# useMemo

## 基本

 把“创建”函数和依赖项数组作为参数传入 `useMemo`，它仅会在某个依赖项改变时才重新计算 memoized 值。这种优化有助于避免在每次渲染时都进行高开销的计算。

```javascript
const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);
```

传入 `useMemo` 的函数会在渲染期间执行。请不要在这个函数内部执行与渲染无关的操作，诸如副作用这类的操作属于 `useEffect` 的适用范畴，而不是 `useMemo`。

如果没有提供依赖项数组，`useMemo` 在每次渲染时都会计算新的值。


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