![]() Or maybe you are depending on this value from useEffect. For example, maybe another useMemo calculation value depends on it. The value you’re passing is later used as a dependency of some Hook.Memoization lets your component re-render only when dependencies are the same. ![]() You want to skip re-rendering if the value hasn’t changed. You pass it as a prop to a component wrapped in memo.The calculation you’re putting in useMemo is noticeably slow, and its dependencies rarely change.Optimizing with useMemo is only valuable in a few cases: On the other hand, if your app is more like a drawing editor, and most interactions are granular (like moving shapes), then you might find memoization very helpful. If your app is like this site, and most interactions are coarse (like replacing a page or an entire section), memoization is usually unnecessary. For example, if this TodoList updates its state or receives new props from its parent, the filterTodos function will re-run: ![]() Let’s walk through an example to see when this is useful.īy default, React will re-run the entire body of your component every time that it re-renders. In other words, useMemo caches a calculation result between re-renders until its dependencies change. Otherwise, React will re-run your calculation and return the new value. If none of the dependencies have changed (compared with Object.is), useMemo will return the value you already calculated before. On every subsequent render, React will compare the dependencies with the dependencies you passed during the last render. On the initial render, the value you’ll get from useMemo will be the result of calling your calculation. A list of dependencies including every value within your component that’s used inside your calculation. ![]()
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