ReactJS Q & A


What is useMemo and useCallback?

React’s performance optimizations often hinge on its ability to avoid unnecessary computations or re-renders. Both `useMemo` and `useCallback` are hooks that play pivotal roles in this arena.

useMemo and useCallback are React hooks introduced to optimize the performance of functional components, especially when rendering large lists or using complex calculations. They both assist in preventing unnecessary re-renders and recalculations by memoizing values and functions.

useMemo is used to memoize computationally expensive calculations. When working with functional components, certain computations might be recalculated every time the component re-renders, even if the inputs to those computations haven’t changed. `useMemo` takes in a function and a list of dependencies, and it returns a memoized value. The provided function only re-computes the memoized value when one of its dependencies changes. This ensures that heavy calculations aren’t unnecessarily repeated, leading to more efficient performance.

On the other hand, useCallback is tailored for functions. In React, function components re-render every time their parent renders or when state or props change. During these re-renders, any inline callback functions are re-created. While this might not be an issue for many scenarios, it can cause problems when these callbacks are dependencies for other effects or when passed to performance-sensitive child components. `useCallback` memoizes the provided function, ensuring that it doesn’t get recreated unless one of its dependencies changes. This can be particularly beneficial when working with React’s `PureComponent` or `React.memo`, where a new function reference could lead to unnecessary renders.

While both `useMemo` and `useCallback` deal with memoization, their primary use-cases differ: `useMemo` is for values and computationally intensive calculations, while `useCallback` is for functions. Properly employing these hooks can drastically improve performance in specific scenarios, making React applications smoother and more responsive.