What happens if you omit the dependencies array in useEffect?

What happens if you omit the dependencies array in useEffect?

• A. It never runs.
• B. It runs only on mount.
• C. It runs after every render.
• D. It runs only when a dependency changes.

Answer: (C)

Omitting the dependencies array makes the effect run after every render. useEffect runs after React has updated the UI, so without a dependencies list React will execute the effect’s callback on the initial mount and after every subsequent re-render. If you want it to run only once on mount, provide an empty array. If you want it to re-run when specific values change, include those values in the array. Just be mindful that because it executes so often, any state updates inside the effect can trigger more renders, which can lead to performance issues or even an infinite loop if not guarded.

Omitting the dependencies array makes the effect run after every render. useEffect runs after React has updated the UI, so without a dependencies list React will execute the effect’s callback on the initial mount and after every subsequent re-render. If you want it to run only once on mount, provide an empty array. If you want it to re-run when specific values change, include those values in the array. Just be mindful that because it executes so often, any state updates inside the effect can trigger more renders, which can lead to performance issues or even an infinite loop if not guarded.