What is the purpose of rerunning the effects?

Difficulty: 5/10

useEffect, dependency array, cleanup

1
Strict Mode runs an extra setup+cleanup cycle for every Effect.
2
If the Effect has no cleanup logic, so it creates an extra connection but doesn’t destroy it. This is a hint that you’re missing a cleanup function.
3
Strict Mode lets you notice such mistakes early in the process. When you fix your Effect by adding a cleanup function in Strict Mode, you also fix many possible future production bugs
4
Without Strict Mode, it was easy to miss that your Effect needed cleanup. By running setup → cleanup → setup instead of setup for your Effect in development, Strict Mode made the missing cleanup logic more noticeable.

0-2 years experience

2-5 years experience