What is state in React - Practice Questions & Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this React component?

Consider this React component that uses state. What will be displayed when the button is clicked once?

React

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increase</button>
    </>
  );
}

export default Counter;

A Count: 1

B Count: 0

C Count: undefined

D Count: NaN

💡 Hint

Think about what happens when the button's onClick updates the state from 0 to 1.

The initial state count is 0. When the button is clicked, setCount(count + 1) updates it to 1. The component re-renders showing Count: 1.

❓ state_output

intermediate

What is the value of count after two button clicks?

Given this React component, what will be the value of count after clicking the button twice quickly?

React

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  const handleClick = () => {
    setCount(count + 1);
    setCount(count + 1);
  };
  return (
    <>
      <p>Count: {count}</p>
      <button onClick={handleClick}>Increase Twice</button>
    </>
  );
}

export default Counter;

A Count: 0

B Count: 2

C Count: 1

D Count: NaN

💡 Hint

Remember that state updates may be batched and the count variable does not update immediately inside the function.

Both setCount(count + 1) calls use the same count value (0). So the state updates to 1, not 2.

📝 Syntax

advanced

Which option correctly initializes state with an object in React?

You want to store a user's name and age in state as an object. Which code correctly initializes this state?

React

import React, { useState } from 'react';

function UserInfo() {
  // Initialize state here
  return null;
}

export default UserInfo;

A const [user, setUser] = useState({ name: 'Alice', age: 30 });

B const [user, setUser] = useState(['Alice', 30]);

C const [user, setUser] = useState('name: Alice, age: 30');

D const [user, setUser] = useState(name = 'Alice', age = 30);

💡 Hint

State should be initialized with a single value, often an object or primitive, not multiple separate variables.

Option A correctly uses an object literal to initialize state. Options A and C use wrong types, and D is invalid syntax.

🔧 Debug

advanced

Why does this React component not update the displayed count?

Look at this React component. Why does clicking the button not change the displayed count?

React

import React, { useState } from 'react';

function Counter() {
  let count = 0;
  const handleClick = () => {
    count = count + 1;
  };
  return (
    <>
      <p>Count: {count}</p>
      <button onClick={handleClick}>Increase</button>
    </>
  );
}

export default Counter;

A Because useState is missing, causing a syntax error.

B Because count is a local variable, React does not track changes to it for re-rendering.

C Because the button's onClick handler is not correctly bound to the component.

D Because count is declared with let, it cannot be updated inside the function.

💡 Hint

React only re-renders when state or props change, not when local variables change.

Local variables like count do not cause React to re-render. State must be used to trigger updates.

🧠 Conceptual

expert

What is the main purpose of state in React components?

Choose the best description of what state is used for in React components.

A State is only used to store event handler functions inside components.

B State is used to store static configuration values that never change.

C State is a global variable accessible by all components without passing props.

D State stores data that changes over time and affects what the component displays.

💡 Hint

Think about what changes in a component cause it to update its display.

State holds data that can change and causes the component to update its output. It is local to the component and not global.

Practice

1. What is the main purpose of state in a React component?

easy

A. To create static content that never changes

B. To define the component's style and layout

C. To import external libraries into the component

D. To store data that can change and affect the component's display

Solution

1. Step 1: Understand what state represents

   State holds information that can change over time and affects what the component shows.

2. Step 2: Compare options with the definition

   Only To store data that can change and affect the component's display correctly describes state as data that changes and affects display.

3. Final Answer:

   To store data that can change and affect the component's display -> Option D

4. Quick Check:

   State = data that changes and updates UI [OK]

Hint: State holds changing data that updates the screen [OK]

Common Mistakes:

• Confusing state with styling or layout
• Thinking state is for static content
• Mixing state with importing libraries

2. Which of the following is the correct way to declare a state variable called count with initial value 0 in a React functional component?

easy

A. const count = useState(0);

B. let count = 0;

C. const [count, setCount] = useState(0);

D. var count = useState(0);

Solution

1. Step 1: Recall the useState syntax

   useState returns an array with current value and a setter function, usually destructured as [value, setValue].

2. Step 2: Check each option's syntax

   const [count, setCount] = useState(0); correctly uses array destructuring with useState(0). Options A, B, and C do not follow the correct pattern.

3. Final Answer:

   const [count, setCount] = useState(0); -> Option C

4. Quick Check:

   useState syntax = [value, setValue] [OK]

Hint: useState returns [value, setter] array, destructure it [OK]

Common Mistakes:

• Using var or let without useState
• Not destructuring the array returned by useState
• Assigning useState directly to a single variable

3. Consider this React component code:

import React, { useState } from 'react';

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

      
Count: {count}
       setCount(count + 1)}>Increase
    
  );
}

What will be displayed after clicking the button three times?

medium

A. Count: 3

B. Count: 0

C. Count: 1

D. Count: undefined

Solution

1. Step 1: Understand initial state and button action

   Initial count is 0. Each button click calls setCount(count + 1), increasing count by 1.

2. Step 2: Calculate count after 3 clicks

   After 3 clicks, count increments 3 times: 0 + 3 = 3.

3. Final Answer:

   Count: 3 -> Option A

4. Quick Check:

   3 clicks increase count from 0 to 3 [OK]

Hint: Each click adds 1 to count starting from 0 [OK]

Common Mistakes:

• Thinking count resets on each click
• Confusing initial value with updated value
• Ignoring that setCount updates state asynchronously

4. What is wrong with this React state update code?

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

function increment() {
  count = count + 1;
}

medium

A. Missing initial value in useState

B. Directly modifying state variable instead of using setter

C. Using const instead of let for count

D. Setter function name is incorrect

Solution

1. Step 1: Identify how state should be updated

   React state must be updated using the setter function returned by useState, not by direct assignment.

2. Step 2: Analyze the code's mistake

   The code tries to assign directly to count, which is a constant and does not update React state.

3. Final Answer:

   Directly modifying state variable instead of using setter -> Option B

4. Quick Check:

   Use setter function to update state [OK]

Hint: Never assign state variable directly; use setter function [OK]

Common Mistakes:

• Trying to assign value directly to state variable
• Ignoring that useState returns a setter function
• Confusing const with let for state variables

5. You want to create a React component that shows a list of items and lets the user add new items. Which approach correctly uses state to update the list when a new item is added?

hard

A. Use const [items, setItems] = useState([]); and update with setItems([...items, newItem])

B. Use let items = []; and push new items with items.push(newItem)

C. Use const items = []; and reassign with items = items.concat(newItem)

D. Use const [items, setItems] = useState([]); and update with items.push(newItem)

Solution

1. Step 1: Understand state immutability in React

   State should not be mutated directly; instead, create a new array with the new item added.

2. Step 2: Check each option's update method

   Use const [items, setItems] = useState([]); and update with setItems([...items, newItem]) uses spread syntax to create a new array and calls setter, which is correct. Options A and D mutate the array directly. Use const items = []; and reassign with items = items.concat(newItem) tries to reassign a const variable.

3. Final Answer:

   Use const [items, setItems] = useState([]); and update with setItems([...items, newItem]) -> Option A

4. Quick Check:

   Update state immutably with setter [OK]

Hint: Always create new array and use setter to update state [OK]

Common Mistakes:

• Mutating state array directly with push
• Reassigning const variables
• Not using setter function to update state