how component is rendered when the state is changed in react

In React, setState is a crucial function used to update a component's state. When setState is called, React automatically triggers a re-render of the component. Here's a detailed explanation of how this process works:

How setState Triggers a Re-Render

1. Initial State:

   • When a React component is first created, it has an initial state defined, usually in the constructor or as a class field.

   class MyComponent extends React.Component {
     constructor(props) {
       super(props);
       this.state = {
         count: 0
       };
     }
   }
   

2. State Update with setState:

   • When the component's state needs to be updated, setState is called with the new state values.

   incrementCount = () => {
     this.setState({ count: this.state.count + 1 });
   }
   

3. Scheduling an Update:

   • setState is asynchronous. When it is called, React schedules an update to the component's state and marks the component for re-rendering.
   • React batches multiple state updates to improve performance.

   this.setState({ count: this.state.count + 1 });
   this.setState({ count: this.state.count + 2 });
   // Only one re-render is triggered with count being incremented by 2.
   

4. Component Re-Render:

   • React re-renders the component by calling the render method. The new state is passed to the component, and the component is re-rendered with the updated state.

   render() {
     return (
       <div>
         <p>Count: {this.state.count}</p>
         <button onClick={this.incrementCount}>Increment</button>
       </div>
     );
   }
   

5. Virtual DOM Diffing:

   • React compares the new virtual DOM with the previous virtual DOM to determine what has changed.
   • This process is called "reconciliation."
   • Only the parts of the DOM that have changed are updated, making the update process efficient.

6. Updating the Real DOM:

   • Based on the differences found during the reconciliation process, React updates the real DOM.
   • This ensures minimal updates to the DOM, which improves performance.

Example

Here's a complete example to illustrate this process:

import React from 'react';

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    };
  }

  incrementCount = () => {
    this.setState({ count: this.state.count + 1 });
  };

  render() {
    console.log('Rendering...');
    return (
      <div>
        <p>Count: {this.state.count}</p>
        <button onClick={this.incrementCount}>Increment</button>
      </div>
    );
  }
}

export default Counter;