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Rendering in react

Rendering in React is the process of transforming your component structures and data into the actual user interface displayed in the browser. It's the bridge between your application's logic and the visual elements users interact with.

The Rendering Process​

React rendering can be broken down into two key phases:

  1. Render Phase: This phase happens whenever a component's state or props change, or when the component is initially mounted. During this phase:

    • React calls the render method of the component (or the function itself for functional components).
    • The render method returns a description of the UI using JSX (JavaScript XML). This JSX code represents the virtual DOM, a lightweight in-memory representation of the UI.

  2. Commit Phase: Once React has the updated virtual DOM, it compares it to the previous virtual DOM using a diffing algorithm. This algorithm efficiently identifies the minimal changes necessary to reflect the updates in the actual browser DOM.

    • Based on the calculated differences, React efficiently modifies the real DOM elements, ensuring the UI on the screen accurately reflects the current state of your application.

Here's a simplified representation of the rendering process:

Component State/Props Change ->
Render Phase (Component Renders JSX) ->
Virtual DOM Update ->
Diffing Algorithm ->
Commit Phase (DOM Updates) ->
Updated UI in Browser

Key Concepts in Rendering​

  • JSX: Provides a syntax extension for writing HTML-like structures within JavaScript, making UI definitions more intuitive.
  • Components: Reusable building blocks that encapsulate UI, logic, and state. The render method within components defines how the UI is displayed based on props and state.
  • Virtual DOM: An in-memory representation of the UI, allowing for efficient updates by calculating minimal changes before applying them to the real DOM.
  • Diffing Algorithm: Compares the previous and updated virtual DOM representations to identify the most efficient way to update the actual DOM.
  • DOM Updates: The final step where React manipulates the real DOM elements based on the calculated differences from the diffing algorithm.

Rendering Optimizations​

While React's virtual DOM and diffing algorithm are powerful, there are ways to further optimize rendering performance:

  • Memoization: Techniques like React.memo or useMemo can help avoid unnecessary re-renders of components by caching expensive calculations or component outputs.
  • Pure Components: Functional components without side effects can be declared as pure using React.memo, ensuring they only re-render when their props change.
  • Conditional Rendering: Use logical expressions within JSX to conditionally render elements based on state or props, avoiding unnecessary UI elements from being created.