Using the functional updates pattern with React hooks

07 April 2020
·
react
·

The functional updates pattern can be used whenever you need to update state using the previous state.

For example, if you were storing in state the number of times a button was clicked, you might do it by referring to the previous count state:

const App = () => {
    const [count, setCount] = useState(0);
    return (
        <button onClick={() => setCount(count + 1)}>
            Click
        </button>
    );
};

Instead of doing count + 1, you can pass in a function to setCount:

<button onClick={() => setCount(prevCount => prevCount + 1)}>
    Click
</button>

What's the benefit of using functional updates?

In the above example, there's no immediate benefit to using this pattern (that I know of). However if you were setting state inside of a useEffect hook:

useEffect(() => {
    setCount(count + 1);
}, [setCount, count]);

Since count is a dependency, each time count was changed by setCount, useEffect would be called again causing an infinite number of re-renders.

Using the functional updates pattern in this case makes sense:

useEffect(() => {
    setCount(prevCount => prevCount + 1);
}, [setCount]);

If you have any better use cases that you can think of, I'd love to hear about them!

Thanks for reading!

References

https://reactjs.org/docs/hooks-reference.html#functional-updates

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