React フック: 詳細な説明

React Hooks are functions that allow you to "hook into" React state and lifecycle features from functional components. Introduced in React 16.8, Hooks enable you to use state and other React features without writing a class component.

Let’s break down the core concepts behind Hooks:

1. Why React Hooks?

Before hooks, stateful logic could only be implemented in class components. Functional components were stateless, making them less versatile. Hooks were introduced to:

• Allow stateful logic in functional components.
• Share stateful logic across components easily.
• Avoid boilerplate code (e.g., lifecycle methods in class components).
• Enhance component reuse and modularity.

2. Basic Rules of Hooks

There are two key rules to follow when using hooks:

• Only call Hooks at the top level: Hooks should be called at the top of the function, not inside loops, conditions, or nested functions. This ensures that React calls the Hooks in the same order each time a component renders.
• Only call Hooks from React functions: You can use Hooks in functional components or custom Hooks, but not in regular JavaScript functions.

3. Built-in React Hooks

Let’s explore some of the key built-in hooks in React:

useState
useState allows you to add state to a functional component.

Syntax:

const [state, setState] = useState(initialState);

• state: The current state value.
• setState: Function that allows you to update the state.
• initialState: The initial value of the state.

useEffect
useEffect is the hook used to handle side effects in functional components. This could include data fetching, subscriptions, or directly interacting with the DOM.

Syntax:

useEffect(() => {
  // Side effect code
  return () => {
    // Cleanup (optional)
  };
}, [dependencies]);

• The first argument is a function where you place your side effect logic.
• The second argument is an optional array of dependencies. If any of these dependencies change, the effect will run again.

4. Other Useful Hooks

useMemo: Memoizes a computed value to avoid recalculating on every render.
useCallback: Memoizes a function to avoid re-creating it on every render.
useLayoutEffect: Similar to useEffect, but fires synchronously after all DOM mutations.

Benefits of Hooks

• Cleaner code: Hooks make components easier to read and understand.
• Reusability: Hooks allow you to share logic between components without HOCs or render props.
• Better state management: More granular control of state and side effects.
• Functional component advantages: Enables the full use of functional programming in React components.

Hooks revolutionized the way we write React components, moving away from class-based components and towards a more functional, concise, and reusable approach to state management and side effects.

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