useMemo vs. useEffect + useState

Question

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:

useEffect & useState

import { expensiveCalculation } from "foo";

const useCalculate = someNumber => {
    const [result, setResult] = useState<number>(null);

    useEffect(() => {
        setResult(expensiveCalculation(someNumber));
    }, [someNumber]);

    return result;
};

useMemo

import { expensiveCalculation } from "foo";

const useCalculateWithMemo = someNumber => {
    return useMemo(() => {
        return expensiveCalculation(someNumber);
    }, [someNumber]);
};

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

Answer 1

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:

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

Lets suppose someNumber 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 someNumber 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.

Answer 2

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.

2. 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.