Dmitri Pavlutin
Thoughts on Frontend development

Use React.memo() wisely

Users enjoy fast and responsive user interfaces (UI). An UI response delay of less than 100 milliseconds feels instant to the user. A delay between 100 and 300 milliseconds is already perceptible.

To improve user interface performance, React offers a higher-order component React.memo(). When React.memo() wraps a component, React memoizes the rendered output then skips unnecessary rendering.

This post describes the situations when React.memo() improves the performance, and, not less important, warns when its usage is useless.

Plus I’ll describe some useful memoization tips you should be aware of.

1. React.memo()

When deciding to update DOM, React first renders your component, then compares the result with the previous render result. If the render results are different, React updates the DOM.

Current vs previous render results comparison is fast. But you can speed up the process under some circumstances.

When a component is wrapped in React.memo(), React renders the component and memoizes the result. Before the next render, if the new props are the same, React reuses the memoized result skipping the next rendering.

Let’s see the memoization in action. The functional component Movie is wrapped in React.memo():

export function Movie({ title, releaseDate }) {
  return (
    <div>
      <div>Movie title: {title}</div>
      <div>Release date: {releaseDate}</div>
    </div>
  );
}

export const MemoizedMovie = React.memo(Movie);

React.memo(Movie) returns a new memoized component MemoizedMovie. It will output the same content as the original Movie component, but with one difference.

MemoizedMovie render output is memoized. The memoized content is reused as long as title or releaseDate props are the same on next renderings.

// First render. React calls MemoizedMovie function.
<MemoizedMovie 
  title="Heat"   releaseDate="December 15, 1995" />

// On next round React does not call MemoizedMovie function,
// preventing rendering
<MemoizedMovie
  title="Heat"   releaseDate="December 15, 1995" />

Open the demo, then expand the console. You will see that React renders <MemoizedMovie> just once, while <Movie> re-renders every time.

You gain a performance boost: by reusing the memoized content, React skips rendering the component and doesn’t perform a virtual DOM difference check.

The same functionality for class components is implemented by PureComponent.

1.1 Custom equality check of props

By default React.memo() does a shallow comparison of props and objects of props.

You can use the second argument to indicate a custom equality check function:

React.memo(Component, [areEqual(prevProps, nextProps)]);

areEqual(prevProps, nextProps) function must return true if prevProps and nextProps are equal.

For example, let’s manually calculate if Movie component props are equal:

function moviePropsAreEqual(prevMovie, nextMovie) {
  return prevMovie.title === nextMovie.title
    && prevMovie.releaseDate === nextMovie.releaseDate;
}

const MemoizedMovie2 = React.memo(Movie, moviePropsAreEqual);

moviePropsAreEqual() function returns true if prev and next props are equal.

2. When to use React.memo()

Inforgraphic explaining when to use React.memo()

2.1 Component renders often with the same props

The best case of wrapping a component in React.memo() is when you expect the functional component to render often and usually with the same props.

A common situation that makes a component render with the same props is being forced to render by a parent component.

Let’s reuse Movie component defined above. A new parent component MovieViewsRealtime displays the number of views of a movie, with realtime updates:

function MovieViewsRealtime({ title, releaseDate, views }) {
  return (
    <div>
      <Movie title={title} releaseDate={releaseDate} />      Movie views: {views}
    </div>
  );
}

The application regularly polls the server in background (every second), updating views property of MovieViewsRealtime component.

// Initial render
<MovieViewsRealtime 
  views={0}   title="Forrest Gump" 
  releaseDate="June 23, 1994" 
/>

// After 1 second, views is 10
<MovieViewsRealtime 
  views={10}   title="Forrest Gump" 
  releaseDate="June 23, 1994" 
/>

// After 2 seconds, views is 25
<MovieViewsRealtime 
  views={25}   title="Forrest Gump" 
  releaseDate="June 23, 1994" 
/>

// etc

Every time views prop is updated with a new number, MovieViewsRealtime renders. This triggers Movie rendering too, regardless of title and releaseDate being the same.

That’s the right case to apply memoization on Movie component.

Improve MovieViewsRealtime to use the memoized component MemoizedMovie:

function MovieViewsRealtime({ title, releaseDate, views }) {
  return (
    <div>
      <MemoizedMovie title={title} releaseDate={releaseDate} />      Movie views: {views}
    </div>
  )
}

As long as title and releaseDate props are the same, React skips rendering MemoizedMovie. This improves the performance of MovieViewsRealtime component.

The more often the component renders with the same props, the heavier and the more computationally expensive the output is, the more chances are that component needs to be wrapped in React.memo()

Anyways, use profiling to measure the benefits of applying React.memo().

3. When to avoid React.memo()

If the component doesn’t “re-render often with the same props”, most likely you don’t need React.memo().

Use the following rule of thumb: don’t use memoization if you can’t quantify the performance gains.

Performance-related changes applied incorrectly can even harm performance. Use React.memo() wisely.

While possible, wrapping class-based components in React.memo() is undesirable. Extend PureComponent class or define a custom implementation of shouldComponentUpdate() method if you need memoization for class-based components.

3.1 Useless props comparison

Imagine a component that typically renders with different props. In this case, memoization doesn’t provide benefits.

Even if you wrap such a volatile component in React.memo(), React does 2 jobs on every rendering:

  1. Invokes the comparison function to determine whether the previous and next props are equal
  2. Because props comparison almost always returns false, React performs the diff of previous and current render results

The invocation of the comparison function is useless because it almost always returns false.

4. React.memo() and callback functions

The function object equals only to itself. Let’s see that by comparing some functions:

function sumFactory() {
  return (a, b) => a + b;
}

const sum1 = sumFactory();
const sum2 = sumFactory();

console.log(sum1 === sum2); // => falseconsole.log(sum1 === sum1); // => true
console.log(sum2 === sum2); // => true

sumFactory() is a factory function. It returns functions that sum 2 numbers.

The functions sum1 and sum2 are created by the factory. Both functions sum 2 numbers. However, sum1 and sum2 are different function objects (sum1 === sum2 is false).

Every time a parent component defines a callback for its child, it might generate new function instances. Let’s study how this can break memoization, and how to fix it.

The following component Logout accepts a callback prop onLogout:

function Logout({ username, onLogout }) {
  return (
    <div onClick={onLogout}>
      Logout {username}
    </div>
  );
}

const MemoizedLogout = React.memo(Logout);

A component that accepts a callback must be handled with care when applying memoization. The parent component could provide different instances of the callback function on every render:

function MyApp({ store, cookies }) {
  return (
    <div className="main">
      <header>
        <MemoizedLogout
          username={store.username}
          onLogout={() => cookies.clear('session')}        />
      </header>
      {store.content}
    </div>
  );
}

Even if provided with the same username value, MemoizedLogout renders every time because it receives new instances of onLogout callback.

Memoization is broken.

To fix it, onLogout prop must receive the same callback instance. Let’s apply useCallback() to preserve the callback instance between renderings:

const MemoizedLogout = React.memo(Logout);

function MyApp({ store, cookies }) {
  const onLogout = useCallback(    () => cookies.clear('session'),     [cookies]  );  return (
    <div className="main">
      <header>
        <MemoizedLogout
          username={store.username}
          onLogout={onLogout}        />
      </header>
      {store.content}
    </div>
  );
}

useCallback(() => cookies.clear('session'), [cookies]) always returns the same function instance as long as cookies is the same. Memoization of MemoizedLogout is fixed.

5. React.memo() is a performance hint

Strictly, React uses memoization as a performance hint.

While in most situations React avoids rendering a memoized component, you shouldn’t count on that to prevent rendering.

6. React.memo() and hooks

Components using hooks can be freely wrapped in React.memo() to achieve memoization.

In case of useState(), React always makes sure to render the component if the state changes. Even if the component is wrapped in React.memo().

7. Conclusion

React.memo() is a great tool to memoize functional components. When applied correctly, it prevents component useless rendering when the next props equal to previous.

Take precaution when memoizing components that use props as callbacks. Make sure to provide the same callback function instance between renderings.

Don’t forget to use profiling to measure the performance gains of memoization.

Do you know interesting use cases of React.memo()? If so, please write a comment below!

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Dmitri Pavlutin

About Dmitri Pavlutin

I'm a software developer and tech writer specialized in Frontend technologies. I like to read books, run and travel the world.