Understanding React’s Component Re-Rendering Process: When and Why it Occurs

React is a popular Javascript library for building user interfaces. One of the key features of React is its ability to efficiently re-render components when the underlying data changes. This makes it possible to build fast and responsive applications without having to worry too much about optimizing performance.

In this article, we’ll take a closer look at how React’s component re-rendering process works, including when and why it occurs.

• How React’s Component Re-Rendering Process Works
• When React Re-Renders Components
• Few examples of when a React component might need to be re-rendered
• Why React Re-Renders Components
• Conclusion

How React’s Component Re-Rendering Process Works

When you build a React application, you typically break down the UI into a hierarchy of components. Each component is responsible for rendering a part of the UI, and can contain other components as children.

When a component’s state or props change, React automatically re-renders that component and any child components that depend on its data. React does this by comparing the new props and state to the previous ones, and then determining which parts of the UI need to be updated.

React uses a process called “reconciliation” to determine which parts of the UI have changed. During reconciliation, React compares the new and old versions of the component tree and generates a list of changes that need to be applied to the DOM. React then applies those changes in a batch, minimizing the number of actual DOM updates.

When React Re-Renders Components

React re-renders components whenever their state or props change. This can happen for a variety of reasons, such as:

• User interaction: For example, when a user clicks a button or inputs some data into a form.
• Data changes: For example, when data is loaded from a server or when a timer fires.
• Parent component re-renders: When a parent component re-renders, all of its children will also re-render.

It’s worth noting that React doesn’t always re-render components immediately when their state or props change. Instead, React batches updates together and performs them in a single pass for efficiency. This means that in some cases, components may not re-render until later, or may be updated multiple times in a single pass.

Few examples of when a React component might need to be re-rendered

1. When a component’s state changes: Let’s say you have a component that displays a counter, and you want the counter to increment every time a button is clicked. You would need to use setState to update the component’s state, which would trigger a re-render of the component:

class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      count: 0
    };
  }
  handleClick() {
    this.setState({
      count: this.state.count + 1
    });
  }
  render() {
    return (
      <div>
        <p>Count: {this.state.count}</p>
        <button onClick={() => this.handleClick()}>Increment</button>
      </div>
    );
  }
}

2. When a component’s props change: Let’s say you have a component that displays a user’s name and avatar image, and you want to update the component’s props when a new user is selected. When the props change, React will automatically re-render the component:

class UserProfile extends React.Component {
  render() {
    return (
      <div>
        <img src={this.props.avatar} alt="Avatar" />
        <h2>{this.props.name}</h2>
      </div>
    );
  }
}
class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      currentUser: {
        name: 'Alice',
        avatar: 'https://example.com/avatar1.jpg'
      }
    };
  }
  handleUserChange(user) {
    this.setState({
      currentUser: user
    });
  }
  render() {
    return (
      <div>
        <UserProfile
          name={this.state.currentUser.name}
          avatar={this.state.currentUser.avatar}
        />
        <UserSelector onUserChange={(user) => this.handleUserChange(user)} />
      </div>
    );
  }
}

3. When a component’s parent re-renders: Let’s say you have a component that displays a list of items, and you want to update the list when a new item is added. If the list component is a child of another component that re-renders (e.g. because its state changes), the list component will also be re-rendered:

class ItemList extends React.Component {
  render() {
    return (
      <ul>
        {this.props.items.map((item) => (
          <li key={item.id}>{item.name}</li>
        ))}
      </ul>
    );
  }
}
class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      items: [
        { id: 1, name: 'Item 1' },
        { id: 2, name: 'Item 2' },
        { id: 3, name: 'Item 3' },
      ]
    };
  }
  handleAddItem() {
    const newItem = { id: Date.now(), name: `Item ${this.state.items.length + 1}` };
    this.setState({
      items: [...this.state.items, newItem]
    });
  }
  render() {
    return (
      <div>
        <button onClick={() => this.handleAddItem()}>Add Item</button>
        <ItemList items={this.state.items} />
      </div>
    );
  }
}

These are just a few examples, but in general, any time a component’s state or props change, or its parent component re-renders, it will need to be re-rendered as well.

Why React Re-Renders Components

React re-renders components to ensure that the UI stays up-to-date with the underlying data. By re-rendering only the parts of the UI that have changed, React minimizes the number of DOM updates required and helps to keep the application fast and responsive.

In addition, React’s component re-rendering process is what makes it possible to build complex, data-driven applications with ease. By automatically re-rendering components whenever data changes, React takes care of a lot of the plumbing that would otherwise be required to keep the UI in sync with the data.

Conclusion

React’s component re-rendering process is a key feature of the library that makes it possible to build fast, responsive, and data-driven applications with ease. By understanding when and why React re-renders components, you can build more efficient and effective React applications that deliver a great user experience.