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React Hooks

[[Why React Hooks]]

2 Rules of Hooks

  1. Hooks must be used in a React component
  2. Hooks must be declared at the top of the function
    1. must be called in the same order every time
    2. can't be in an if statement
    3. see implementation of useState


  • accepts a function which is a lazy initializer!
    • useState ignores the param afterward
    • for computationally expensive stuff (async, like getFromLocalStorage)
    • so that the expensive function only runs once

const [item, setItem] = useState(() => getFromLocalStorage());

Problem with this

const [count, setCount] = useState(0);
setCount(count + 1);
setCount(count + 1);


const [count, setCount] = useState(0)
- setCount(count + 1)
+ setCount(currentCount => currentCount + 1)

Implementing useState


  • dependency array does a === comparison
    • will always re-render if you pass it an array/object

Implementing useEffect


ate range greaterd

function reducer(state, action) {
  if (action.type === 'increment') {
    return {
      age: state.age + 1
  throw Error('Unknown action.');
const [state, dispatch] = useReducer(reducer, initialValue);

dispatch({type: "increment"})
  • first prop: the old state
  • second prop: whatever you pass to dispatch
    • usually action
  • doesn't have to follow the Redux style of having actions with a type prop

Implementing useState with useReducer

const [count, setCount] = useState(0);
function countReducer(state, newState) {
    return newState;

const [count, setCount] = useReducer(countReducer, 0);

useReducer vs useState

  • useReducer might be good for more complex logic


Why can't we have a direct pointer to a ref?

Why is the API like

const ref = useRef();
node = ref.current;

The pointer will change

We have an object so that when the ref changes, it just changes the object pointer

Uses of useRef

  1. pointing to DOM elements
    1. the pointers to the DOM elements are re-rendered all the time
  2. Store state where we don't need to rerender when it changes

useRef to access the previous props/state

Stack Overflow

const usePrevious = <ValueType,>(value: ValueType): ValueType | undefined => {
    const ref = useRef<ValueType>();
    useEffect(() => {
        ref.current = value;
    return ref.current;
export default usePrevious;


const [count, setCount] = useState(0);
const prevCount = usePrevious(count);

useCallback and useMemo

Use cases (

  1. not have a function re-render every time so that it can be used in a useEffect dep array
  2. avoiding unnecessary re-renders when re-rendering is super expensive (Graphs, Charts, Animations)
  3. useMemo can be passed a function (just like useState) and lazily calculate computationally expensive items
import { useCallback } from 'react';

const MyParent = ({ seardchTerm }) => {
  const onItemClick = useCallback(event => {
  }, [term]);
  return (
  • Without useCallback, the onItemClick would be redefined whenever a useState or a parent's state changes
    • which means <MyBigList /> would be re-rendered every time useState or a parent's state is called

useAsync custom hook using useCallback

advanced-react-hooks/ at main · advanced-react-hooks


what's the run function that we return in useAsync?

  • instead of the user putting their fetch in a useCallback
  • the useEffect in useAsync becomes run which is memoized with useCallback
  • they can put it in a useEffect and run(result)

safeDispatch (safe fetch) and useEffect cleanup

  • Memory leak when unmounting before the fetch finishes
  • it's not a class component so we can't just have a componentWillMount and abort the fetch
  • I have to cancel subscriptions and async tasks in the useEffect cleanup function

How do you know if a component has been unmounted?

React.useLayoutEffect(() => {
    mountedRef.current = true;
    return () => {
        mountedRef.current = false;
}, []);


useEffect vs useLayoutEffect

  • same API
  • useEffect is run after pixels are painted
  • useLayoutEffect is run before so it will delay painting
    • can update the DOM before it paints


  • ????


  • can pass a ref as a second param
    • instead of the first param with all the other regular props

Could you put ref as a prop? in the first param of the component?

  • yeah but that would cause a re-render
  • you might not want to always re-render when the ref that's passed down to the component changes

Do you need to always use useImperativeHandle when using React.forwardRef?


  • feels like __repr__ in Python for custom hooks

Last update: 2023-04-24