Great question. The term "expensive calculation" sometimes sounds vague so let me make it real with examples.

In React (or any UI-driven application), an expensive calculation typically refers to any operation that takes a significant amount of time or resources relative to rendering. It may not seem like a big deal in isolation, but when React needs to re-run these calculations frequently-for example, on every render-it can lead to noticeable performance issues.

Examples of Expensive Calculations:

1. Complex Loops or Algorithms

Sorting or filtering large datasets, such as sorting thousands of products according to price.

Computing statistical data, such as averages, medians, or aggregations over huge arrays.

2. Intensive Math Operations

Executing intensive mathematical calculations, like matrix multiplications for animations or effects.

3. Formatting Huge Data

Converting a huge dataset to a new format, for example, formatting thousands of dates or restructuring JSON.

4. Heavy Component Derivations

Deriving values that are dependent on multiple layers of props and state, like calculating a derived value for nested data structures.

- Why UseMemo Helps

With such scenarios, React will otherwise recompute the same thing multiple times when there are re-renders. The useMemo will prevent React from recalculating the result unless dependencies change and thus saves time and resources.

That being said, expensive depends on the context. If you have a very simple app with very minimal data, you will never find this. However, apps that deal with large datasets, real-time processing, or heavy user interaction require these optimizations to be a game-changer.

What do you think? When have you encountered a situation where you thought you should worry about performance optimizations? We can chat about it and it will clear any doubt anyone has, Thanks