Mounting phase in React - Step-by-Step Execution

Concept Flow - Mounting phase

Component Function Called

↓

JSX Evaluated -> Virtual DOM Created

↓

React DOM Mounts Virtual DOM to Real DOM

↓

useEffect with empty deps runs

↓

Component Ready on Screen

The mounting phase starts when React calls the component function, creates the virtual DOM from JSX, mounts it to the real DOM, and runs effects for setup.

Execution Sample

React

import React, { useState, useEffect } from 'react';

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

export default Counter;

This React component shows a button with a count starting at 0 and logs 'Mounted' once when first shown.

Execution Table

Step	Trigger	State Before	Action	State After	What Re-renders	DOM Change
1	Initial render	count: undefined	Call Counter(), useState(0) sets count=0	count: 0	Counter component	Button with text '0' created
2	JSX evaluated	count: 0	Return <button> with count 0	count: 0	Counter component	Virtual DOM button with '0' ready
3	React mounts	count: 0	Mount virtual DOM to real DOM	count: 0	Counter component	Button with '0' appears on screen
4	useEffect runs	count: 0	Run effect callback, console.log('Mounted')	count: 0	No re-render	No DOM change
5	User clicks button	count: 0	setCount(1) triggers state update	count: 1	Counter component	Button text updates to '1'
6	Re-render after click	count: 1	Re-evaluate JSX with count=1	count: 1	Counter component	Button text changes to '1'
7	No more mounting	count: 1	No further mounting phase	count: 1	No re-render	No DOM change

💡 Mounting phase ends after initial render and useEffect with empty dependencies runs.

Variable Tracker

Variable	Start	After Step 1	After Step 5	Final
count	undefined	0	1	1

Key Moments - 3 Insights

Why does useEffect run only once during mounting?

Because useEffect has an empty dependency array [], React runs it only after the first render (see step 4 in execution_table).

Is the component function called again during mounting?

Yes, the component function is called once initially to create the virtual DOM (step 1), but not again until state changes.

What triggers the DOM to update after mounting?

State changes like setCount trigger re-renders and DOM updates, but these happen after mounting (step 5 and 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'count' after step 1?

A 1

B 0

C undefined

D null

💡 Hint

Check the 'State After' column in row for step 1.

At which step does React mount the virtual DOM to the real DOM?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for the row where 'React mounts virtual DOM to real DOM' in the 'Action' column.

If the useEffect dependency array was not empty, when would the effect run?

A Every time the component re-renders

B Only once during mounting

C Never

D Only when the component unmounts

💡 Hint

Recall how useEffect runs with dependencies on state changes, not just mounting.

Concept Snapshot

Mounting phase in React:
- Component function runs first time
- JSX creates virtual DOM
- React mounts virtual DOM to real DOM
- useEffect with [] runs once after mount
- Component appears on screen ready for interaction

Full Transcript

The mounting phase in React starts when the component function is called for the first time. React runs useState to set initial state, then evaluates the JSX to create a virtual DOM representation. Next, React mounts this virtual DOM to the real DOM, making the component visible on the screen. After mounting, useEffect hooks with empty dependency arrays run once for setup tasks. This phase ends when the component is fully rendered and ready. Later state changes cause re-renders but are not part of mounting.