Optimizing Re-Rendering in React: Best Practices for Beginners

React's component-based architecture enables efficient UI updates, but developers often encounter challenges with unnecessary re-renders, which can hinder application performance. Understanding and optimizing React's rendering behavior is crucial for creating fast and responsive applications. This article outlines best practices for beginners to minimize unnecessary re-renders and enhance the performance of React applications, ensuring a smoother user experience.

These are the following topics that we are going to discuss:

What Causes Re-Renders in React?

Re-renders in React occur when a component's state or props change. React compares the new state or props to the previous ones, and if a change is detected, it re-renders the component to reflect the updates in the UI. While this behavior ensures that your app stays up-to-date, excessive re-renders can lead to performance bottlenecks, especially in larger applications.

Some common causes of unnecessary re-renders include:

• State changes in parent components: When a parent component's state changes, all of its child components re-render, even if their props haven’t changed.
• Passing non-primitive props: Objects, arrays, and functions always create new references during re-render, causing components receiving these as props to re-render.
• Updating context: When context values change, all components consuming that context re-render, potentially affecting performance.

Key Strategies to Optimize Re-Renders in React

Use React.memo() for Component Memorization

React provides the React.memo() higher-order component to optimize re-renders. React.memo() works by memorizing a component and preventing it from re-rendering unless its props change. This is particularly effective for functional components that receive primitive props like strings, numbers, or booleans.

Example: In this case, the ChildComponent will only re-render if the count prop changes. This helps reduce the number of re-renders, particularly in deeply nested component trees.

 const ChildComponent = React.memo(({ count }) => {
     console.log('Child component is rendering');
      return (<> <div>Count: {count}</div> </>);
   });

Optimize Functions Passed as Props with useCallback

When a function is passed as a prop to a child component, it causes re-renders because functions in JavaScript are non-primitive and create a new reference every time. The useCallback hook can help by memorizing the function so that it only changes when its dependencies do.

Example: By using useCallback, the increment function will only be redefined when the count value changes, reducing unnecessary re-renders in child components.

const increment = useCallback(() => setCount(count + 1), [count]);

Memorize Expensive Computations with useMemo

For expensive calculations, React’s use Memo hook ensures that the result of a function is cached and only recalculated when dependencies change. This is useful when you want to avoid recalculating complex logic on every render.

const computedValue = useMemo(() => expensiveFunction(data), [data]);

By memorizing the computedValue, React prevents unnecessary recalculations, improving performance in components with heavy computational workloads.

Control Context Re-Renders

React’s Context API is a powerful way to share data across components. However, when a context value changes, all components consuming the context re-render, which can lead to performance issues. To mitigate this, you can:

• Split contexts to minimize the number of consumers affected by changes.
• Use React.memo() or useMemo to memorize context values and avoid unnecessary re-renders.

Use shouldComponentUpdate in Class Components

For class components, the shouldComponentUpdate() lifecycle method allows you to control whether a component should re-render based on changes in props or state. By default, React re-renders a component on every state or prop change, but with shouldComponentUpdate(), you can return false if the update isn't necessary.

Example: This is particularly useful in class-based components where you need finer control over the render cycle.

shouldComponentUpdate(nextProps, nextState) {
   return nextProps.value !== this.props.value;
}

Optimize Lists with key Props

Lists in React are rendered efficiently when each item is given a unique key. The key helps React identify which items have changed, been added, or been removed, ensuring that only the modified items are re-rendered.

Providing a unique key ensures React can correctly track the items in the list, reducing unnecessary re-renders of unchanged list items.

return (
   <ul>
     {items.map (item => (
         item.id}>{item.name}</li>
     ))}
   </ul>
 );

Concurrent Rendering in React 18

React 18 introduces concurrent rendering, which allows the app to prioritize tasks and prevent blocking the main thread. Using the useTransition hook, for instance, you can mark some state updates as low-priority, which lets the UI remain responsive even during complex state transitions.

This helps improve the user experience by ensuring that rendering expensive UI updates does not block more important interactions, like typing in a search field.

const [isPending, startTransition] = useTransition();
   startTransition(() => {
     setSearchQuery(query);
  });

Avoid Inline Functions and Objects

Defining functions or objects directly inside JSX can lead to unnecessary re-renders because they create new references on every render. Moving them outside the component or memorizing them with useCallback or useMemo prevents this issue.

Defining objects or functions outside the component or memorizing them ensures that they are not recreated on each render.

const style = { color: 'blue' };  // Defined outside the component

Conclusion

Optimizing re-renders in React is crucial for building fast, efficient applications, especially as they grow in complexity. By leveraging memorization techniques like React.memo(), useCallback, and useMemo, along with careful management of state and props, you can significantly reduce unnecessary re-renders and improve your app’s performance. Remember to profile your React app regularly to detect performance bottlenecks and make informed optimization decisions.

For further learning, check out GFG React Rendering to explore more in-depth articles on React performance optimization.