The useEffect hook in React Native is a powerful tool for managing side effects in functional components. It allows developers to perform actions like fetching data, subscribing to events, or updating the DOM in response to changes in component state or props.

1. What is useEffect?

• useEffect is a React hook that executes a function after the component renders or when specified dependencies change.
• It replaces lifecycle methods like componentDidMount, componentDidUpdate, and componentWillUnmount in class components.

2. How useEffect Works

• The hook takes two arguments:
  1. A callback function that contains the side effect logic.
  2. A dependency array that determines when the effect should run.
• The callback function runs:
  • After the initial render.
  • Whenever one of the dependencies changes (if specified).

3. Dependency Array

• Empty Array ([]): The effect runs only once, after the initial render.
• Dependencies ([var1, var2]): The effect runs whenever the listed variables change.
• No Array: The effect runs after every render, which can lead to performance issues.

4. Cleaning Up Effects

• useEffect can return a cleanup function to handle tasks like unsubscribing from events or canceling network requests.
• Cleanup runs:
  • Before the component unmounts.
  • Before the effect re-runs due to dependency changes.

5. Common Use Cases

• Data Fetching: Fetch data from an API when the component mounts.
• Subscriptions: Listen for events, such as keyboard visibility changes or network status updates.
• Timers: Set intervals or timeouts and clean them up when they’re no longer needed.

Best Practices

• Always specify a dependency array to avoid unnecessary re-renders or side effects.
• Keep the effect logic clean and focused on a single responsibility.
• Use multiple useEffect hooks if different effects depend on different dependencies.

Understanding and using useEffect effectively is essential for building dynamic and responsive React Native applications, as it provides control over side effects in functional components.