Introduction

React.memo is a higher order component provided by react that will return a memoized version of the component that only changes if one of the props has changed. It is same as PureComponent but instead of classes React.memo is used for functional components.

Why use React.memo?

React.memo memoizes the rendered output then skips unnecessary rendering. This helps to prevent unnecessary re-rendering of components and computations needed for component rendering.

React.memo in action

As an example implementation lets create a component which will:

• Greet user
• Show number of times user has greeted
• Let user greet using button

Let's create and add a function/method on GreetUser component that does the work of simulating some heavy computations while rendering the component.

// userGreeting.js

const UserGreeting = () => {
  const getUserName = () => {
    let i = 0;
    while (i < 3000000000) i++;

    return 'John Doe';
  };

  return <div>Hello {getUserName()},</div>;
};

GreetingCount and Button components will show the count and increment greet count on click respectively and do not have any heavy computations.

// greetingCount.js

const GreetingCount = ({ count }) => (
  return <div>You greeted me {count} times.</div>;
);

// button.js

const Button = ({ title, onClick }) => (
  <button onClick={onClick}>{title}</button>
);

And the parent component will import all these components and have method to update the greetings count.

//App.js

const App = () => {
  const [greetCount, setGreetCount] = useState(0);
  const onGreet = () => {
    setGreetCount(greetCount + 1);
  };

  return (
    <div className='App'>
      <UserGreeting />
      <GreetingCount count={greetCount} />
      <Button title='Hi' onClick={onGreet} />
    </div>
  );
};

Problem

As you can see that there is delay for certain interval before the UI updates after the button is clicked. This is because when we click on the button the state changes so every components are rerendered and the GreetUser component is rerendered as well. The getUserName method is executed again due to re-render of GreetUser component thus causing delay on UI update.

Solution

So the solution for the above problem is to use React.memo(). The React.memo() method will memoize the component and does a shallow comparison of the component and since none of the props in GreetUser component has been changed, it will skip re-rendering of this component. This will prevent the recomputation during the render and the UI updates quickly. For this we will wrap the component with React.memo() and export it.

const UserGreeting = () => {
  // code here
};

export default React.memo(UserGreeting);

Result:

As you can see now that the component does not re-render GreetUser component and the UI is updated without any delay.

You can find complete example on CodeSandbox