How does React.memo, useMemo, and useCallback affect performance?

React.memo memoizes a component’s output by shallow-comparing props. useMemo memoizes a computed value. useCallback memoizes a function identity. Each trades memory and complexity for fewer re-renders or fewer expensive recalculations—but they only help if child work is actually expensive or if referential equality prevents downstream renders.

Premature memoization can hide bugs (stale props) and add overhead. Good targets: pure list items receiving stable props, expensive derived data, callbacks passed to memoized children that would otherwise re-render due to new function identities.