useMemo

const cachedValue = useMemo(calculateValue, dependencies)

• Reference
  • useMemo(calculateValue, dependencies)
• Usage
  • Skipping expensive recalculations
  • Skipping re-rendering of components
  • Memoizing a dependency of another Hook
  • Memoizing a function
• Troubleshooting
  • My calculation runs twice on every re-render
  • My useMemo call is supposed to return an object, but returns undefined
  • Every time my component renders, the calculation in useMemo re-runs
  • I need to call useMemo for each list item in a loop, but it’s not allowed

Reference

useMemo(calculateValue, dependencies)

Call useMemo at the top level of your component to cache a calculation between re-renders:

import { useMemo } from 'react';

function TodoList({ todos, tab }) {
  const visibleTodos = useMemo(
    () => filterTodos(todos, tab),
    [todos, tab]
  );
  // ...
}

See more examples below.

Parameters

• calculateValue: The function calculating the value that you want to cache. It should be pure, should take no arguments, and should return a value of any type. React will call your function during the initial render. On next renders, React will return the same value again if the dependencies have not changed since the last render. Otherwise, it will call calculateValue, return its result, and store it so it can be reused later.

• dependencies: The list of all reactive values referenced inside of the calculateValue code. Reactive values include props, state, and all the variables and functions declared directly inside your component body. If your linter is configured for React, it will verify that every reactive value is correctly specified as a dependency. The list of dependencies must have a constant number of items and be written inline like [dep1, dep2, dep3]. React will compare each dependency with its previous value using the Object.is comparison.

Returns

On the initial render, useMemo returns the result of calling calculateValue with no arguments.

During next renders, it will either return an already stored value from the last render (if the dependencies haven’t changed), or call calculateValue again, and return the result that calculateValue has returned.

Caveats

• useMemo is a Hook, so you can only call it at the top level of your component or your own Hooks. You can’t call it inside loops or conditions. If you need that, extract a new component and move the state into it.
• In Strict Mode, React will call your calculation function twice in order to help you find accidental impurities. This is development-only behavior and does not affect production. If your calculation function is pure (as it should be), this should not affect your logic. The result from one of the calls will be ignored.
• React will not throw away the cached value unless there is a specific reason to do that. For example, in development, React throws away the cache when you edit the file of your component. Both in development and in production, React will throw away the cache if your component suspends during the initial mount. In the future, React may add more features that take advantage of throwing away the cache—for example, if React adds built-in support for virtualized lists in the future, it would make sense to throw away the cache for items that scroll out of the virtualized table viewport. This should be fine if you rely on useMemo solely as a performance optimization. Otherwise, a state variable or a ref may be more appropriate.

Usage

Skipping expensive recalculations

To cache a calculation between re-renders, wrap it in a useMemo call at the top level of your component:

import { useMemo } from 'react';

function TodoList({ todos, tab, theme }) {
  const visibleTodos = useMemo(() => filterTodos(todos, tab), [todos, tab]);
  // ...
}

You need to pass two things to useMemo:

1. A calculation function that takes no arguments, like () =>, and returns what you wanted to calculate.
2. A list of dependencies including every value within your component that’s used inside your calculation.

On the initial render, the value you’ll get from useMemo will be the result of calling your calculation.

On every subsequent render, React will compare the dependencies with the dependencies you passed during the last render. If none of the dependencies have changed (compared with Object.is), useMemo will return the value you already calculated before. Otherwise, React will re-run your calculation and return the new value.

In other words, useMemo caches a calculation result between re-renders until its dependencies change.

Let’s walk through an example to see when this is useful.

By default, React will re-run the entire body of your component every time that it re-renders. For example, if this TodoList updates its state or receives new props from its parent, the filterTodos function will re-run:

function TodoList({ todos, tab, theme }) {
  const visibleTodos = filterTodos(todos, tab);
  // ...
}

Usually, this isn’t a problem because most calculations are very fast. However, if you’re filtering or transforming a large array, or doing some expensive computation, you might want to skip doing it again if data hasn’t changed. If both todos and tab are the same as they were during the last render, wrapping the calculation in useMemo like earlier lets you reuse visibleTodos you’ve already calculated before.

This type of caching is called memoization.

console.time('filter array');
const visibleTodos = filterTodos(todos, tab);
console.timeEnd('filter array');

console.time('filter array');
const visibleTodos = useMemo(() => {
  return filterTodos(todos, tab); // Skipped if todos and tab haven't changed
}, [todos, tab]);
console.timeEnd('filter array');

Skipping re-rendering of components

In some cases, useMemo can also help you optimize performance of re-rendering child components. To illustrate this, let’s say this TodoList component passes the visibleTodos as a prop to the child List component:

export default function TodoList({ todos, tab, theme }) {
  // ...
  return (
    <div className={theme}>
      <List items={visibleTodos} />
    </div>
  );
}

You’ve noticed that toggling the theme prop freezes the app for a moment, but if you remove <List /> from your JSX, it feels fast. This tells you that it’s worth trying to optimize the List component.

By default, when a component re-renders, React re-renders all of its children recursively. This is why, when TodoList re-renders with a different theme, the List component also re-renders. This is fine for components that don’t require much calculation to re-render. But if you’ve verified that a re-render is slow, you can tell List to skip re-rendering when its props are the same as on last render by wrapping it in memo:

import { memo } from 'react';

const List = memo(function List({ items }) {
  // ...
});

With this change, List will skip re-rendering if all of its props are the same as on the last render. This is where caching the calculation becomes important! Imagine that you calculated visibleTodos without useMemo:

export default function TodoList({ todos, tab, theme }) {
  // Every time the theme changes, this will be a different array...
  const visibleTodos = filterTodos(todos, tab);
  return (
    <div className={theme}>
      {/* ... so List's props will never be the same, and it will re-render every time */}
      <List items={visibleTodos} />
    </div>
  );
}

In the above example, the filterTodos function always creates a different array, similar to how the {} object literal always creates a new object. Normally, this wouldn’t be a problem, but it means that List props will never be the same, and your memo optimization won’t work. This is where useMemo comes in handy:

export default function TodoList({ todos, tab, theme }) {
  // Tell React to cache your calculation between re-renders...
  const visibleTodos = useMemo(
    () => filterTodos(todos, tab),
    [todos, tab] // ...so as long as these dependencies don't change...
  );
  return (
    <div className={theme}>
      {/* ...List will receive the same props and can skip re-rendering */}
      <List items={visibleTodos} />
    </div>
  );
}

By wrapping the visibleTodos calculation in useMemo, you ensure that it has the same value between the re-renders (until dependencies change). You don’t have to wrap a calculation in useMemo unless you do it for some specific reason. In this example, the reason is that you pass it to a component wrapped in memo, and this lets it skip re-rendering. There are a few other reasons to add useMemo which are described further on this page.

export default function TodoList({ todos, tab, theme }) {
  const visibleTodos = useMemo(() => filterTodos(todos, tab), [todos, tab]);
  const children = useMemo(() => <List items={visibleTodos} />, [visibleTodos]);
  return (
    <div className={theme}>
      {children}
    </div>
  );
}

Memoizing a dependency of another Hook

Suppose you have a calculation that depends on an object created directly in the component body:

function Dropdown({ allItems, text }) {
  const searchOptions = { matchMode: 'whole-word', text };

  const visibleItems = useMemo(() => {
    return searchItems(allItems, searchOptions);
  }, [allItems, searchOptions]); // 🚩 Caution: Dependency on an object created in the component body
  // ...

Depending on an object like this defeats the point of memoization. When a component re-renders, all of the code directly inside the component body runs again. The lines of code creating the searchOptions object will also run on every re-render. Since searchOptions is a dependency of your useMemo call, and it’s different every time, React knows the dependencies are different, and recalculate searchItems every time.

To fix this, you could memoize the searchOptions object itself before passing it as a dependency:

function Dropdown({ allItems, text }) {
  const searchOptions = useMemo(() => {
    return { matchMode: 'whole-word', text };
  }, [text]); // ✅ Only changes when text changes

  const visibleItems = useMemo(() => {
    return searchItems(allItems, searchOptions);
  }, [allItems, searchOptions]); // ✅ Only changes when allItems or searchOptions changes
  // ...

In the example above, if the text did not change, the searchOptions object also won’t change. However, an even better fix is to move the searchOptions object declaration inside of the useMemo calculation function:

function Dropdown({ allItems, text }) {
  const visibleItems = useMemo(() => {
    const searchOptions = { matchMode: 'whole-word', text };
    return searchItems(allItems, searchOptions);
  }, [allItems, text]); // ✅ Only changes when allItems or text changes
  // ...

Now your calculation depends on text directly (which is a string and can’t “accidentally” become different).

Memoizing a function

Suppose the Form component is wrapped in memo. You want to pass a function to it as a prop:

export default function ProductPage({ productId, referrer }) {
  function handleSubmit(orderDetails) {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails
    });
  }

  return <Form onSubmit={handleSubmit} />;
}

Just as {} creates a different object, function declarations like function() {} and expressions like () => {} produce a different function on every re-render. By itself, creating a new function is not a problem. This is not something to avoid! However, if the Form component is memoized, presumably you want to skip re-rendering it when no props have changed. A prop that is always different would defeat the point of memoization.

To memoize a function with useMemo, your calculation function would have to return another function:

export default function Page({ productId, referrer }) {
  const handleSubmit = useMemo(() => {
    return (orderDetails) => {
      post('/product/' + productId + '/buy', {
        referrer,
        orderDetails
      });
    };
  }, [productId, referrer]);

  return <Form onSubmit={handleSubmit} />;
}

This looks clunky! Memoizing functions is common enough that React has a built-in Hook specifically for that. Wrap your functions into useCallback instead of useMemo to avoid having to write an extra nested function:

export default function Page({ productId, referrer }) {
  const handleSubmit = useCallback((orderDetails) => {
    post('/product/' + productId + '/buy', {
      referrer,
      orderDetails
    });
  }, [productId, referrer]);

  return <Form onSubmit={handleSubmit} />;
}

The two examples above are completely equivalent. The only benefit to useCallback is that it lets you avoid writing an extra nested function inside. It doesn’t do anything else. Read more about useCallback.

Troubleshooting

My calculation runs twice on every re-render

In Strict Mode, React will call some of your functions twice instead of once:

function TodoList({ todos, tab }) {
  // This component function will run twice for every render.

  const visibleTodos = useMemo(() => {
    // This calculation will run twice if any of the dependencies change.
    return filterTodos(todos, tab);
  }, [todos, tab]);

  // ...

This is expected and shouldn’t break your code.

This development-only behavior helps you keep components pure. React uses the result of one of the calls, and ignores the result of the other call. As long as your component and calculation functions are pure, this shouldn’t affect your logic. However, if they are accidentally impure, this helps you notice and fix the mistake.

For example, this impure calculation function mutates an array you received as a prop:

  const visibleTodos = useMemo(() => {
    // 🚩 Mistake: mutating a prop
    todos.push({ id: 'last', text: 'Go for a walk!' });
    const filtered = filterTodos(todos, tab);
    return filtered;
  }, [todos, tab]);

React calls your function twice, so you’d notice the todo is added twice. Your calculation shouldn’t change any existing objects, but it’s okay to change any new objects you created during the calculation. For example, if the filterTodos function always returns a different array, you can mutate that array instead:

  const visibleTodos = useMemo(() => {
    const filtered = filterTodos(todos, tab);
    // ✅ Correct: mutating an object you created during the calculation
    filtered.push({ id: 'last', text: 'Go for a walk!' });
    return filtered;
  }, [todos, tab]);

Read keeping components pure to learn more about purity.

Also, check out the guides on updating objects and updating arrays without mutation.

My useMemo call is supposed to return an object, but returns undefined

This code doesn’t work:

  // 🔴 You can't return an object from an arrow function with () => {
  const searchOptions = useMemo(() => {
    matchMode: 'whole-word',
    text: text
  }, [text]);

In JavaScript, () => { starts the arrow function body, so the { brace is not a part of your object. This is why it doesn’t return an object, and leads to mistakes. You could fix it by adding parentheses like ({ and }):

  // This works, but is easy for someone to break again
  const searchOptions = useMemo(() => ({
    matchMode: 'whole-word',
    text: text
  }), [text]);

However, this is still confusing and too easy for someone to break by removing the parentheses.

To avoid this mistake, write a return statement explicitly:

  // ✅ This works and is explicit
  const searchOptions = useMemo(() => {
    return {
      matchMode: 'whole-word',
      text: text
    };
  }, [text]);

Every time my component renders, the calculation in useMemo re-runs

Make sure you’ve specified the dependency array as a second argument!

If you forget the dependency array, useMemo will re-run the calculation every time:

function TodoList({ todos, tab }) {
  // 🔴 Recalculates every time: no dependency array
  const visibleTodos = useMemo(() => filterTodos(todos, tab));
  // ...

This is the corrected version passing the dependency array as a second argument:

function TodoList({ todos, tab }) {
  // ✅ Does not recalculate unnecessarily
  const visibleTodos = useMemo(() => filterTodos(todos, tab), [todos, tab]);
  // ...

If this doesn’t help, then the problem is that at least one of your dependencies is different from the previous render. You can debug this problem by manually logging your dependencies to the console:

  const visibleTodos = useMemo(() => filterTodos(todos, tab), [todos, tab]);
  console.log([todos, tab]);

You can then right-click on the arrays from different re-renders in the console and select “Store as a global variable” for both of them. Assuming the first one got saved as temp1 and the second one got saved as temp2, you can then use the browser console to check whether each dependency in both arrays is the same:

Object.is(temp1[0], temp2[0]); // Is the first dependency the same between the arrays?
Object.is(temp1[1], temp2[1]); // Is the second dependency the same between the arrays?
Object.is(temp1[2], temp2[2]); // ... and so on for every dependency ...

When you find which dependency breaks memoization, either find a way to remove it, or memoize it as well.

I need to call useMemo for each list item in a loop, but it’s not allowed

Suppose the Chart component is wrapped in memo. You want to skip re-rendering every Chart in the list when the ReportList component re-renders. However, you can’t call useMemo in a loop:

function ReportList({ items }) {
  return (
    <article>
      {items.map(item => {
        // 🔴 You can't call useMemo in a loop like this:
        const data = useMemo(() => calculateReport(item), [item]);
        return (
          <figure key={item.id}>
            <Chart data={data} />
          </figure>
        );
      })}
    </article>
  );
}

Instead, extract a component for each item and memoize data for individual items:

function ReportList({ items }) {
  return (
    <article>
      {items.map(item =>
        <Report key={item.id} item={item} />
      )}
    </article>
  );
}

function Report({ item }) {
  // ✅ Call useMemo at the top level:
  const data = useMemo(() => calculateReport(item), [item]);
  return (
    <figure>
      <Chart data={data} />
    </figure>
  );
}

Alternatively, you could remove useMemo and instead wrap Report itself in memo. If the item prop does not change, Report will skip re-rendering, so Chart will skip re-rendering too:

function ReportList({ items }) {
  // ...
}

const Report = memo(function Report({ item }) {
  const data = calculateReport(item);
  return (
    <figure>
      <Chart data={data} />
    </figure>
  );
});