React Component Re-rendering

Overview

• Previous video covered the concept of rendering in React.
• Today's topic: When does a React component re-render?

Purpose of Re-rendering

• React components consist of multiple state variables and props.
• UI is computed based on these variables, requiring re-computation when values change.
• Without re-rendering, the UI would remain static, similar to early web pages.

When Does a Component Re-render?

• Common misconception: A component re-renders whenever its state or props change.
• This explanation is overly simplistic and not entirely accurate.

Example: App Component

• App Component: Displays a counter with an increment button.
• Counter Component: Contains a state variable count that updates on button press.
  • On pressing the increment button, the state variable changes.
  • Only the Counter and its children (e.g., displaying count) re-render, not the entire App component.

Understanding Component Re-rendering

• Big Count Number Component: Re-renders because it receives count as a prop.
• Test Component: Even if it does not depend on count, it still re-renders when the Counter component re-renders.
• Reason: React assumes components are not pure; outputs may depend on external factors (e.g., API calls, current time).

Performance Considerations

• Concerns about performance due to re-rendering many child components.
• React optimizes rendering through:
  • Reconciliation Algorithm: Detects minimal changes needed for the DOM.
  • Virtual DOM: Efficiently updates only the necessary parts of the UI.

Controlling Re-renders with React.memo

• If a component is pure and should not re-render with every parent re-render, use React.memo.
  • Wrap the component in React.memo to memoize the output based on props.
• React will skip re-rendering if props remain the same.

Conclusion

• Clarified that components re-render when their state changes and trigger re-renders of child components.
• The rendering process is well-optimized, but developers can control it using React.memo.
• Suggested further exploration of React.memo in future content.
• Link to an article by Josh for deeper understanding provided.