Use React.memo() wisely

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

To improve user interface performance, React offers a higher-order component React.memo() . By memoizing the rendered output, memoization helps to avoid unnecessary re-rendering.

While it might be tempting to apply React.memo() on many components, you must use it wisely.

This post helps you distinguish the situations when React.memo() improves the performance, and, not less important, understand when its usage is useless.

Plus I’ll describe some useful 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. If the render results are different, React decides to update 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 props are the same, React reuses the memoized content.

Let’s define a functional component Movie , then wrap it 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 during the next rendering rounds.

This is where you gain performance benefit: by reusing the memoized result, 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()

2.1 Component renders often with the same props

React.memo() is applied on pure functional components. These are components that given the same props, always render the same output.

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

A common situation that makes a component re-render with the same props is being forced to re-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 every second:

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 re-renders. This triggers Movie re-render 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 re-rendering of MemoizedMovie . This improves the performance of MovieViewsRealtime component.

The more often the component re-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 see the benefits of applying React.memo() .

Do you know other circumstances when React.memo() improves performance? If so, please write a comment below!

3. When to avoid React.memo()

If your component’s rendering situation doesn’t fit into the case described above, most likely you don’t need React.memo() .

Use the following rule of thumb: if you don’t see the gains of memoization, don’t use it.

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

Of course, you cannot use React.memo() on non-pure components, e.g. components that have state, etc.

3.1 Useless props comparison

Suppose a case when the component typically renders with different props. In this case, memoization doesn’t provide benefits.

Even if you try to use React.memo() , React will have to do 2 jobs on every re-render:

1. Invoke the comparison function to determine whether the previous and next props are equal
2. Because the props are different, perform the diff of previous and current render output

Invocation of the comparison function is useless.

4. React.memo() and callback functions

Function objects follow the same principles of comparison as any object. The function object equals only to itself.

Let’s compare some functions:

function sum1(a, b) {
  return a + b;
}
function sum2(a, b) {
  return a + b;
}

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

The functions sum1 and sum2 both sum 2 numbers. However, sum1 and sum2 are different function objects.

Now, let’s define a component that accepts a callback prop:

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

const MemoizedLogout = React.memo(Logout);

Because of function equality pitfall, a component that accepts a callback must be handled with care when applying memoization. There’s a chance that the parent component provides different instances of the callback function on every re-render:

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

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

Memoization is broken.

To fix it, the same callback instance must be used to set onLogout prop. 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() }, []);  return (
    <div className="main">
      <header>
        <MemoizedLogout
          username={store.username}
          onLogout={onLogout}        />
      </header>
      {store.content}
    </div>
  );
}

useCallback(() => { cookies.clear() }, []) always returns the same function instance. 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 re-rendering a memoized component, you shouldn’t count on that to prevent rendering.

6. Conclusion

React.memo() is a great tool to gain the benefits of memoization for pure functional components. When applied correctly, it prevents component re-render when the next props are the same as previous.

Take precaution when memoizing components that use props with callback functions. 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!