Lesson 14 — useEffect: timers

The dependency array

The second argument of useEffect is the dependency array: React re-runs the effect whenever a value in it changes (first running the old cleanup). [] means "run once"; [running] means "run again every time running changes". If the effect reads a state value but does not list it, it keeps seeing the old value.

useEffect(() => {
  document.title = "Count: " + count;
}, [count]); // re-runs when count changes

Why it matters: a wrong dependency array is the number-one useEffect bug — effects that never update, or run forever.