React ships with a whole bunch of hooks that can be a bit tough to grasp when you're learning them all at once. This post should help you understand the differences and use-cases of three of them.

UseEffect

A pure component only interacts with itself and its children. Any time you need to interact with the world outside your component, you are dealing with side-effects.

React gives us a handy hook for dealing with these. the React.useEffect hook lets us specify a function that deals with external forces, provide a second function to clean up after it, and drop a list of dependencies so we can re-run the effect when one of the dependencies change.

Examples of useEffect

Updating the page title

This effect will run the first time the component is rendered, and then only ever run again if the title has changed.

const [title, setTitle] = React.useState("Hooks 101");

React.useEffect(() => {
    document.title = title;
}, [title]);

Fetching data from an API into local state.

Since our state changing will not affect the list of products that is returned, we can pass an empty array [] as our dependency so that the effect will only run when the component is first mounted.

const [products, setProducts] = React.useState([]);

React.useEffect(() => {
    getProducts()
    .then(products => {
        setProducts(products);
    })
}, []);

Fetching data from an API into local state, based on a query.

If we have a query or filter to modify the set of API data we want, then we can pass it as a dependency to make sure that React runs this effect every time the component renders using a new query.

const [products, setProducts] = React.useState([]);
const [query, setQuery] = React.useState("");

React.useEffect(() => {
    getProducts({name: query})
    .then(products => {
        setProducts(products);
    })
}, [query]);

Dispatching a Redux action.

If your GET action already reduces into your Redux state, then you don't need to maintain any of that locally.

By passing products.length as a dependency, you only run this

const dispatch = Redux.useDispatch();
const products = Redux.useSelector(state => state.products);

React.useEffect(() => {
    dispatch(GetProducts())
}, []);

UseMemo

Unlike useEffect, React.useMemo does not trigger every time you change one of its dependencies.

A memoized function will first check to see if the dependencies have changed since the last render. If so, it executes the function and returns the result. If false, it simply returns the cached result from the last execution.

This is good for expensive operations like transforming API data or doing major calculations that you don't want to be re-doing unnecessarily

Example of useMemo

const posts = Redux.useSelector(state => state.posts);

const tags = React.useMemo(() => {
    return getTagsFromPosts(posts)
}, [posts]);

UseCallback

This is a special case for memoizing functions. Since javascript compares equality by reference, the function you create the first time a component renders will be different than the one created in subsequent renders.

If you try passing a function as props or state, this means that it will be treated as a prop change every single time. By wrapping it in useCallback, React will know that it's the same function. You can still add a dependency array to trigger a recalculation if the dependencies change.

A strong use-case here to avoid child component re-renders

Example of useCallback

Every time this component renders, it will also trigger a whole re-render of the Button component because the removeFromCart function is unique every time.

const dispatch = useDispatch();

const removeFromCart = () => dispatch(removeItem(product.id));

return (
    <Button onClick={removeFromCart}>Delete</Button>
);

Replacing our callback with this will avoid that problem entirely. Now the Button will only re-render when our product ID changes, so that it will function to remove the new product from our cart.

const removeFromCart = React.useCallback(() => {
    dispatch(removeItem(product.id))
}, [product.id]);

Blog

React Hooks: UseEffect, UseMemo, UseCallback

Jacob Paris