Enhance React Performance: Avoid Unnecessary Re-renders Using React.memo, useMemo, and useCallback

React is a powerful library for building dynamic user interfaces, but sometimes its default behavior can lead to inefficiencies. One such behavior is the re-rendering of child components whenever a parent component updates. While this is intentional to ensure consistent rendering, it can be optimized to prevent unnecessary re-renders and improve performance. In this blog, we’ll dive into React’s rendering behavior and explore techniques like React.memo, useMemo, and useCallback to optimize your application.

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1. Understanding React’s Default Rendering Behavior

By default, whenever a parent component re-renders, all its child components also re-render — even if the child’s props haven’t changed. React assumes that changes in the parent might affect its children. While this behavior works for most scenarios, it can sometimes lead to inefficiencies when child components unnecessarily re-render.

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2. Using React.memo to Prevent Unnecessary Re-renders

React.memo is a higher-order component (HOC) that can optimize functional components by preventing them from re-rendering unless their props change.

Example:

const ChildComponent = React.memo(({ count }) => {
  console.log("Child Component Rendered");
  return <div>Count: {count}</div>;
});

How It Works:

• React.memo performs a shallow comparison of the props.

  • If the new props are the same as the old props (for primitives by value, and for objects/arrays by reference), the component does not re-render.

  • If the props differ (e.g., an object with a new reference), the component re-renders.

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3. Common Pitfalls with Objects and Arrays as Props

The Problem:

In JavaScript, objects and arrays are compared by reference. This means even if their content is the same, passing them as props will cause the child component to re-render because their reference changes on every render.

const ParentComponent = () => {
  const [count, setCount] = useState(0);
  const data = { value: "Hello" }; // New object reference on every render
  console.log("Parent Component Rendered");

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <ChildComponent data={data} />
    </div>
  );
};

const ChildComponent = React.memo(({ data }) => {
  console.log("Child Component Rendered");
  return <div>{data.value}</div>;
});

Console Output:

Parent Component Rendered
Child Component Rendered

When setCount is called, it triggers a re-render of ParentComponent. During this re-render, the data object is redefined, creating a new object with a new reference (even though its contents are the same).

Even though the data object hasn’t changed, its reference changes on every render, causing the child component to re-render even if it is wrapped inside the react memo.

The Solution:

Use useMemo to memoize objects or arrays so their reference remains stable unless their content changes.

const ParentComponent = () => {
  const [count, setCount] = useState(0);

  const data = useMemo(() => ({ value: "Hello" }), []); // Memoized object
  console.log("Parent Component Rendered");

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <ChildComponent data={data} />
    </div>
  );
};

const ChildComponent = React.memo(({ data }) => {
  console.log("Child Component Rendered");
  return <div>{data.value}</div>;
});

Updated Console Output:

Parent Component Rendered

The child component no longer re-renders unnecessarily because the data reference remains stable.

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4. Avoiding Inline Functions as Props

The Problem:

Passing a function as a prop, especially an inline function, creates a new reference on every render, even if the function's implementation doesn’t change. This leads to unnecessary re-renders of child components, even if the props themselves have not changed.

Consider the following example:

const ParentComponent = () => {
  const [count, setCount] = useState(0);

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <ChildComponent onClick={() => console.log("Clicked")} />
    </div>
  );
};

const ChildComponent = React.memo(({ onClick }) => {
  console.log("Child Component Rendered");
  return <button onClick={onClick}>Click Me</button>;
});

Console Output:

Parent Component Rendered
Child Component Rendered

Even though ChildComponent is wrapped with React.memo to prevent unnecessary re-renders, it still re-renders because the inline function passed as onClick is a new reference on every render. React.memo only prevents re-renders when props have not changed, but the function reference changes on each render of the parent component.

The Solution:

To fix this, we can memoize the function using useCallback. This ensures that the function's reference remains stable across renders, and the child component won’t re-render unnecessarily.

const ParentComponent = () => {
  const [count, setCount] = useState(0);
  console.log("Parent Component Rendered");

  const handleClick = useCallback(() => console.log("Clicked"), []); // Memoized function

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <ChildComponent onClick={handleClick} />
    </div>
  );
};

const ChildComponent = React.memo(({ onClick }) => {
  console.log("Child Component Rendered");
  return <button onClick={onClick}>Click Me</button>;
});

Updated Console Output:

Parent Component Rendered

By using useCallback, the handleClick function reference remains stable, and the child component no longer re-renders unnecessarily. This approach works because the function is only recreated when its dependencies change (in this case, there are no dependencies).

Summary:

This issue arises when passing functions as props because they create new references every time the parent re-renders. By using useCallback to memoize the function, we ensure that the function’s reference remains stable across renders, preventing unnecessary child re-renders.

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5. Final Optimized Example

Here’s an all-in-one example combining React.memo, useMemo, and useCallback to prevent unnecessary re-renders:

import React, { useState, useMemo, useCallback, memo } from "react";

const ParentComponent = () => {
  const [count, setCount] = useState(0);

  const data = useMemo(() => ({ value: "Hello" }), []); // Memoized object
  const handleClick = useCallback(() => console.log("Button Clicked"), []); // Memoized function

  console.log("Parent Component Rendered");

  return (
    <div>
      <h1>Parent Component</h1>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <ChildComponent data={data} onClick={handleClick} />
    </div>
  );
};

const ChildComponent = memo(({ data, onClick }) => { // ChildComponent is wrapped in React.memo
  console.log("Child Component Rendered");
  return (
    <div>
      <p>{data.value}</p>
      <button onClick={onClick}>Click Me</button>
    </div>
  );
});

export default ParentComponent;

Console Output:

1. On initial render:

    Parent Component Rendered
    Child Component Rendered
   

2. On incrementing the count:

    Parent Component Rendered
   

3. On clicking the button in the child component:

    Button Clicked
   

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6. When to Use React.memo

While React.memo is a great optimization tool, it’s not always necessary. Here’s a quick guide:

Use React.memo When:

1. Child components are expensive to render (e.g., heavy computations, complex DOM).

2. Props are stable and don’t change frequently.

3. Reusable components are rendered in multiple places.

4. Functions passed as props are memoized with useCallback.

Avoid React.memo When:

1. Child components are lightweight and fast to render.

2. Parent re-renders are infrequent, making optimization unnecessary.

3. Dynamic or unstable props (e.g., arrays, objects) are not memoized.

4. Premature optimization adds complexity without noticeable performance benefits.

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7. Key Takeaways

• React.memo prevents child components from re-rendering unless their props change.

• useMemo memoizes objects and arrays to ensure stable references.

• useCallback memoizes functions to prevent new references on every render.

• Optimize context and state updates to reduce unnecessary re-renders.

By understanding React’s rendering behavior and using tools like React.memo, useMemo, and useCallback, you can build applications that are not only functional but also highly performant. Remember, optimization should always be driven by actual performance issues — don’t over-optimize prematurely!

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