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useEffect and Side Effects

useEffect and Side Effects

Explore the core concepts of state management in React, including props, state, and the powerful use of hooks. You'll learn how to manage component states effectively and handle side effects, ensuring your applications are responsive and dynamic.

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What does useEffect do?

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useEffect is a hook that lets you run side effects in functional components, such as data fetching, subscriptions, or manually changing the DOM. It runs after the component renders and can optionally clean up when the component unmounts.

What is the dependency array in useEffect?

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The dependency array is the second argument to useEffect. It tells React which values the effect depends on. The effect re-runs only when one of those values changes. An empty array means the effect runs only once after the first render.

What happens if you omit the dependency array in useEffect?

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If the dependency array is omitted, the effect runs after every render of the component. This is rarely desired because it can cause performance problems and infinite loops if the effect updates state.

How do you clean up an effect in useEffect?

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You return a cleanup function from the effect. React calls this function before the next effect runs and when the component unmounts. It is commonly used to remove event listeners, cancel timers, or close subscriptions.

What is useLayoutEffect and how does it differ from useEffect?

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useLayoutEffect has the same signature as useEffect but runs synchronously after the DOM is updated and before the browser paints. Use it when you need to measure the DOM or make visual changes that must happen before paint to avoid flicker.

What is a common cause of infinite loops with useEffect?

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A common cause is updating state inside an effect without correct dependencies. If the effect depends on the state it updates and does not guard the update, each update triggers another effect run, creating an infinite loop.

How do functional components replicate componentDidMount?

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You use useEffect with an empty dependency array. The effect runs once after the first render, which is equivalent to componentDidMount in a class component.

How do functional components replicate componentWillUnmount?

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You return a cleanup function from useEffect. React calls that function when the component unmounts, which replaces componentWillUnmount from class components.

What is useSyncExternalStore?

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useSyncExternalStore is a hook designed for libraries that integrate with external state sources. It lets React subscribe to the store and read its current value in a way that is safe with concurrent rendering and prevents tearing.

What is tearing in concurrent React and how is it prevented?

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Tearing is when different parts of the UI display inconsistent values from the same external state during a single render. React 18 prevents it for built-in state and provides useSyncExternalStore for safely integrating external stores. ---