Скачать или смотреть useMemo - what on earth is it?

useMemo is a React Hook used to memoize the result of a calculation so it doesn’t get recomputed on every render. By default, React re-runs all calculations inside a component whenever it re-renders, even if the inputs haven’t changed. This usually isn’t an issue, but it matters when the calculation is expensive or when its result is passed to a memoized child component. useMemo tells React to recompute the value only when its dependencies change, otherwise it reuses the previously calculated result.

When we say useMemo memoizes a value, we mean React remembers the computed result and reuses it as long as the dependencies stay the same. This memoization is temporary and scoped to the component’s lifecycle—it exists only while the component is mounted and is recalculated when dependencies change.