Published on

Epic React Course Notes: Advanced React Hooks

7 min read

Continuing my learning from the EpicReactDev course by Kent C Doods, this is my notes from the third part of the course. In this post, I'll post my notes for the Advanced React Hooks course.

Github Repo: https://github.com/kentcdodds/advanced-react-hooks

Lesson Notes

useReducer: Simple Counter

The course start with a brief comparison on useState and useReducer and how you can use one to replace another. Kent’s blogpost explain the trade-offs between two and use case for each.

  • TLDR; from the blogpost

    • When it's just an independent element of state you're managing: useState
    • When one element of your state relies on the value of another element of your state in order to update: useReducer
    • starting with useState , and moving to **useReducer**when you notice elements of state need to change together.
  • My take: if you’re familiar with redux, the pattern in useReducer kind of similar, where you write reducer function that consists of state and action.

  • useReducer will return an array where first index is the state and second index is the dispatcher function

  • Reducer function → func(state, action)

    • The structure that you most likely see, which newState may includes an action is a convention for the reducer function and not an actual API from useReducer
    • The second argument for the reducer function is the value that the dispatcher function arguments receive
  • We can also simulate the dispatcher function to accept a function with the argument is the previous state, similar to the dispatcher function in useState.

    • By constructing the reducer function like this
    const countReducer = (state, action) => {
      return {
        ...(typeof action === 'function' ? action(state) : action),

    The useReducer dispatch function now can be called with

    • dispatch(newValue) or
    • dispatch((prevState) => { ...prevState, newValue })

useCallback: custom hooks

  • It’s a hook that’s used for memoization. In react you can use useCallback or useMemo for memoization. Kent’s has a good article discuss about this in detail

  • In this section, I learn to refactor a logic within a component to be a custom hook. In the exercise, it’s to refactor the API call function that previously run within a component to a custom hook.

    • In this exercise, Kent put a reminder on how important to list out the dependencies array for the useCallback hook or otherwise potentially we’ll get the indefinite re-render or unexpected re-render.
  • The exercise in this section teach me how various kind of use case where the useCallback can be suitable to use, especially at the custom hook level.

    • The exercise also teach a simple trick to make sure dispatch function in useReducer can be called safely, for a case there’s a long operation happening at the dispatch and the component has an unexpected unmount. This require us to use useRef to determine the mount/unmount state. Code below for reference
    function useSafeDispatch(dispatch) {
      const mountedRef = React.useRef(false)
      React.useEffect(() => {
        mountedRef.current = true
        return () => {
          mountedRef.current = false
      }, [])
      return React.useCallback(
        (...args) => (mountedRef.current ? dispatch(...args) : void 0),

I’d say I spent some time in this section to fully understand the pattern here, and still try to think how can I apply this in my work.

useContext: simple Counter

This section of the course gives a brief intro to the React context and how we can

This section also revisit some of the concepts like:

  • He will touch more on this composition pattern later in the Advanced React Pattern course

This section also teach about how to wrap a context into a custom hook that can be easily shared to a consumer.

This section also teach me one trick that I end up use in the Goal Tree. Have a context initialise with just undefined (because the initial value is undetermined initially)

import React from 'react'

interface GoalTreeState {
  expandedNodes: string[]
  updateExpandedNodes: (newExpandedNodes: string[]) => void

const GoalTreeStateContext = React.createContext<GoalTreeState | undefined>(undefined)

interface GoalTreeStateProps {
  children: React.ReactNode

export function GoalTreeStateProvider({ children }: GoalTreeStateProps) {
  const [expandedNodes, setExpandedNodes] = React.useState<string[]>([])

  const updateExpandedNodes = (newExpandedNodes: string[]) => {

  return (
    <GoalTreeStateContext.Provider value={{ expandedNodes, updateExpandedNodes }}>

export function useGoalTreeState() {
  const context = React.useContext(GoalTreeStateContext)

  if (context === undefined) {
    throw new Error('useGoalTreeState must be used within GoalTreeStateProvider')

  return context

Another examples in this section is to use React context to cache the response from API call.

useLayoutEffect: auto-growing textarea

  • Read Kent’s article on the comparison for useEffect vs useLayoutEffect https://kentcdodds.com/blog/useeffect-vs-uselayouteffect
    • TLDR;
      • Use useLayoutEffect when there’s a need to mutate the DOM or do measurement to the actual DOM that’s rendered to the browser
      • Use useEffect for any other side-effect that’s not required mutating the DOM or getting measurement from the DOM element.
  • Simple rule from the course If you are making observable changes to the DOM, then it should happen in useLayoutEffect, otherwise useEffect.
  • This section doesn’t give much explanation as it should be obvious enough on the use case for this particular hook.

useImperativeHandle: scroll to top/bottom

This is react hook that allows us to expose imperative methods to a component consumer which pass a ref prop to our component and it needs to be handled imperatively.

  • The exercise from this section teach about how to attach a function to a ref object and call it from the component consumer.

I will never know about this hook if it’s not from this course. Kent also said, there’s a little use case for this hook for a problem that you probably can solve with React native “declarative” way.


It’s a react hook that is useful in conjunction with React Devtools. It can only be used inside a custom hook.

This hook basically allow you to give a label to a custom hook so that it gives more context for debugging in the React dev tools

  • Without useDebugValue

without useDebugValue

  • With useDebugValue

with useDebugValue

Basically it’s like put a console.log, but you can see the print out directly from the React Dev Tools