useCallback

useCallback returns a memoized callback.

it accepts a function, and returns the same instance of the function being passed instead of creating a new one when a component re-renders, which is the default behavior.

useCallback allows you to cache an instance of a function between renders.

by default, all variables (including functions) defined in a component body will be regenerated on each rendering of the component. useCallback allows us to say "I know that this function body will not change unless this prop (or state) changes. Therefore, I want to opt-out of the automatic regeneration of the function unless that prop changes."

The purpose of useCallback is to prevent unnecessarily re-defining functions between component renders, making our code more efficient.

useMemo "caches" the value, while useCallback "caches" the function itself.
In the days of class components, we had a render() function. Anything placed inside would get re-calculated on every render, but we also had the option of defining functions as class methods (by putting functions outside of render()). These methods would not get recalculated on each render, and would lead to performance gains. Since functional components naturally call everything (since render() method is implicit), they inherently have a performance issue. useCallback and useMemo exist to alleviate this problem.

You should consider using useCallback and/or useMemo hooks on the following situations:

const additionResult = useCallback(add(firstVal, secondVal), [firstVal, secondVal])

In this example, the additionResult function only gets re-defined if either firstVal or secondVal have changed between those 2 renders.

Imagine we had a function that we passed down to a child component.

const getItems = () => {
	return [number, number + 1, number + 2]
}

This can get expensive. Instead we can memoize the callback:

const getItems = useCallback(() => {
	return [number, number + 1, number + 2]
}, [number])

Now, the getItems function:

() => {
	return [number, number + 1, number + 2]
}

Only gets redefined when the number prop changes.

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