</p>
      </div>
    </div>
  )
}

Example 3: Using a forwarded ref for the target element

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.

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 (
    <div ref={targetRef}>
      <p>{isInViewport ? 'In viewport' : 'Out of viewport'}</p>
    </div>
  )
})

Example 4: Using a forwarded ref for the viewport element

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

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 (
    <div ref={viewportRef}>
      <div ref={targetRef}>
        <p>{isInViewport ? 'In viewport' : 'Out of viewport'}</p>
      </div>
    </div>
  )
})

Example 5: Tracking visibility in a custom viewport of multiple elements

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.

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 (
    <div ref={viewportRef}>
      <div ref={divOneTargetRef}>
        <p>{isDivOneInViewport ? 'Div one In viewport' : 'Div one out of viewport'}</p>
      </div>
      <div ref={divTwoTargetRef}>
        <p>{isDivTwoInViewport ? 'Div two in viewport' : 'Div two out of viewport'}</p>
      </div>
    </div>
  )
}

Example 6: Composing with useState to create a custom hook that clamps to true once the target element is in viewport

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]
}