When does a component re-render in React? Does the entire tree re-render or when does it decide which component to re-render which not to?

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🔍 What is it?

  • In React, re-rendering refers to the process of updating the user interface (UI) of a component to reflect changes in its state or props. When the data that a component depends on changes, React automatically re-renders the component to ensure that the UI stays in sync with the updated data.

❓ How is it used?

  • React manages re-rendering internally based on changes in state and props. When the state of a component changes (via setState()) or when new props are received from a parent component, React triggers a re-render of the component and its children. Developers don't need to explicitly call the render() method; React handles this automatically.

Why is it needed?

  • Re-rendering is essential for maintaining a responsive and dynamic user interface. It ensures that the UI accurately reflects the underlying data and state of the application. Without re-rendering, the UI could become out of sync with the data, leading to inconsistencies and rendering errors. By automatically re-rendering components when their state or props change, React ensures a smooth and consistent user experience.

Examples:

  • Imagine a <Counter> component that displays a count value. When the user clicks a button to increment the count, the component's state changes (setState() is called with the new count value), triggering a re-render of the <Counter> component. This re-render updates the displayed count in the UI to reflect the updated state.
  • Similarly, consider a parent component <App> that passes new props to a child component <Child>. When <App> updates its state or receives new data, it re-renders, causing <Child> to receive new props. React then re-renders <Child> with the new props, updating its UI based on the changes. This allows for dynamic updates in the UI based on changes in data or user interactions.

When does the entire tree re-render?

React's reconciliation algorithm is designed to optimize performance by minimizing unnecessary re-renders.

Here's how it works:

  1. Virtual DOM Comparison: When a component's state or props change, React constructs a new virtual DOM tree representing the updated UI.
  2. Element Comparison: React then compares the new virtual DOM tree with the previous one to determine which elements have changed. This comparison is performed recursively, starting from the root of the tree and traversing down to the leaf nodes.

  3. Component Reconciliation: React uses several strategies to determine whether a component needs to be re-rendered:

    • If a component's state or props have changed, React considers the component and its subtree to be dirty and in need of re-rendering.
    • If a component's state or props have not changed, React skips the re-rendering of the component and its subtree. This is known as "shallow comparison."
    • React also uses a mechanism called "shouldComponentUpdate" or "PureComponent" to optimize re-rendering further. Components can implement these methods to specify whether they should re-render based on the changes in their props or state.
    • Re-rendering Decision: Based on the comparison results, React decides which components to re-render and which ones to skip. Only the components and elements that have changed or need to be updated are re-rendered, while the rest of the tree remains unchanged.