What is reconciliation in React?

Reconciliation is the process by which React updates the browser's DOM to match the latest React element tree. It's how React determines what parts of the UI need to change in response to state or prop updates, doing so in an efficient manner.

What is Reconciliation?

When a component's state or props change, React needs to update the UI to reflect these changes. Instead of directly manipulating the browser's DOM, which can be slow, React uses a concept called the Virtual DOM. Reconciliation is the algorithm that React uses to compare the new Virtual DOM tree with the previous one, identify the differences, and then apply only the necessary updates to the real DOM.

The Virtual DOM and Diffing Algorithm

React maintains an in-memory representation of the UI, known as the Virtual DOM. When an update occurs, React performs the following steps:

• A new Virtual DOM tree is created by React, reflecting the latest state and props.
• React then 'diffs' (compares) this new Virtual DOM tree with the previous one.
• Based on the differences, it computes the minimal set of changes needed to update the real DOM.
• Finally, these changes are batched and applied to the browser's DOM, optimizing performance by avoiding unnecessary re-renders and direct DOM manipulations.

Diffing Rules

To make the diffing process efficient, React makes two main assumptions:

• When comparing two React elements of different types, React will tear down the old tree and build the new tree from scratch. For example, changing a <div> to a <span> will result in destroying the old div and all its children, then mounting the new span.
• When comparing two React DOM elements of the same type, React looks at the attributes of both and only updates what has changed. For example, changing className on a <div> will only update that attribute on the real DOM element.

When processing lists of elements, React relies on keys to efficiently identify which items have changed, been added, or been removed. Providing stable and unique keys is crucial for optimizing list reconciliation.

Benefits

• Performance: By minimizing direct manipulation of the actual DOM, which is a slow operation, React significantly improves application performance.
• Declarative UI: Developers can describe how their UI should look based on the current state, and React handles the complexities of updating the underlying DOM.
• Cross-Browser Compatibility: React abstracts away browser-specific DOM implementation details, providing a consistent development experience.