What is state in React - Interactive Quiz & Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a state variable called count with initial value 0.

React

const [count, setCount] = [1](0);

Drag options to blanks, or click blank then click option'

AuseContext

BuseRef

CuseEffect

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Using useEffect instead of useState

Trying to update state directly without a setter function

The useState hook creates a state variable and a function to update it.

2fill in blank

medium

Complete the code to update the count state by adding 1 when the button is clicked.

React

<button onClick={() => setCount([1])}>Add 1</button>

Drag options to blanks, or click blank then click option'

Acount - 1

B0

Ccount + 1

Dcount * 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition

Setting count to zero instead of increasing

To increase the count by 1, use count + 1 inside the setter function.

3fill in blank

hard

Fix the error in the code to correctly initialize state with an object having a name property.

React

const [user, setUser] = useState([1]);

Drag options to blanks, or click blank then click option'

A{name: 'Alice'}

B"name: 'Alice'"

C[name: 'Alice']

D'name: "Alice"'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of an object

Using square brackets instead of curly braces

State should be initialized with an object literal: {name: 'Alice'}.

4fill in blank

hard

Fill both blanks to create a state variable called message with initial value 'Hello' and update it to 'Hi'.

React

const [message, [1]] = useState([2]);
setMessage('Hi');

Drag options to blanks, or click blank then click option'

AsetMessage

Bmessage

C'Hello'

D'Hi'

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the setter function and state variable

Using the wrong initial value

The setter function is usually named starting with 'set'. Initial state is the first value.

5fill in blank

hard

Fill all three blanks to create a counter component with state count starting at 0, a button to increment, and display the count.

React

import React, { [1] } from 'react';

function Counter() {
  const [count, [2]] = [3](0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </div>
  );
}

Drag options to blanks, or click blank then click option'

AuseState

BsetCount

DuseEffect

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to import useState

Using wrong setter function name

Calling wrong hook to initialize state

Import useState, use setCount as setter, and call useState(0) to initialize.

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