Virtual DOM vs Real DOM

 okay it mean in case of useState there is initial virtual dom when state is updated a new instance of vdom is created and both vdom are compared and only minimal sort of change required in particular node of real dom is done.

Venice

Yes, you've got the core idea exactly right, but with one crucial clarification that's a common point of confusion.

Let's refine your statement slightly for technical precision.

Your statement: "in case of useState there is initial virtual dom when state is updated a new instance of vdom is created and both vdom are compared and only minimal sort of change required in particular node of real dom is done."

The Refined Version: When useState updates, a new React Element tree is created, which is then used to generate a new Virtual DOM (VDOM) representation. This new VDOM is then compared (diffed) against the VDOM from the previous render. React calculates the minimal set of changes and applies them to the real DOM.

You were 99% there. The key distinction is that it's not a "new instance of VDOM" in isolation, but a new VDOM representation of the entire component tree, which is then diffed against the old one.

Let's break down the full technical flow:

The useState Render Cycle: A Step-by-Step

Step 1: Initial Render

1. You call YourComponent() for the first time.
2. Inside, const [count, setCount] = useState(0) is executed. React initializes a state cell for count with the value 0.
3. Your component returns a React Element tree (e.g., <div><p>0</p><button>...</button></div>).
4. React builds a Virtual DOM from this tree.
5. React compares this VDOM to the "real" DOM (which is empty for this component) and renders the entire structure to the screen.

Step 2: State Update

1. The user clicks the button, which calls setCount(1).
2. This is the critical part: setCount does not change the count variable yet. Instead, it tells React: "Schedule a re-render for this component. The new value for count is 1."

Step 3: The Re-Render (The "Diffing" Process)

1. React starts the re-render process. It calls YourComponent() again from scratch.
2. useState(0) is called again. But this time, React sees it already has a state cell for this hook. It doesn't use the initial value 0. Instead, it returns the new value, 1.
3. Your component runs with count now being 1 and returns a new React Element tree: <div><p>1</p><button>...</button></div>.
4. React builds a new Virtual DOM from this new tree.

Step 4: Reconciliation (The Comparison)

Now React has two Virtual DOMs:

• Old VDOM: <div><p>0</p><button>...</button></div>
• New VDOM: <div><p>1</p><button>...</button></div>

The Reconciliation algorithm (often called "diffing") compares them, node by node:

• div vs div: Same type, no change.
• p vs p: Same type. Let's check the children.
• "0" vs "1": Different! This is a change.
• button vs button: Same type, no change.

Step 5: Commit (The Real DOM Update)

React has calculated the minimal change: the text content inside the <p> tag needs to be updated.

It then performs a single, highly-optimized operation on the real DOM:
pNode.textContent = '1';

It does not destroy the <div> or the <button>. It surgically updates only the part that changed. This is far more efficient than re-rendering the entire component and its children in the browser.

Summary

Phase	What Happens	Key Concept
Trigger	setCount(newVal) is called.	Scheduling
Render	React calls the component function again, generating a new React Element Tree. useState returns the new value.	Re-render
Reconciliation	React creates a new VDOM and diffs it against the previous VDOM.	Diffing
Commit	The calculated minimal changes are applied to the real DOM.	Updating the UI