React Reconciliation, Fiber, and Virtual DOM Explained (Without the Jargon)

When you write React code, you usually just think in terms of state → UI.

But have you ever wondered how React efficiently updates the UI without repainting everything?

The secret sauce is Reconciliation, Fiber, and the Virtual DOM.

Let’s break it down step by step 👇

🧠 1. Virtual DOM: The Memory Blueprint

React doesn’t touch the real DOM directly every time you update state.

Instead, it builds a Virtual DOM — a lightweight JS object tree representing your UI.

Example UI:

<div id="app">
  <h1>Hello</h1>
  <button>Click</button>
</div>

Virtual DOM Representation:

{
  type: "div",
  props: { id: "app" },
  children: [
    { type: "h1", props: {}, children: ["Hello"] },
    { type: "button", props: {}, children: ["Click"] }
  ]
}

Each node has:

type → element/component type
props → attributes
children → nested nodes or text

Why it’s faster:

Updating JS objects in memory is much cheaper than updating the real DOM, which triggers:

style recalculation
layout reflow
repainting pixels

⚡ 2. React Diffing and Minimal DOM Updates

When you call setState, React doesn’t immediately touch the DOM.

It follows these steps:

Builds a new Virtual DOM.
Compares it with the old Virtual DOM (diffing).
Generates a patch with only the differences.

Example:

<!-- Before -->
<button>Click</button>

<!-- After -->
<button>Submit</button>

React sees only the text changed and updates button.textContent in the DOM.

The DOM is dumb — it just follows commands.

✅ You could manually update the DOM, but in complex apps that quickly becomes unmanageable.

🧩 3. Batching Updates

React groups multiple updates together in a process called batching.

This prevents unnecessary re-renders.

Pre-React 18:

Batching happened only inside React event handlers.

Example:

function handleClick() {
  setCount(c => c + 1);
  setName("Alex");
}

Both updates happen together → one render

React 18+ (Automatic Batching):

Now, batching works everywhere, even inside async code.

Example:

setTimeout(() => {
  setCount(c => c + 1);
  setName("Alex");
}, 0);

Both updates inside the timeout are batched → one render, not two.

Bottom line: fewer renders → smoother UI.

🧵 4. Fiber: Making Large Trees Responsive

For huge UI trees, diffing everything at once can freeze the page.

React Fiber fixes that.

How Fiber works:

Breaks work into units: Each component is a “unit of work.”
Incremental rendering: React processes a few nodes, pauses, then continues.
Yielding: React temporarily pauses work so the browser can handle input, scrolling, or animations.
Priority system:
- High: typing, animations
- Low: timers, transitions (startTransition() for low-priority updates)

💡 Analogy:

Old React = lifting 1000 kg of UI in one go → UI freezes.

Fiber = lifting 50 kg, take a break, continue → smooth experience.

🚀 5. Virtual DOM Skip / Compile Updates Directly

Some frameworks like Svelte or Solid go beyond React’s Virtual DOM idea.

They skip the diffing step entirely.

They:

Track exactly which state affects which node
Compile your code to direct DOM updates

Example:

function updateCount(newCount) {
  button.textContent = newCount;
}

No runtime diffing → direct DOM manipulation → blazing fast.

🧾 Key Takeaways

Virtual DOM → memory-efficient blueprint of the UI
Reconciliation → minimal DOM updates after diffing
Batching → multiple state updates grouped into one render
Fiber → incremental rendering, yielding, and priority control
Virtual DOM skip / compile updates → direct DOM updates for peak performance

React may seem like magic, but now you know the mechanics behind its efficiency.

Understanding this helps you write faster, smoother, and more predictable React apps.

Short summary: