Reconciliation is the process React uses to compare the current Virtual DOM with the previous one and determine the minimal set of changes needed to update the real DOM. It ensures React applications stay fast and efficient even when rendering complex UI trees.
What is React reconciliation and how does it work?
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Answer
Overview
Reconciliation is the core mechanism that makes React performant. Whenever the state or props of a component change, React generates a new Virtual DOM tree. It then compares this new tree with the previous one to detect differences. This comparison process is called reconciliation, and the optimized algorithm React uses to perform it is known as the diffing algorithm.
Step | Description |
|---|---|
Render Phase | React creates a new Virtual DOM representation of the UI based on the latest component state and props. |
Diffing Phase | React compares the new Virtual DOM with the old one to detect what has changed. |
Commit Phase | React updates only the changed elements in the real DOM, leaving the rest untouched. |
JSX
// Example: Reconciliation in action
function Counter() {
const [count, setCount] = useState(0);
return (
<div>
<h2>Count: {count}</h2>
<button onClick={() => setCount(count + 1)}>Increment</button>
</div>
);
}
// Only <h2> changes when count updates; React reuses <div> and <button>.
Why It Matters
Direct manipulation of the DOM is expensive. React’s reconciliation process ensures that only necessary updates happen, minimizing reflows and improving performance. This approach makes React declarative — you describe the UI, and React efficiently figures out how to update it.
Rules of Reconciliation
- If an element type changes (e.g.,
<div><span> - Keys help React identify elements across renders, especially in lists, ensuring stable updates instead of re-creating entire subtrees.
<li>Elements of the same type are compared recursively.Reconciliation and Keys
- Keys allow React to match old and new elements efficiently.
- When keys are missing or inconsistent, React may re-render entire lists instead of updating only changed items.
- Stable, unique keys (like IDs) help React maintain state between renders.
Example
If you render a list and change one item’s position, React uses the key attribute to identify which element moved instead of destroying and re-creating all items.
JSX
{items.map(item => (
<li key={item.id}>{item.name}</li>
))}
Reconciliation is React’s secret engine — it ensures your UI stays fast by updating only what’s necessary, not everything that changed.
Summary
- Reconciliation is React’s process for updating the DOM efficiently.
- It compares old and new Virtual DOM trees using the diffing algorithm.
- Only changed parts of the real DOM are updated, ensuring optimal performance.
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