[Web Dev Simplified
My name is Kyle and I have been a full stack web developer since 2015. I have a Computer Engineering degree, but am…courses.webdevsimplified.com](https://courses.webdevsimplified.com/view/courses/react-hooks-simplified "courses.webdevsimplified.com/view/courses/r..")

useState

Initial State Computation

useState(() => {
/* Slow computation */
})

Updating State Objects

setPreferences(prevPreferences => {
return { ...prevPreferences, theme: 'dark' }
})

useEffect

useEffect(() => {
console.log('This is my side effect')

return () => {
console.log('This is my clean up')
}
})

// MOUNTED
// This is my side effect

// RE-RENDER 1:
// This is my clean up
// This is my side effect

// RE-RENDER 2:
// This is my clean up
// This is my side effect

// UN-MOUNT:
// This is my clean up

This is because the cleanup is run directly before the side effect is run as long as the side effect has occurred at least once. Also, the cleanup is run when a component un-mounts as well.

useContext

you can put Context inside other Context

useRef

storing the previous value of a state variable

function Component() {
const [name, setName] = useState('Kyle')
const previousName = useRef(null)

useEffect(() => {
previousName.current = name
}, [name])

return (
<>
setName(e.target.value)} />
{previousName.current} => {name}
</>
)
}

useMemo

Referential Equality : comparing objects in dependency arrays

function Component({ param1, param2 }) {
const params = useMemo(() => {
return { param1, param2, param3: 5 }
}, [param1, param2])

useEffect(() => {
callApi(params)
}, [params])
}

the reference for params will only change if param1, or param2 change.

useCallback

Referential Equality : Just like with useMemo, useCallback is used to maintain referential equality.

function Parent() {
const [items, setItems] = useState([])
const handleLoad = useCallback((res) => setItems(res), [])

return
}

function Child({ onLoad }) {
useEffect(() => {
callApi(onLoad)
}, [onLoad])
}

Now the handleLoad function will never change, thus the useEffect in the Child component will not be called on each re-render.

useReducer

const ACTIONS = {
INCREMENT: 'increment',
DECREMENT: 'decrement',
RESET: 'reset',
CHANGE_COUNT: 'change-count'
}

function reducer(count, action) {
switch (action.type) {
case ACTIONS.INCREMENT:
return count + 1
case ACTIONS.DECREMENT:
return count - 1
case ACTIONS.RESET:
return 0
case ACTIONS.CHANGE_COUNT:
return count + action.payload.amount
default:
return count
}
}

function Counter() {
const [count, dispatch] = useReducer(reducer, 0)

return (
<>
{count}
dispatch({ type: ACTIONS.INCREMENT })}>

• 
  dispatch({ type: ACTIONS.DECREMENT })}>
• 
  {
  dispatch({
  type: ACTIONS.CHANGE_COUNT,
  payload: { amount: 5 }
  })
  }}>
  Add 5
  
  dispatch({ type: ACTIONS.RESET })}>
  Reset
  
  </>
  )
  }