Intern level: Lifecycle Methods and Hooks in React
Tags: webdev, javascript, programming, react
Published: 2024-07-16 18:39:31

## Introduction to React Hooks

React Hooks are functions that let you use state and other React features in functional components. Before hooks, stateful logic was only available in class components. Hooks provide a more direct API to the React concepts you already know, such as state, lifecycle methods, and context.

### Key Hooks in React

#### useState

`useState` is a hook that lets you add state to functional components.

Example:
```jsx
import React, { useState } from 'react';

const Counter = () => {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click me</button>
    </div>
  );
};

export default Counter;
```

In this example, `useState` initializes the `count` state variable to 0. The `setCount` function is used to update the state when the button is clicked.

#### useEffect

`useEffect` is a hook that lets you perform side effects in functional components, such as fetching data, directly interacting with the DOM, and setting up subscriptions. It combines the functionality of several lifecycle methods in class components (`componentDidMount`, `componentDidUpdate`, and `componentWillUnmount`).

Example:
```jsx
import React, { useState, useEffect } from 'react';

const DataFetcher = () => {
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch('https://api.example.com/data')
      .then(response => response.json())
      .then(data => setData(data));
  }, []);

  return (
    <div>
      {data ? <pre>{JSON.stringify(data, null, 2)}</pre> : 'Loading...'}
    </div>
  );
};

export default DataFetcher;
```

In this example, `useEffect` fetches data from an API when the component mounts.

#### useContext

`useContext` is a hook that lets you access the context value for a given context.

Example:
```jsx
import React, { useContext } from 'react';

const ThemeContext = React.createContext('light');

const ThemedComponent = () => {
  const theme = useContext(ThemeContext);

  return <div>The current theme is {theme}</div>;
};

export default ThemedComponent;
```

In this example, `useContext` accesses the current value of `ThemeContext`.

#### useReducer

`useReducer` is a hook that lets you manage complex state logic in a functional component. It is an alternative to `useState`.

Example:
```jsx
import React, { useReducer } from 'react';

const initialState = { count: 0 };

const reducer = (state, action) => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    default:
      return state;
  }
};

const Counter = () => {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
    </div>
  );
};

export default Counter;
```

In this example, `useReducer` manages the `count` state with a reducer function.

## Custom Hooks

Custom hooks let you reuse stateful logic across multiple components. A custom hook is a function that uses built-in hooks.

Example:
```jsx
import { useState, useEffect } from 'react';

const useFetch = (url) => {
  const [data, setData] = useState(null);

  useEffect(() => {
    fetch(url)
      .then(response => response.json())
      .then(data => setData(data));
  }, [url]);

  return data;
};

const DataFetcher = ({ url }) => {
  const data = useFetch(url);

  return (
    <div>
      {data ? <pre>{JSON.stringify(data, null, 2)}</pre> : 'Loading...'}
    </div>
  );
};

export default DataFetcher;
```

In this example, `useFetch` is a custom hook that fetches data from a given URL.

## Advanced Hook Patterns

### Managing Complex State with useReducer

When dealing with complex state logic involving multiple sub-values or when the next state depends on the previous one, `useReducer` can be more appropriate than `useState`.

Example:
```jsx
import React, { useReducer } from 'react';

const initialState = { count: 0 };

const reducer = (state, action) => {
  switch (action.type) {
    case 'increment':
      return { count: state.count + 1 };
    case 'decrement':
      return { count: state.count - 1 };
    default:
      return state;
  }
};

const Counter = () => {
  const [state, dispatch] = useReducer(reducer, initialState);

  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
    </div>
  );
};

export default Counter;
```

In this example, `useReducer` manages the `count` state with a reducer function.

### Optimizing Performance with useMemo and useCallback

#### useMemo

`useMemo` is a hook that memoizes a computed value, recomputing it only when one of the dependencies changes. It helps optimize performance by preventing expensive calculations on every render.

Example:
```jsx
import React, { useState, useMemo } from 'react';

const ExpensiveCalculation = ({ number }) => {
  const computeFactorial = (n) => {
    console.log('Computing factorial...');
    return n <= 1 ? 1 : n * computeFactorial(n - 1);
  };

  const factorial = useMemo(() => computeFactorial(number), [number]);

  return <div>Factorial of {number} is {factorial}</div>;
};

const App = () => {
  const [number, setNumber] = useState(5);

  return (
    <div>
      <input
        type="number"
        value={number}
        onChange={(e) => setNumber(parseInt(e.target.value, 10))}
      />
      <ExpensiveCalculation number={number} />
    </div>
  );
};

export default App;
```

In this example, `useMemo` ensures that the factorial calculation is only recomputed when `number` changes.

#### useCallback

`useCallback` is a hook that memoizes a function, preventing its recreation on every render unless one of its dependencies changes. It is useful for passing stable functions to child components that rely on reference equality.

Example:
```jsx
import React, { useState, useCallback } from 'react';

const Button = React.memo(({ onClick, children }) => {
  console.log(`Rendering button - ${children}`);
  return <button onClick={onClick}>{children}</button>;
});

const App = () => {
  const [count, setCount] = useState(0);

  const increment = useCallback(() => setCount((c) => c + 1), []);

  return (
    <div>
      <Button onClick={increment}>Increment</Button>
      <p>Count: {count}</p>
    </div>
  );
};

export default App;
```

In this example, `useCallback` ensures that the `increment` function is only recreated if its dependencies change, preventing unnecessary re-renders of the `Button` component.

## Conclusion

Understanding React Hooks is essential for modern React development. They enable you to write cleaner, more maintainable code in functional components. By mastering hooks like `useState`, `useEffect`, `useContext`, and `useReducer`, as well as advanced patterns like custom hooks and performance optimizations with `useMemo` and `useCallback`, you can build robust and efficient React applications. As an intern, gaining a solid grasp of these concepts will set a strong foundation for your journey in React development.