Effect dependencies

The dependency array tells React which values the effect reads. When any dependency changes, React re-runs the effect after the next render.

Rules of thumb

• Include every prop and state variable used inside the effect
• Stable setters from useState do not need to be listed (React guarantees stability)
• Objects and arrays as dependencies compare by reference—memoize or depend on primitives

Stale closures

Missing dependencies can show outdated values—the effect “closes over” old state. ESLint’s react-hooks/exhaustive-deps rule catches most mistakes in real projects.

Important interview questions and answers

1. Q: Empty deps vs no deps?
   A: [] runs once per mount; omitting the array runs after every render.
2. Q: Infinite loop symptom?
   A: Effect sets state that is in its dependency array without a guard—fix logic or split effects.

Self-check

1. Why does changing userId require listing it in deps?
2. What happens if you pass a new object literal in deps every render?

Tip: ESLint react-hooks/exhaustive-deps warns about stale closures—fix deps or refactor, do not silence blindly.