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Are there any benefits in using useMemo (e.g. for an intensive function call) instead of using a combination of useEffect and useState?

Here are two custom hooks that work exactly the same on first sight, besides useMemo's return value being null on the first render:

See on CodeSandbox

useEffect & useState

import { expensiveCalculation } from "foo"; function useCalculate(someNumber: number): number | null { const [result, setResult] = useState<number | null>(null); useEffect(() => { setResult(expensiveCalculation(someNumber)); }, [someNumber]); return result; }

useMemo

import { expensiveCalculation } from "foo"; function useCalculateWithMemo(someNumber: number): number { return useMemo(() => { return expensiveCalculation(someNumber); }, [someNumber]); };

Both calculate the result each time their parameter someNumber changes, where is the memoization of useMemo kicking in?

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    The first will be null on the first render, while the second won't? Commented May 7, 2019 at 19:08
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    Are there any benefits in using useMemo (e. g. for an intensive function call) - yes. You're using a hook that was designed specifically for this purpose. The example you listed is most common real world example for useMemo. Commented May 7, 2019 at 19:42

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The useEffect and setState will cause extra renders on every change: the first render will "lag behind" with stale data and then it'll immediately queue up an additional render with the new data.

Suppose we have:

// Maybe I'm running this on a literal potato function expensiveCalculation(x) { return x + 1; };

Lets suppose x is initially 0:

  • The useMemo version immediately renders 1.
  • The useEffect version renders null, then after the component renders the effect runs, changes the state, and queues up a new render with 1.

Then if we change x to 2:

  • The useMemo runs and 3 is rendered.
  • The useEffect version runs, and renders 1 again, then the effect triggers and the component reruns with the correct value of 3.

In terms of how often expensiveCalculation runs, the two have identical behavior, but the useEffect version is causing twice as much rendering which is bad for performance for other reasons.

Plus, the useMemo version is just cleaner and more readable, IMO. It doesn't introduce unnecessary mutable state and has fewer moving parts.

So you're better off just using useMemo here.

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15 Comments

I think the useEffect could be useful in some long running synchronous scenarios too. Check out the sandbox below. It takes 5 seconds to load because of the useMemo holding the render thread while the long calc runs, while the useEffect/useState one can show a 'spinner' while the calc is running so doesn't hold up the render: codesandbox.io/s/usememo-vs-useeffect-usestate-ye6qm @Retsam
besides optimization, I use useMemo instead of the useState + useEffect pattern because debugging is harder with more renders.
It’s worth noting that the React API docs mention that useMemo doesn’t guarantee that the memoized function won’t be executed again if the dependencies don’t change because React may, in the future, discard cache to improve performance. So if the memoized function has side effects of some kind, it might be smarter to use a custom hook.
@Abhi Changing a prop triggers a re-render. But the value that is rendered is based on the [result, setResult] state, and setResult won't be called until the useEffect runs, which happens after the render.
Can we conclude from this that useEffect + useState is the right solution in situations where the thing to compute is async, because the value cannot be available in the current render anyway? Related question.
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I think there are two main points you should consider when choosing between them.

  1. Time when function called.

useEffect called after component has been rendered, so you can access DOM from it. For example, this is important if you want to access DOM elements via refs.

  1. Semantic guarantees.

useEffect guarantees that it will not be fired if dependencies have not changed. useMemo does not give such guarantees.

As stated in the React documentation, you should consider useMemo as pure optimization technique. Your program should continue to work correctly even if you replace useMemo with regular function call.

useEffect + useState can be used to control updates. Even to break-up circular dependencies and prevent infinite update loops.

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useEffect does NOT warranty that it will not re-render when dependencies have not changed. useMemo is the one that does not fire when dependencies have not changed. I have never had issue of extra re-renders on useMemo, contrary to useEffect that sometimes re-renders when dependencies have not changed. Instead, you want to watch out for the fact that useMemo struggles to detect some changes in some dependencies that are complex in structure. (for example, a change in an object that is inside of an array).
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I would say other than the async nature, there might be some difference in terms how they are designed.

useEffect is a collective call, async or not, it's collected after all components are rendered.

useMemo is a local call, which has only something to do with this component. You could just think of useMemo as another assignment statement with benefits to use the assignment from last update.

This means, useMemo is more urgent, and then useLayoutEffect and the last being useEffect.

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useState + useEffect is not necessarily a good pattern.

const [state, setState] = useState(initialValue)

provides you with setState to explicitly change the state. In your example, call setState where some change to someNumber is made instead of using useEffect.

I know, this is not always possible. E.g. someNumber might belong to some outer context and changes are not made by the parent component but by siblings or cousins.

In case it's a child component that causes a change to someNumber, passing down setState to the child as prop isn't the worst idea.

Before slapping useEffect on useState, consider changes to your component tree. For an expensive calculation within some component, you can use simple Javascript laziness to make sure, the calculation won't happen on every re-render:

const [state, setState] = useState(() => { return /* expensive calculation */ }) ^^^^^ lazy

As pointed out in a comment by @mark-adamson, when you want to display some component before the expensive calculation (e.g. with a spinner), you need useState, e.g. with a nullable state, because the absence of the result of the calculation is a legitimate state and needs some representation.

useEffect is still optional even in this case.

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This is a very interesting perspective. Thank you for this.

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