What are React Hooks?

React Hooks are functions that allow you to "hook into" React state and lifecycle features from function components. They were introduced in React 16.8 to allow you to use state and other React features without writing class components.

Most Commonly Used Hooks

1. useState

The useState hook lets you add state to functional components.

import React, { useState } from 'react';

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

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>
        Increment
      </button>
    </div>
  );
}

2. useEffect

useEffect lets you perform side effects in function components. It's similar to componentDidMount, componentDidUpdate, and componentWillUnmount combined.

import React, { useState, useEffect } from 'react';

function WindowWidth() {
  const [width, setWidth] = useState(window.innerWidth);

  useEffect(() => {
    const handleResize = () => setWidth(window.innerWidth);
    window.addEventListener('resize', handleResize);

    // Cleanup function
    return () => {
      window.removeEventListener('resize', handleResize);
    };
  }, []); // Empty dependency array means this runs once on mount

  return <div>Window width: {width}px</div>;
}

3. useContext

useContext lets you subscribe to React context without introducing nesting.

import React, { useContext } from 'react';
const ThemeContext = React.createContext('light');

function ThemedButton() {
  const theme = useContext(ThemeContext);
  return <button className={theme}>Themed Button</button>;
}

4. useRef

useRef returns a mutable ref object that persists for the full lifetime of the component.

function TextInputWithFocusButton() {
  const inputEl = useRef(null);

  const onButtonClick = () => {
    inputEl.current.focus();
  };

  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

Custom Hooks

You can create your own Hooks to reuse stateful logic between different components.

function useLocalStorage(key, initialValue) {
  const [storedValue, setStoredValue] = useState(() => {
    try {
      const item = window.localStorage.getItem(key);
      return item ? JSON.parse(item) : initialValue;
    } catch (error) {
      return initialValue;
    }
  });

  const setValue = value => {
    try {
      setStoredValue(value);
      window.localStorage.setItem(key, JSON.stringify(value));
    } catch (error) {
      console.log(error);
    }
  };

  return [storedValue, setValue];
}

Rules of Hooks