One of the biggest reasons React is popular is its ability to update the user interface efficiently.
When your application’s data changes, React updates what users see on the screen. But React does not usually rebuild the entire DOM from scratch every time something changes.
Instead, React compares the previous UI with the new UI and determines what actually needs to change.
This process is called reconciliation.
Understanding reconciliation helps you answer important questions such as:
- Why does a React component re-render?
- What happens after state changes?
- How does React decide what to update?
- Why are
keyprops important in lists? - What is the difference between rendering and updating the DOM?
In this article, we’ll explore React reconciliation in a simple and practical way.
What Is React Reconciliation?
Reconciliation is the process React uses to compare the previous UI representation with the new one and determine the necessary changes to update the actual DOM.
A simplified flow looks like this:
State or Props Change
│
▼
Component Re-renders
│
▼
New React Element Tree
│
▼
React Compares Old and New Trees
│
▼
Reconciliation
│
▼
Only Necessary DOM Updates
The important idea is that a component re-render does not automatically mean the entire DOM is rebuilt.
React may run your component again, compare the result with the previous result, and update only the parts that actually changed.
The Difference Between Rendering and Updating the DOM
These two concepts are often confused.
Suppose you have a component that displays a counter.
When the counter state changes:
State Changes
↓
React Renders the Component Again
↓
React Compares the Result
↓
React Updates the DOM if Needed
A re-render means React calculates what the UI should look like now.
A DOM update means React actually changes something in the browser.
These are not exactly the same thing.
For example:
Component Re-rendered
│
▼
New Output Is Same
│
▼
No Meaningful DOM Change Needed
This distinction is important when learning React performance.
A Simple Example
Imagine this UI:
Hello, Manjeet
After a state change, the new UI becomes:
Hello, John
React compares the previous and new UI:
Previous
Hello, Manjeet
VS
New
Hello, John
React sees that only the text content changed.
So conceptually:
Keep:
Hello,
Update:
Manjeet → John
React does not need to remove the entire page and create everything again.
It identifies the required change and updates the UI accordingly.
This comparison process is part of reconciliation.
How React Represents Your UI
When you write JSX like this:
<h1>Hello, {name}</h1>
React creates a description of the UI.
You can think about it as a tree:
App
│
└── h1
│
└── Hello, Manjeet
After the state changes, React creates a new description:
App
│
└── h1
│
└── Hello, John
React then compares the previous tree with the new tree.
This comparison helps React decide what should stay, what should change, and what should be removed.
The Basic Reconciliation Process
At a high level, React follows this flow:
1. Something Changes
↓
2. React Re-renders
↓
3. New UI Tree Is Created
↓
4. Previous and New Trees Are Compared
↓
5. Differences Are Identified
↓
6. Required Changes Are Committed to the DOM
Let’s understand each step.
Step 1: Something Changes
Reconciliation usually begins when something causes React to update.
Common reasons include:
- State changes
- Props change
- Context changes
- A parent component re-renders
For example:
Button Click
│
▼
setCount(...)
│
▼
State Changes
│
▼
React Starts Update Process
Step 2: React Re-renders
React runs the component again to determine what the UI should look like with the latest data.
For example:
Old State: count = 1
↓
Component Renders
New State: count = 2
↓
Component Renders Again
The component produces a new UI description.
Step 3: React Creates a New UI Tree
React now has two versions of the UI:
Previous Tree
App
└── Counter
└── Count: 1
And:
New Tree
App
└── Counter
└── Count: 2
React compares these versions.
Step 4: React Compares the Trees
React uses reconciliation to determine the differences.
Conceptually:
Old UI Tree
│
▼
Compare
▲
│
New UI Tree
React uses heuristics to make this comparison efficient.
It does not perform an unlimited, perfect comparison of every possible UI structure. Instead, React follows practical rules that allow it to identify updates efficiently.
What Happens When Element Types Change?
One important rule is based on the type of an element.
Suppose the previous UI is:
<div>
Profile
</div>
And the new UI becomes:
<section>
Profile
</section>
The element type has changed:
div
↓
section
React treats this as a significant change.
Conceptually:
Remove Old Tree
↓
Create New Tree
This can also affect the state of components inside that part of the tree.
The reason is simple: React sees these as different element types.
What Happens When the Element Type Stays the Same?
Now consider:
Old:
<button class="primary">Save</button>
Then:
New:
<button class="secondary">Save</button>
The element type is still:
button
Only an attribute changed.
React can update the relevant DOM property instead of replacing the entire element.
Conceptually:
Same Element
│
▼
Compare Props
│
▼
Update Changed Values
This is one reason React can efficiently handle many UI updates.
Reconciliation and Components
The same idea applies to React components.
Suppose the tree looks like this:
App
├── Header
├── Sidebar
└── Content
If Content receives updated data, React can reconcile the new tree with the previous one and determine which parts need changes.
Conceptually:
App Update
│
▼
New Component Tree
│
▼
Compare Previous Tree
│
├── Header
├── Sidebar
└── Content ← Changed
The exact rendering behavior depends on the component tree, state, props, context, and React optimizations.
The key point is:
React determines what needs to change by comparing the previous and new UI structure.
Why Are key Props Important?
Keys are especially important when rendering lists.
Consider a list:
Users
1. Alice
2. Bob
3. Charlie
React needs to understand which item represents which user.
Without stable keys, list updates can become harder for React to track correctly.
With keys:
Alice → key="alice"
Bob → key="bob"
Charlie → key="charlie"
React can identify each item.
Conceptually:
Previous List New List
Alice [1] Alice [1]
Bob [2] → David [4]
Charlie[3] Bob [2]
Charlie[3]
React can use the keys to understand which items are the same, which are new, and which have moved.
What Happens Without Stable Keys?
Imagine a list:
Apple
Banana
Orange
Now a new item is inserted at the beginning:
Mango
Apple
Banana
Orange
If React cannot reliably identify the items, it may match them based on their position in ways that can lead to unnecessary updates or incorrect state preservation.
Stable keys provide identity.
Item
│
▼
Stable Key
│
▼
React Can Track It
This is why keys are important for dynamic lists.
Why Using Array Index as a Key Can Be Risky
Developers sometimes write:
items.map((item, index) => (
<Item key={index} />
))
Using an array index can work in simple lists, especially when the list is static.
However, it can cause problems when items are:
- Inserted
- Removed
- Reordered
For example:
Before
0 → Alice
1 → Bob
2 → Charlie
After inserting a user at the beginning:
0 → David
1 → Alice
2 → Bob
3 → Charlie
The positions have changed.
If the index is used as identity, React may associate component state with positions rather than the actual logical items.
A stable unique ID is generally a better choice:
Alice → user.id = 101
Bob → user.id = 102
Charlie → user.id = 103
Reconciliation and Component State
Reconciliation also affects whether React preserves or resets component state.
React tracks state based on a component’s position and identity in the UI tree.
Imagine:
App
└── Profile
└── Form
If the same component remains in the same logical position, React can generally preserve its state.
But if the component identity changes significantly, React may treat it as a new component.
Conceptually:
Same Position + Same Identity
↓
State Can Be Preserved
While:
Different Identity
↓
Old Component Removed
↓
New Component Created
↓
State Resets
This behavior becomes particularly important when working with conditional rendering and keys.
Reconciliation in Lists
Lists are one of the most common places where reconciliation becomes easier to understand.
Imagine:
Before
Products
├── Laptop
├── Phone
└── Watch
After an update:
Products
├── Laptop
├── Tablet
├── Phone
└── Watch
React compares the old and new children.
With stable keys, it can conceptually determine:
Laptop → Keep
Tablet → Create
Phone → Keep
Watch → Keep
The key helps React understand that Phone and Watch did not suddenly become completely different items just because their positions changed.
Reconciliation vs the Virtual DOM
The terms Virtual DOM and reconciliation are often used together, but they are not exactly the same thing.
A simple way to think about them is:
UI Representation
↓
Previous Version
+
New Version
↓
Reconciliation
↓
Determine Changes
↓
Update Actual DOM
The UI representation gives React a way to describe what the interface should look like.
Reconciliation is the process of comparing the relevant previous and new representations and determining the required changes.
So, in simple terms:
- Virtual DOM/UI representation → describes the UI
- Reconciliation → compares and determines changes
- DOM update → applies the required changes to the browser
The Render Phase and Commit Phase
React updates can be understood in two broad stages.
Render Phase
During this stage, React determines what the UI should look like.
Conceptually:
State Changes
↓
Components Render
↓
React Calculates New UI
↓
Reconciliation Work
This phase is about determining the changes.
Commit Phase
During the commit phase, React applies the required changes.
Conceptually:
Changes Identified
↓
Commit Phase
↓
Update DOM
↓
Browser Displays New UI
A simple mental model is:
Render Phase
"Figure out what changed"
↓
Commit Phase
"Apply the changes"
Why React Reconciliation Is Useful
Imagine manually updating the DOM.
You would need to determine:
What changed?
What should be removed?
What should be added?
Which element should be updated?
Which list item moved?
React handles this process for you.
You focus on describing the UI based on your current state:
UI = function of state
React handles the process of updating the interface when that state changes.
This declarative approach is one of React’s biggest advantages.
Common Misunderstanding: React Re-renders Everything in the DOM
This is not how you should think about React.
A component may re-render because React needs to calculate the new UI.
But that does not mean the browser DOM is completely recreated.
The flow is closer to:
Component Re-renders
↓
React Gets New Output
↓
Compare With Previous Output
↓
Find Necessary Changes
↓
Update DOM Where Needed
A re-render is part of React’s update process.
The actual DOM changes depend on the result of reconciliation.
Common Misunderstanding: Re-rendering Is Always Bad
Re-rendering is a normal part of React.
For example:
User Clicks Button
↓
State Changes
↓
Component Re-renders
This is expected behavior.
You should not immediately optimize every re-render.
Optimization becomes useful when repeated rendering causes measurable problems, such as:
- Slow interactions
- Expensive calculations
- Large UI trees
- Performance bottlenecks
The best approach is:
Understand the rendering behavior first, then optimize if there is a real problem.
How Memoization Relates to Reconciliation
Memoization tools such as:
React.memouseMemouseCallback
can sometimes reduce unnecessary work.
For example:
Parent Re-renders
│
▼
React.memo Child
│
▼
Props Changed?
┌──┴──┐
No Yes
│ │
▼ ▼
Skip Render
Possible Again
Work
Memoization can help in specific situations, but it does not replace an understanding of reconciliation.
Before adding memoization, understand:
- What is re-rendering?
- Why is it re-rendering?
- Whether the work is actually expensive
- Whether optimization provides a measurable benefit
A Complete Mental Model
Here is the easiest way to visualize the entire process:
User Interaction
│
▼
State / Props Change
│
▼
React Re-renders Components
│
▼
New UI Description
│
▼
Compare With Previous UI
│
▼
Reconciliation
│
▼
Identify Required Changes
│
▼
Commit Changes
│
▼
Browser DOM Updates
│
▼
User Sees Updated UI
This is the core idea behind React reconciliation.
Best Practices
Use Stable Keys in Lists
When rendering dynamic lists, use stable and unique IDs whenever possible.
Good:
key={user.id}
Less Reliable for Dynamic Lists:
key={index}
Stable keys help React correctly identify items across updates.
Do Not Fear Normal Re-renders
React components are designed to re-render when necessary.
Focus on actual performance problems instead of trying to prevent every render.
Avoid Unnecessary State
The more unnecessary state you maintain, the more updates your application may need to process.
If a value can be calculated from existing state or props, consider calculating it instead of storing another piece of state.
Keep Components Well Structured
Breaking a large component into meaningful components can make your UI easier to understand and optimize.
For example:
Dashboard
├── Header
├── Sidebar
├── Statistics
└── ProductList
Clear component boundaries often make rendering behavior easier to reason about.
Optimize Only When Needed
Tools like React.memo, useMemo, and useCallback can be useful, but they should solve a specific problem.
Avoid adding memoization everywhere by default.
Final Thoughts
React reconciliation is the process that helps React efficiently update the user interface when your application’s data changes.
The most important idea to remember is:
A React re-render does not necessarily mean the entire DOM is recreated.
React generally follows a process like this:
State Changes
↓
Components Re-render
↓
New UI Is Generated
↓
Old and New UI Are Compared
↓
Reconciliation Finds Differences
↓
Necessary Changes Are Committed
↓
DOM Updates
Keys play an important role when reconciling lists, while element types and component identity affect whether React can preserve existing parts of the UI and their state.
Once you understand reconciliation, many React concepts become easier to understand—including:
- Component re-rendering
- The Virtual DOM
- List keys
- Component state preservation
- Memoization
- React performance optimization
The simplest way to remember React reconciliation is:
React renders the new UI, compares it with the previous UI, and updates only what needs to change.
That mental model will help you understand how React works behind the scenes and write more predictable React applications.
FAQs
What is reconciliation in React?
Reconciliation is the process React uses to compare the previous UI with the new UI and determine the changes required to update the DOM.
Does a component re-render mean the entire DOM is recreated?
No. A re-render allows React to calculate the new UI. React then reconciles the result and commits only the DOM changes that are needed.
Why are keys important in React?
Keys help React identify items in a list between renders. Stable keys are especially important when items are added, removed, or reordered.
What happens if an element type changes?
React treats different element types as different UI structures. The old element tree may be replaced with a new one.
What is the difference between the render phase and commit phase?
During the render phase, React determines what the UI should look like. During the commit phase, React applies the necessary changes to the DOM.
Is React re-rendering always bad for performance?
No. Re-rendering is a normal part of React. Optimization is usually necessary only when rendering causes a measurable performance problem.
How does reconciliation relate to the Virtual DOM?
React uses its UI representations to compare previous and new output. Reconciliation is the process of determining the differences and deciding what changes should be applied to the actual DOM.
