Updating phase in React - Step-by-Step Execution

Concept Flow - Updating phase

Event or State Change

↓

React schedules update

↓

Component re-renders

↓

JSX evaluated with new state/props

↓

React compares new and old DOM (diff)

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DOM updates applied

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Browser shows updated UI

When state or props change, React re-renders the component, compares the new output with the old, and updates the browser view.

Execution Sample

React

import { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return <button onClick={() => setCount(count + 1)}>{count}</button>;
}

A button that shows a number and increases it by 1 each time you click.

Execution Table

Step	Trigger	State Before	Action	State After	What Re-renders	DOM Change
1	Initial render	count=0	Render component	count=0	Counter component	Button shows '0'
2	Click button	count=0	setCount(count+1)	count=1	Counter component	Button updates to '1'
3	Re-render after state update	count=1	Render component	count=1	Counter component	Button shows '1'
4	Click button	count=1	setCount(count+1)	count=2	Counter component	Button updates to '2'
5	Re-render after state update	count=2	Render component	count=2	Counter component	Button shows '2'
6	No further clicks	count=2	No action	count=2	No re-render	No DOM change

💡 No more events trigger updates, so React stops re-rendering.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
count	0	1	1	2	2	2

Key Moments - 3 Insights

Why does React re-render the component after setCount is called?

React schedules an update when state changes (see step 2 in execution_table). This triggers a re-render to update the UI.

Does React re-render the whole page when state changes?

No, React only re-renders the component where state or props changed (see 'What Re-renders' column). It updates only the necessary DOM parts.

Why does the DOM update only after React compares new and old output?

React uses a diffing process to update only what changed for efficiency (see concept_flow). This avoids unnecessary DOM changes.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'count' after step 4?

A 1

B 0

C 2

D Undefined

💡 Hint

Check the 'State After' column at step 4 in execution_table.

At which step does React apply the DOM update to show '1' on the button?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look for the 'DOM Change' column showing button updates to '1'.

If the button is clicked again after step 5, what will happen next?

A count becomes 3 and component re-renders

B count stays 2 and no re-render

C count resets to 0

D Component unmounts

💡 Hint

Refer to the pattern of state updates and re-renders in execution_table steps 2-5.

Concept Snapshot

React Updating Phase:
- Triggered by state or props change
- React schedules re-render
- Component function runs again
- JSX output compared to previous
- Only changed DOM parts updated
- Browser shows new UI
- Efficient and fast updates

Full Transcript

The React updating phase starts when an event or state change happens, like clicking a button. React schedules an update and re-renders the component by running its function again with new state or props. It compares the new JSX output with the old one to find differences. Then React updates only the changed parts of the DOM. This process repeats for each state change, making the UI update smoothly and efficiently without reloading the whole page.