Q186- What does the dependency array of useEffect affect?

The dependency array in useEffect determines when the effect runs or re-runs in a React component.

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⚙️ 1. No Dependency Array

useEffect(() => {
  console.log("Runs after every render");
});

🔸 The effect runs after every render (both initial and updates).

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⚙️ 2. Empty Dependency Array []

useEffect(() => {
  console.log("Runs only once");
}, []);

🔸 The effect runs only once — after the initial render, like componentDidMount.

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⚙️ 3. With Dependencies

useEffect(() => {
  console.log("Runs when count changes");
}, [count]);

🔸 The effect runs only when the specified dependencies (count here) change.

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⚙️ 4. Cleanup Function

If your effect returns a function, React runs that cleanup before re-running the effect or when the component unmounts.

useEffect(() => {
  const timer = setInterval(() => console.log("Tick"), 1000);
  return () => clearInterval(timer); // Cleanup
}, [count]);

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💡 In Short:

The dependency array controls when and how often useEffect executes —
✅ Empty [] → Run once
✅ With deps [x, y] → Run when x or y changes
✅ Without array → Run after every render