Difference between rendering and committing phases?

React's reconciliation process, which updates the UI in response to state or prop changes, is fundamentally divided into two main phases: the rendering phase and the committing phase. Understanding these distinct phases is crucial for grasping how React works under the hood and for effective debugging and performance optimization.

1. The Rendering Phase

The rendering phase is where React calculates what changes need to happen to the UI. When a component's state or props change (or a parent component re-renders), React schedules a re-render for that component and its children.

• Purpose: To call your component functions (functional components) or render methods (class components) and compare the new JSX output (React elements) with the previous one. This process identifies what has changed within the component tree.
• Process: React traverses the component tree from the root or the changed component downwards, executing the component logic. It performs a 'diffing' algorithm between the newly generated React elements and the existing fiber tree (React's internal representation of the UI).
• Output: A 'snapshot' of the UI as React elements (an updated fiber tree). This is a description of what *should* be rendered, but no actual DOM manipulation occurs at this stage.
• Characteristics:
• - Pure and Idempotent: Rendering functions should be pure; they should not cause side effects (like direct DOM manipulation, network requests, or modifying objects outside their scope). Calling them multiple times with the same inputs should produce the same output. React's Strict Mode helps enforce this purity.
• - Interruptible: React can pause, interrupt, or restart the rendering process (e.g., if a higher-priority update comes in, or when using concurrent features like startTransition). This is why purity is essential.
• - No DOM Interaction: The rendering phase solely prepares the changes; it does not directly interact with the browser's Document Object Model (DOM).

2. The Committing Phase

The committing phase is where React takes the changes identified during rendering and physically applies them to the actual host environment (e.g., the browser's DOM for web applications, or native views for React Native). This is the stage where the UI is visually updated.

• Purpose: To perform the necessary operations to update the UI, making the changes visible to the user.
• Process: React walks the fiber tree (generated or updated during the rendering phase) and executes the actual host-specific operations: inserting new nodes, updating existing attributes/props, and deleting old nodes from the DOM.
• Output: The actual, updated UI reflected in the browser, ready for painting.
• Characteristics:
• - Impure: This phase directly interacts with the host environment (like the browser's DOM API), which is inherently a side effect.
• - Uninterruptible: Once the commit phase begins, it runs to completion without interruption to ensure UI consistency and prevent partial updates. It cannot be paused or restarted.
• - Side Effects: This is the phase where useLayoutEffect callbacks are executed synchronously immediately after DOM mutations, but before the browser has a chance to paint. useEffect callbacks are scheduled to run asynchronously after the browser has painted the updated UI.

Key Differences Summarized