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Exam details
Strict Mode mounts → unmounts → remounts to catch bugs from missing cleanups. Production runs effects once per mount.
Updating state inside the effect with a dep that changes when state updates. Memoize the value or remove from deps if it doesn't need to trigger.
Returns the same function reference unless deps change — avoids re-rendering children that take it as a prop and use memo().
useMemo(f, d): memoizes a value (the result of f()).
useCallback(f, d): memoizes the function itself. Equivalent to useMemo(() => f, d).
Mutable container that doesn't trigger re-render: DOM nodes, timers, latest-value pattern, instance variables.