React Rendering Deep Dive — Part 2: Inside the Virtual DOM Lifecycle

In Part 1, we explored what the Virtual DOM is and how React uses it to efficiently render UI. Now, let’s go deeper into the lifecycle of the Virtual DOM and how React Fiber enables concurrent rendering.

🧠 What Happens During Rendering?

Rendering in React is a two-phase process:

1. Render Phase (a.k.a. Reconciliation)

• React builds a new Virtual DOM tree based on state/props changes.
• It compares it with the previous tree using the diffing algorithm.
• It determines the minimum set of changes required.

1. Commit Phase

• React applies those changes to the real DOM.
• This is where mutations (DOM updates, refs, componentDidMount) happen.

🔄 React Fiber: The Engine Behind Concurrent Rendering

React Fiber is a reimplementation of React's core algorithm, enabling the splitting of work into interruptible units.

✅ Benefits of Fiber:

• Pause and resume rendering (non-blocking updates)
• Prioritize updates (e.g. input field > analytics)
• Better support for animations and transitions

🧩 Visualizing the Lifecycle

• Trigger: A state or prop changes.
• Reconciliation: React builds a new virtual tree.
• Diffing: It compares old vs. new tree.
• Fiber Scheduling: React decides when to commit.
• Commit Phase: DOM updates are applied.

Each update becomes a “fiber” — an individual unit of work. React walks through these fibers, allowing pausing/resuming when needed.

🧪 Tools to Explore the Lifecycle

• React DevTools Profiler – visualize render phases.
• why-did-you-render – debug unnecessary renders.
• Lighthouse – check app responsiveness.

💡 Pro Tips

• Use React.memo, useMemo, and useCallback to minimize recalculations.
• Keep components small and predictable.
• Avoid blocking the main thread with heavy operations.

React’s Fiber architecture makes the Virtual DOM lifecycle more flexible and performant — empowering apps to feel snappy even during intensive updates.

👉 Stay tuned for the next blogs.