One of the primary causes of performance bottlenecks in React, particularly in large apps, is re-rendering. React's virtual DOM mechanism effectively updates the DOM, but needless re-renders can still cause performance issues. Re-rendering can be optimized to make sure that only the components that require it re-render, which improves application performance and responsiveness.

The best practices for cutting down on pointless re-renders in React apps will be discussed in this post, along with some helpful hints and methods to help you maximize the efficiency of your React components.

Understanding Re-Rendering in React

React's rendering process revolves around its component tree. When a component's state or props change, React re-renders that component and its child components. However, if not managed properly, this can lead to unnecessary re-renders, where components that didn’t experience any real changes also get re-rendered, wasting resources.

Common Causes of Unnecessary Re-Renders

Props or state changes that don’t affect the component's output.
Parent component re-renders causing child components to re-render, even if their props haven’t changed.
Anonymous functions or object references being re-created on every render.

Key Techniques to Optimize Re-Rendering in React

1. Use React.memo for Functional Components

React.memo is a higher-order component (HOC) that helps prevent unnecessary re-renders in functional components by memoizing the result. If the props of a component haven’t changed, React.memo prevents it from re-rendering.

const MyComponent = React.memo(({ data }) => {
  console.log('Component rendered');
  return <div>{data}</div>;
});

With this optimization, MyComponent will only re-render when its data prop changes, improving performance.

2. Optimize Re-renders with useCallback and useMemo

👉🏻 useCallback: Used to memoize functions so that they don’t get recreated on every render. This is useful when passing functions down to child components that depend on specific values.

const handleClick = useCallback(() => {
  // handle button click
}, []);

👉🏻 useMemo: Used to memoize expensive calculations or derived data to prevent recalculating on every render.

const expensiveCalculation = useMemo(() => {
  return someHeavyFunction(input);
}, [input]);

These hooks ensure that only when the dependencies change will the function or value be recomputed, reducing unnecessary renders.

3. Avoid Inline Functions and Object Creation in JSX

Inline functions or objects created inside JSX are a common source of re-renders. Since new function or object references are created each time the component renders, it triggers unnecessary re-renders in child components.

// Avoid this pattern
<MyChildComponent onClick={() => doSomething()} />

// Instead, define the function outside the JSX
const handleClick = () => doSomething();
<MyChildComponent onClick={handleClick} />

By avoiding inline function or object creation in JSX, you help prevent child components from re-rendering unnecessarily.

4. Split Large Components into Smaller Components

Another technique to avoid unnecessary re-renders is breaking down large components into smaller, more focused components. This allows React to perform more granular updates and prevents re-rendering of entire components when only a part of the component has changed.

For example, if you have a component that renders a form and a list, you can split them into two separate components. This way, updating the form won’t trigger re-rendering of the list, and vice versa.

function ParentComponent() {
  return (
    <>
      <FormComponent />
      <ListComponent />
    </>
  );
}

5. Use the key Prop Correctly

When rendering lists, React uses the key prop to identify elements. Incorrect usage of the key prop can cause React to incorrectly update or re-render components.

Make sure to use a unique and stable key for each element in a list:

const items = ['apple', 'banana', 'orange'];
items.map((item) => <li key={item}>{item}</li>);

Avoid using index as a key when items can be reordered or modified, as it can lead to unexpected re-renders or incorrect rendering.

6. Use shouldComponentUpdate and PureComponent in Class Components

For class components, shouldComponentUpdate is a lifecycle method that allows you to control whether a component should re-render based on changes in props or state.

Alternatively, you can use PureComponent, which automatically implements shouldComponentUpdate with a shallow comparison of props and state.

class MyComponent extends React.PureComponent {
  render() {
    return <div>{this.props.data}</div>;
  }
}

PureComponent is a simpler alternative to shouldComponentUpdate and helps avoid unnecessary re-renders by comparing previous and current props and state.

7. Optimize Context API Usage

When using React's Context API, be cautious about overuse, as it can lead to unnecessary re-renders if every consumer re-renders when the context value changes. To avoid this:

Break down context providers into smaller ones, so only the necessary part of the state triggers updates.
Memoize values passed to the context provider using useMemo to avoid unnecessary re-renders.

const value = useMemo(() => ({ state, updateState }), [state]);

return (
  <MyContext.Provider value={value}>
    {children}
  </MyContext.Provider>
);

8. Lazy Load Components

For performance improvements, especially in large applications, you can lazy load components that are not immediately needed. This can reduce the initial rendering time and the load on the main thread.

React’s React.lazy and Suspense can help in lazy-loading components:

const LazyComponent = React.lazy(() => import('./LazyComponent'));

<Suspense fallback={<div>Loading...</div>}>
  <LazyComponent />
</Suspense>

By lazy-loading components, you delay their rendering until they are actually needed, which reduces unnecessary renders and improves the user experience.

Conclusion

Maintaining performance in React apps requires optimizing re-rendering, especially as the application grows. Avoid needless re-renders by employing strategies like using React.memo, useCallback, useMemo, and breaking up big components into smaller ones. By putting these tactics to good use, you can make sure that your React application stays light-weight, responsive, and simple to update.