Top 10 React.js useState() Interview Questions

What is useState()?
A React hook to manage state in functional components with a state variable and update function.

How to use useState()?

import { useState } from 'react';
const [count, setCount] = useState(0);
<button onClick={() => setCount(count + 1)}>{count}</button>;

What does useState() return?
An array with the current state and its update function.

Can useState() hold objects/arrays?
Yes, it supports any data type (objects, arrays, strings, etc.).

How to update an object in useState()?

const [user, setUser] = useState({ name: '', age: 0 });
setUser({ ...user, name: 'John' });

Why isn’t state updated immediately?
setState is asynchronous; React batches updates for performance.

How to use previous state in setState?

setCount(prevCount => prevCount + 1);

Can useState initialize with a function?
Yes, for lazy initialization:

const [value, setValue] = useState(() => expensiveComputation());

How to handle multiple state variables?
Use multiple useState calls:

const [name, setName] = useState('');
const [age, setAge] = useState(0);

Why does state update cause an infinite loop?
setState in useEffect or render without conditions/dependencies causes re-renders; use dependency arrays or conditionals.