useState is a React Hook used to store and update data in a component. When the state changes, React updates the UI.

1. Basic syntax

const [value, setValue] = useState(initialValue);

value — current state value.
setValue — function to update the state.
useState(initialValue) — sets the initial value.

2. Example: Counter

import { useState } from "react"; export default function App() { const [count, setCount] = useState(0); return ( <div> <h1>{count}</h1> <button onClick={() => setCount(count + 1)}> Increment </button> <button onClick={() => setCount(count - 1)}> Decrement </button> <button onClick={() => setCount(0)}> Reset </button> </div> ); }

How it works:

  1. useState(0) sets the initial count to 0.
  2. setCount(count + 1) increases the count.
  3. setCount(count - 1) decreases the count.
  4. setCount(0) resets it to zero.