Effect dependencies

What this means

The dependency array tells React which values an effect depends on. When a dependency changes, the effect runs again.

In beginner terms, this topic answers one practical question: "What should I write, and why does React care about it?" Do not try to memorize the syntax first. First understand the idea, then connect the syntax to that idea.

Why it matters

Dependencies prevent stale values and unnecessary work. They also make your effect honest about what it reads.

When you build real React screens, this idea helps you decide where data should live, what the user should see, and what should happen after an interaction. That is why this lesson is part of the main path instead of being an optional detail.

Step by step

1. Notice the UI problem this topic solves. 2. Look at the smallest possible example. 3. Change one value and predict what should appear. 4. Run the example and compare the result with your prediction. 5. Use the practice task before moving on.

Small example

React.useEffect(() => {
  console.log(count);
}, [count]);

Common mistake

Do not ignore dependency warnings without understanding them. Missing dependencies can make effects use old values.

Practice task

Write an effect that logs a search term whenever the search term changes.

Remember this

Effects should list the values they depend on.