Can you describe the reconciliation process?

by Pawan Kumar @jsartisan

**Reconciliation** is the algorithm React uses to figure out what _real_ DOM changes are needed after a component’s state or props change. ### 1. Triggering a re-render Calling `setState` (or its Hook equivalent) makes React call the component function again, producing a new virtual-DOM tree in memory. No DOM work happens yet. Consider the following code: ```jsx import { useState } from "react"; export default function App() { const [showTextarea, setShowTextarea] = useState(false); return showTextarea ?

: <input />;
}
```

### 2. Building “before” and “after” trees

After the first render React has a _previous_ virtual-DOM (Fiber) tree. When state changes it produces a _next_ tree, e.g.

**Before**

```json
{ "type": "input", "props": {} }
```

**After**

```json
{ "type": "textarea", "props": {} }
```

### 3. React’s reconciliation heuristics

1. **Same element type & key**  
   Reuse the existing DOM node and update its props and children.
2. **Different element type or key**  
   Unmount the old node (running cleanup effects), then create and mount a brand-new DOM node.

Because “input” and “textarea” are different types React chooses the second path: it removes the `<input>` and mounts a new `<textarea>`.

### 4. Lists and keys

For arrays of children React matches elements by their `key` prop first and then by index. Supplying stable keys (`id`, not `array.index`) prevents unwanted unmounts and preserves user input.

### 5. Why this matters

These simple rules give React an O(n) diffing algorithm, allowing large UIs to update quickly without a full page reload.

That entire decision-making process is called **reconciliation**.

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