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React Hooks - useEffect()

March 17, 2022

The useEffect() React hook is used to interact with interfaces like APIs that are ‘outside’ the app that may not feed us the data we’re expecting. The basic syntax requires a function that as a first argument, an array as a second argument. A basic example:

function newComponent() {
    useEffect() => {
        // perform side effect

to use useEffect() to fetch a list of posts, we would do so like this:

import { useEffect } from 'react';

function ListOfPosts() {
    const [post, setPosts] = useState([]);

    useEffect(() => {
        .then(response => response.json())
        .then(posts => setPosts(posts));
    }, []);

    return => <Post key={} post={post} />)

However, if we need to use a value from outside the effect function, that value must be included in the dependencies array. If that value changes, the effect function will be re-executed.

In the following, the class overflow-hidden is added or removed from the body element whenever a mobile menu is opened or closed.

function MobileMenu({ open }) {
    useEffect(() => {
        const body = document.querySelector('#__next');

        if(open) {
        } else {

    // . . .

I encountered useEffect() while building out the Uranium City History site:

  useEffect(async () => {
    let mounted = true;
    if(mounted) {
      const newsPostsFromServer = await getAllNewsPostsFromServer();
    return () => (mounted = false);
  }, []);

In this case, useEffect is used to get all the news posts from the getAllNewsPostsFromServer, which is then assigned to newsPosts using setNewsPosts.

Explain How This Works … TBC