Avoiding unnecessary renders in React

Introduction

Avoiding unnecessary renders helps your app run faster and smoother by not updating parts of the screen when nothing has changed.

When a component updates too often and slows down the app

When you have a list of items and only some change at a time

When passing functions or objects as props that cause child components to re-render

When you want to keep your app responsive on slower devices

When you want to save battery life on mobile devices by reducing work

Syntax

React

const MemoizedComponent = React.memo(Component);

const memoizedCallback = React.useCallback(() => {
  // function code
}, [dependencies]);

const memoizedValue = React.useMemo(() => computeValue(), [dependencies]);

React.memo wraps a component to skip re-render if props are the same.

useCallback keeps a function the same between renders unless dependencies change.

Examples

This button only re-renders if onClick or label props change.

React

const Button = React.memo(({ onClick, label }) => {
  console.log('Button rendered');
  return <button onClick={onClick}>{label}</button>;
});

handleClick is memoized so Button won't re-render just because Parent re-renders.

React

const Parent = () => {
  const [count, setCount] = React.useState(0);
  const handleClick = React.useCallback(() => {
    setCount(c => c + 1);
  }, []);

  return <Button onClick={handleClick} label="Click me" />;
};

useMemo caches the result of a heavy calculation until input changes.

React

const expensiveValue = React.useMemo(() => {
  return computeHeavyTask();
}, [input]);

Sample Program

In this example, the Child component only re-renders when the count changes. Typing in the input does not cause Child to re-render because it is wrapped with React.memo. You can see "Child rendered" in the console only when the count changes.

React

import React from 'react';

const Child = React.memo(({ value }) => {
  console.log('Child rendered');
  return <div>Value: {value}</div>;
});

export default function App() {
  const [count, setCount] = React.useState(0);
  const [text, setText] = React.useState('');

  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment Count</button>
      <input
        aria-label="Input text"
        value={text}
        onChange={e => setText(e.target.value)}
        placeholder="Type here"
      />
      <Child value={count} />
    </div>
  );
}

Important Notes

Wrapping components with React.memo only works if props are simple and stable.

Use useCallback and useMemo to keep functions and values stable between renders.

Too much memoization can make code complex, so use it only when you notice performance issues.

Summary

Avoid unnecessary renders to make your app faster and smoother.

Use React.memo to skip re-rendering components when props don't change.

Use useCallback and useMemo to keep functions and values from changing needlessly.

Practice

1. What is the main purpose of using React.memo in a React component?

easy

A. To convert a class component into a functional component

B. To force the component to re-render every time the parent renders

C. To prevent re-rendering when the component's props have not changed

D. To automatically update the component's state

Solution

1. Step 1: Understand React.memo's role

   React.memo is a higher-order component that wraps a functional component to optimize rendering.

2. Step 2: Identify its effect on rendering

   It skips re-rendering the wrapped component if its props remain the same, improving performance.

3. Final Answer:

   To prevent re-rendering when the component's props have not changed -> Option C

4. Quick Check:

   React.memo skips renders if props unchanged = D [OK]

Hint: React.memo skips render if props stay the same [OK]

Common Mistakes:

• Thinking React.memo forces re-render
• Confusing React.memo with state updates
• Believing it converts component types

2. Which of the following is the correct syntax to memoize a function using useCallback in React?

easy

A. const memoizedFn = useCallback(() => { doSomething(); }, []);

B. const memoizedFn = useCallback(doSomething(), []);

C. const memoizedFn = useCallback(() => doSomething);

D. const memoizedFn = useCallback(() => { doSomething(); });

Solution

1. Step 1: Recall useCallback syntax

   useCallback takes a function and a dependency array, returning a memoized version of the function.

2. Step 2: Check each option's correctness

   const memoizedFn = useCallback(() => { doSomething(); }, []); correctly passes a function and an empty dependency array, memoizing the function properly.

3. Final Answer:

   const memoizedFn = useCallback(() => { doSomething(); }, []); -> Option A

4. Quick Check:

   useCallback needs function and dependencies = C [OK]

Hint: useCallback(fn, deps) memoizes function with dependencies [OK]

Common Mistakes:

• Calling the function inside useCallback instead of passing it
• Omitting the dependency array
• Passing function reference without parentheses incorrectly

3. Consider this React component:

const MyComponent = React.memo(({ count, onClick }) => {
  console.log('Rendered');
  return <button onClick={onClick}>Count: {count}</button>;
});

function App() {
  const [count, setCount] = React.useState(0);
  const handleClick = () => setCount(count + 1);
  return <MyComponent count={count} onClick={handleClick} />;
}

What will happen when App re-renders?

medium

A. MyComponent will never re-render because it is wrapped in React.memo

B. MyComponent will re-render every time because handleClick is recreated on each render

C. MyComponent will re-render only when count changes, ignoring onClick

D. MyComponent will re-render only when handleClick changes, ignoring count

Solution

1. Step 1: Understand React.memo and props

   React.memo skips re-render if props are shallowly equal. Here, onClick is a new function each render.

2. Step 2: Analyze function recreation

   Since handleClick is recreated on every render, its reference changes, causing MyComponent to re-render.

3. Final Answer:

   MyComponent will re-render every time because handleClick is recreated on each render -> Option B

4. Quick Check:

   New function prop breaks memo = B [OK]

Hint: New function props cause re-render despite React.memo [OK]

Common Mistakes:

• Assuming React.memo ignores function prop changes
• Thinking React.memo prevents all re-renders
• Ignoring function identity in props

4. You have this React code that causes unnecessary re-renders:

const Parent = () => {
  const [count, setCount] = React.useState(0);
  const increment = () => setCount(count + 1);
  return <Child onClick={increment} />;
};

const Child = React.memo(({ onClick }) => {
  console.log('Child rendered');
  return <button onClick={onClick}>Click</button>;
});

How can you fix the unnecessary re-renders of Child?

medium

A. Wrap increment in useCallback with count as dependency

B. Remove React.memo from Child

C. Move increment outside the Parent component

D. Use useMemo to memoize count

Solution

1. Step 1: Identify cause of re-renders

   The increment function is recreated on every render, changing its reference and causing Child to re-render.

2. Step 2: Apply useCallback to memoize function

   Wrapping increment in useCallback with count dependency keeps the same function reference unless count changes.

3. Final Answer:

   Wrap increment in useCallback with count as dependency -> Option A

4. Quick Check:

   Memoize functions with useCallback to avoid re-renders = A [OK]

Hint: Memoize event handlers with useCallback to prevent re-renders [OK]

Common Mistakes:

• Removing React.memo which disables optimization
• Moving function outside loses access to state
• Using useMemo incorrectly for functions

5. You want to optimize a component that receives a large list and a filter function as props. The filter function is defined inline in the parent component. Which approach best avoids unnecessary renders of the child component?

hard

A. Use useMemo to memoize the filtered list inside the child component

B. Define the filter function inline without memoization and rely on React.memo alone

C. Avoid React.memo and always re-render the child component to ensure fresh data

D. Wrap the child component with React.memo and memoize the filter function using useCallback with proper dependencies

Solution

1. Step 1: Understand props causing re-renders

   The inline filter function changes reference every render, causing child re-renders despite React.memo.

2. Step 2: Use React.memo and useCallback together

   Wrapping child with React.memo and memoizing the filter function with useCallback keeps function reference stable, avoiding unnecessary renders.

3. Final Answer:

   Wrap the child component with React.memo and memoize the filter function using useCallback with proper dependencies -> Option D

4. Quick Check:

   Memoize functions and use React.memo to optimize renders = A [OK]

Hint: Memoize functions and wrap child with React.memo for best optimization [OK]

Common Mistakes:

• Relying on React.memo without memoizing functions
• Memoizing filtered list inside child instead of function
• Avoiding React.memo and causing extra renders