React usePrevious Hook Bug: Why Ref-Based Implementations Can Mislead Developers

React has no built-in way to access a component's previous state or prop values, leading developers to rely on a widely shared pattern that stores values in a ref via useEffect. This classic approach returns the value from the previous render rather than the last distinct value, meaning any unrelated re-render silently overwrites the stored reference. The flaw can cause UI indicators — such as a count-change arrow — to disappear incorrectly after a theme toggle or parent re-render, even when the tracked value has not changed. The @reactuses/core library addresses this with a usePrevious hook built on useState instead of refs and effects, ensuring only genuine value changes update the stored previous value. The alternative twelve-line implementation follows patterns recommended in React's own documentation and avoids the drift and infinite-loop risks associated with the ref-based recipe.