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import { useState, useEffect, useRef } from 'react';
// Let's pretend this <Counter> component is expensive to re-render so ...
// ... we wrap with React.memo, but we're still seeing performance issues :/
// So we add useWhyDidYouUpdate and check our console to see what's going on.
const Counter = React.memo(props => {
useWhyDidYouUpdate('Counter', props);
return <div style={}>{props.count}</div>;
function App() {
const [count, setCount] = useState(0);
const [userId, setUserId] = useState(0);
// Our console output tells use that the style prop for <Counter> ...
// ... changes on every render, even when we only change userId state by ...
// ... clicking the "switch user" button. Oh of course! That's because the
// ... counterStyle object is being re-created on every render.
// Thanks to our hook we figured this out and realized we should probably ...
// ... move this object outside of the component body.
const counterStyle = {
fontSize: '3rem',
color: 'red'
return (
<div className="counter">
<Counter count={count} style={counterStyle} />
<button onClick={() => setCount(count + 1)}>Increment</button>
<div className="user">
<img src={`${userId}`} />
<button onClick={() => setUserId(userId + 1)}>Switch User</button>
// Hook
function useWhyDidYouUpdate(name, props) {
// Get a mutable ref object where we can store props ...
// ... for comparison next time this hook runs.
const previousProps = useRef();
useEffect(() => {
if (previousProps.current) {
// Get all keys from previous and current props
const allKeys = Object.keys({ ...previousProps.current, ...props });
// Use this object to keep track of changed props
const changesObj = {};
// Iterate through keys
allKeys.forEach(key => {
// If previous is different from current
if (previousProps.current[key] !== props[key]) {
// Add to changesObj
changesObj[key] = {
from: previousProps.current[key],
to: props[key]
// If changesObj not empty then output to console
if (Object.keys(changesObj).length) {
console.log('[why-did-you-update]', name, changesObj);
// Finally update previousProps with current props for next hook call
previousProps.current = props;
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davidpn11 commented Feb 28, 2019


I did this small change to check for nested objects at the props. If so, it will use lodash's isEquals to check for equality (I'm lazy haha) .
At the main example, this will avoid logging if you have only changed the avatar and caused a rerender.
I realize this is costy, but since is only for debugging, we don't have to worry since it will not be used in prod =)

Hope you like it!
Link here

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gragland commented Feb 28, 2019

@davidpn11 Maybe I'm missing something, but isn't the point that it does log every time there is a re-render? Why skip doing it when the avatar changes? Also, could you explain rationale behind isEqual? Not seeing why a simple previousProps.current[key] !== props[key] isn't enough, since we just want to compare references to see if they changed (and thus caused a re-render).

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davidpn11 commented Mar 1, 2019

@gragland You are right. I was thinking more of a solution to log exactly which props have changed. So it shows you more clearly how your app is behaving. I was not thinking about reference, but more of the data itself.

In conclusion, I was thinking about a different hook. But is nice to see it can be easily derive from your example.

Great work with useHooks by the way! =)

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gragland commented Mar 1, 2019

@davidpn11: Gotcha, makes sense and seems like a good way to extend this hook. Thanks for the kind words!

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nwatab commented Mar 12, 2022

I'm happy if someone could turn the snippet into typescipt.

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