What is the `useMemo` hook in React and when should it be used?

TL;DR

The useMemo hook in React is used to memoize expensive calculations so that they are only recomputed when one of the dependencies has changed. This can improve performance by avoiding unnecessary recalculations. You should use useMemo when you have a computationally expensive function that doesn't need to run on every render.

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

What is the `useMemo` hook in React and when should it be used?

What is `useMemo`?

The useMemo hook is a built-in React hook that allows you to memoize the result of a function. This means that the function will only be re-executed when one of its dependencies changes. The primary purpose of useMemo is to optimize performance by preventing unnecessary recalculations.

Syntax

The syntax for useMemo is as follows:

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

The first argument is a function that returns the value you want to memoize.
The second argument is an array of dependencies. The memoized value will only be recomputed when one of these dependencies changes.

When should it be used?

Expensive calculations

If you have a function that performs a computationally expensive calculation, you can use useMemo to ensure that this calculation is only performed when necessary.

const expensiveCalculation = (num) => {
  // Some expensive calculation
  return num * 2;
};

const MyComponent = ({ number }) => {
  const memoizedValue = useMemo(() => expensiveCalculation(number), [number]);

  return <div>{memoizedValue}</div>;
};

Avoiding unnecessary renders

useMemo can also be useful for avoiding unnecessary renders of child components. If a child component depends on a value that is expensive to compute, you can use useMemo to ensure that the value is only recomputed when necessary.

const MyComponent = ({ items }) => {
  const sortedItems = useMemo(() => {
    return items.sort((a, b) => a - b);
  }, [items]);

  return <ChildComponent sortedItems={sortedItems} />;
};

Caveats

Overuse: Overusing useMemo can lead to more complex code without significant performance benefits. It should be used judiciously.
Dependencies: Make sure to correctly specify all dependencies. Missing dependencies can lead to stale values, while extra dependencies can lead to unnecessary recalculations.

What is the `useMemo` hook in React and when should it be used?

TL;DR