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BFE.dev solution for React Quiz
1. React re-render 1

1. Initial render(mount)

Let's look at what happens step by step.

const root = createRoot(document.getElementById('root'))root.render(<App />)

This render() call kicks off the rendering of whole app. It first see <App/>, so App() is run and "App" is logged.

function App() {
{...} const [state, setState] = useState(0) useEffect(() => { setState((state) => state + 1) }, []) console.log('App')
return ( <div> <A state={state} /> <D /> </div> )}

Keep in mind that components are blueprints, child components are not rendered directly, but that React runtime renders the children in a synchronous or asynchronous DFS-like approach based on the priorities of tasks it has internally.

When App is rendered, React sees following children that needs to be rendered further.

<div>  <A state={state} />  <D /></div>

React then goes to <div/><A/>, so A is run and "A" is logged. Similarly React goes to <B/><C/>, in the end <D/> is processed.

When all components are rendered and the changes for DOM are committed, this marks the whole journey for the initial render(mount).

So far we get following logs.

'App''A''B''C''D'

2. Re-render

Once initial mount is done, React tries to flush the passive effects(useEffect) in component App.

const [state, setState] = useState(0)useEffect(() => {  setState((state) => state + 1)}, [])

App holds the state, when setState is called, it does NOT update the state immediately but schedules an update internally which tells React that App needs to be re-rendered, and by incrementing the state by 1.

One thing to keep in mind is that on any state updates, React basically tries to re-render everything from root, while it skips rendering components that doesn't receive updates, or even skips the whole sub-tree if there are no updates there.

Check out how useState works internally. and how bailout works in React

So React start re-rendering everything from root, which is App. App is already marked as "needs update", so App is re-run and "App" is logged.

Now App returns a new set of children.

<div>  <A state={state} />  <D /></div>
  1. div receives new props, because children changes.
  2. A receives a new state(actually state doesn't matter)
  3. D receives a new empty object {} for props.

This might doesn't look obvious. It is much easier to understand if we re-write the JSX into object notation and notice that this is a new object being returned every time the function is run.

{  type: "div",  children: [    {      type: A,      props: {        state: state      }    },    {      type: D      props: {}    }  ]}

Even state doesn't change, once App is re-rendered, A and D always get new object for props.

So now React have to go further to to <div/> because it has updates in props. React renders A, B, C and D for the same reason and we get following logs, as the result to the question.

'App''A''B''C''D''App''A''B''C''D'