JIYIK CN >

Check if an element is inside a <div> in React.js Viewport:

1. Sets the attribute on the element ref.
2. Use IntersectionObserverthe API to track whether elements intersect.

App.js

import {useEffect, useRef, useState, useMemo} from 'react';

export default function App() {
  const ref1 = useRef(null);
  const ref2 = useRef(null);

  const isInViewport1 = useIsInViewport(ref1);
  console.log('isInViewport1: ', isInViewport1);

  const isInViewport2 = useIsInViewport(ref2);
  console.log('isInViewport2: ', isInViewport2);

  return (
    <div>
      <div ref={ref1}>Top div {isInViewport1 && '| in viewport ✅'}</div>

      <div style={{height: '155rem'}} />

      <div ref={ref2}>Bottom div {isInViewport2 && '| in viewport ✅'}</div>
    </div>
  );
}

function useIsInViewport(ref) {
  const [isIntersecting, setIsIntersecting] = useState(false);

  const observer = useMemo(
    () =>
      new IntersectionObserver(([entry]) =>
        setIsIntersecting(entry.isIntersecting),
      ),
    [],
  );

  useEffect(() => {
    observer.observe(ref.current);

    return () => {
      observer.disconnect();
    };
  }, [ref, observer]);

  return isIntersecting;
}

This example shows how to check if an element is in the viewport.

IntersectionObserverThe API enables us to check if a given element intersects the document.

useIsInViewportThe hook takes a ref object pointing to the element we want to track.

IntersectionObserverThe constructor takes a function which is called with an array of entries.

Entries are an array of target elements for all observers that become more or less visible than one of the intersection observer ratios.

Each entry describes the degree to which a given element intersects the root element (the document).

We destructure the entries because our IntersectionObserveronly tracks a single element (the one we set ref to).

We call observe()the method, passing it the element we want to track - observer.observe(ref.current).

Every time an element enters or leaves the viewport, IntersectionObserver()the function we passed to the constructor is called and we update the state.

// 👇️ gets called every time element enters or leaves viewport
new IntersectionObserver(([entry]) =>
  setIsIntersecting(entry.isIntersecting),
)

useIsInViewportThe hook will return true if the element we set the ref object to is in the viewport .

If the element is not in the viewport, the hook returns false.

请注意, on the initial render, useIsInViewportthe hook will return false , since that is useStatethe initial value we passed to const [isIntersecting, setIsIntersecting] = useState(false);.

If you want to track changes to the hook's return value, use useEffecta hook and add that value to the hook's dependencies.

const isInViewport1 = useIsInViewport(ref1);
console.log('isInViewport1: ', isInViewport1);

useEffect(() => {
  // 👇️ listen for changes
  console.log(isInViewport1);
}, [isInViewport1]);