Reconciliation is the process by which React determines how to efficiently update the actual DOM to reflect changes in the Virtual DOM. When a component's state or props change, React generates a new Virtual DOM representation of the component and then compares it with the previous Virtual DOM to identify the differences. Once the differences are identified, React applies the minimum number of changes to the actual DOM to bring it in sync with the new Virtual DOM. This process is at the heart of React's efficient rendering and contributes to its performance optimisation.
Virtual DOM Representation: Whenever a component's state or props change, React creates a new Virtual DOM representation of the component's structure and content. This representation is a lightweight JavaScript object tree that mirrors the actual DOM elements.
Diffing Algorithm: React uses a diffing algorithm to compare the new Virtual DOM with the previous one. This algorithm efficiently identifies the differences between the two representations.
Element-Level Comparison: React's diffing process is performed at the element level. It checks whether each element in the new Virtual DOM is the same as the corresponding element in the previous Virtual DOM. If the elements are different, React considers the element as needing an update.
Virtual DOM Reconciliation vs. Actual DOM Manipulation: The reconciliation process, which operates on the Virtual DOM, is significantly faster than directly manipulating the actual DOM. Traditional DOM manipulation requires browser reflows and repaints, which can be resource-intensive and slow down the UI. React's approach minimizes these costly operations, resulting in better performance.
Element Creation: If an element exists in the new Virtual DOM but not in the previous one, React creates the corresponding element in the actual DOM.
Element Removal: If an element exists in the previous Virtual DOM but not in the new one, React removes the corresponding element from the actual DOM.
Element Update: If an element exists in both the previous and new Virtual DOM, React compares the attributes and content of the elements to identify any changes. It then updates only the necessary attributes and content in the actual DOM to reflect the changes.
Batched Updates: React batches multiple updates together and performs them in a single pass. This batching reduces the number of times the actual DOM is accessed and modified, minimizing the performance overhead associated with frequent DOM manipulations.
Efficiency Gains: By identifying and applying only the necessary changes to the actual DOM, React reduces the computational complexity and resource consumption of rendering updates. This approach leads to faster rendering times and smoother user experiences.