React Class Component Lifecycle Methods Explained With Examples
Class component lifecycle methods are core interview material. Here is each phase and method explained with examples.
React Class Component Lifecycle Methods Explained With Examples
Lifecycle methods are how class components run side effects at specific points. Even if you write hooks, understanding these helps you read old code and answer interviews. Here is each phase and method.
The Three Phases
A class component goes through three phases: mounting (added to the DOM), updating (re-rendered), and unmounting (removed from the DOM). Each phase has lifecycle methods that run at specific points.
Mounting
constructor runs first, where you initialize state and bind methods. render produces the JSX. componentDidMount runs after the component is added to the DOM, where you fetch data, set up subscriptions, and timers.
Updating
render produces new JSX when state or props change. componentDidUpdate runs after the update is committed to the DOM, where you can respond to changes, like fetching new data when a prop changes.
Unmounting
componentWillUnmount runs right before the component is removed, where you clean up subscriptions, timers, and listeners to prevent memory leaks.
Rare Methods
Methods like shouldComponentUpdate and getSnapshotBeforeUpdate exist for performance and edge cases. You rarely use them, but knowing they exist helps when reading optimized code.
How They Map to Hooks
constructor maps to useState initial values. componentDidMount maps to useEffect with an empty dependency array. componentDidUpdate maps to useEffect with dependencies. componentWillUnmount maps to the cleanup function returned from useEffect.
The Takeaway
Lifecycle methods run at mount, update, and unmount. Know constructor, render, componentDidMount, componentDidUpdate, and componentWillUnmount, and how each maps to a hook. That covers almost everything you will read or be asked.
Three phases: mounting when the component is added to the DOM, updating when it re-renders due to state or props changes, and unmounting when it is removed. Each phase has lifecycle methods that run at specific points.
It runs after the component is added to the DOM. You use it to fetch data, set up subscriptions, and start timers, similar to useEffect with an empty dependency array in functional components.
It runs right before the component is removed from the DOM. You use it to clean up subscriptions, timers, and listeners to prevent memory leaks, similar to the cleanup function returned from useEffect.
It runs after an update is committed to the DOM, when state or props change. You use it to respond to changes, like fetching new data when a prop changes, similar to useEffect with dependencies.
constructor maps to useState initial values. componentDidMount maps to useEffect with an empty dependency array. componentDidUpdate maps to useEffect with dependencies. componentWillUnmount maps to the cleanup function returned from useEffect.
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