React primarily uses a Diffing Algorithm as part of its Reconciliation process.

This algorithm compares the current Virtual DOM with the previous Virtual DOM and determines the minimal number of changes required to update the real DOM.

This makes React extremely fast and efficient.

Understanding React’s Diffing Algorithm

When a component's state or props change, React performs the following steps:

1. React creates a new Virtual DOM tree
2. It compares this new tree with the previous Virtual DOM
3. It calculates the difference (diff)
4. It updates only the changed elements in the real DOM

This process is called Reconciliation.

Instead of updating the entire page, React updates only the parts that changed.

Why React Needs a Diffing Algorithm

Updating the real DOM directly is slow and expensive.

For example:

Imagine updating a list of 1000 elements.

Without a diffing strategy, the browser would need to:

• Remove all 1000 elements
• Recreate them again

But React does something smarter.

It finds exactly what changed and updates only that element.

How React Optimizes the Diffing Process

React makes two important assumptions to optimize performance.

1. Different Element Types Produce Different Trees

Example:

<div>

changing to

<span>

React destroys the old tree and builds a new one.

2. Keys Help Identify List Elements

When rendering lists, React uses keys to track elements efficiently.

Example:

{items.map(item => (
  <li key={item.id}>{item.name}</li>
))}

Keys help React identify:

• which elements changed
• which moved
• which were removed

This dramatically improves rendering performance.

Why DFS, Dijkstra, or A* Are Not Used

Some developers think React uses algorithms like:

• Depth First Search (DFS)
• Dijkstra’s Algorithm
• A* Search

But these are used for graph traversal and pathfinding, not UI rendering.

React's algorithm is specifically designed for tree comparison and UI updates.

Complexity of React's Diffing Algorithm

A naive tree comparison would take:

O(n³) time complexity.

React optimizes this to approximately:

O(n)

This is why React applications scale well even with large component trees.

Virtual DOM vs Real DOM

React updates the Virtual DOM first, then applies the minimal changes to the Real DOM.

Real World Example

Imagine a React component:

<h1>Hello World</h1>

Later it updates to:

<h1>Hello InterviewKit</h1>

React does not re-render the entire element.

It simply updates:

textContent = "Hello InterviewKit"

This small optimization improves performance significantly.

Key Takeaways

✔ React uses a Diffing Algorithm
✔ It compares Virtual DOM trees
✔ The process is called Reconciliation
✔ Only changed elements are updated
✔ This keeps React apps fast and scalable