What is state in React - Hands-On Activity

What is state

📖 Scenario: You want to build a simple React component that shows a number on the screen. This number can change when you click a button. To do this, you need to keep track of the number inside the component. This is called state.

🎯 Goal: Create a React functional component that uses state to store a number. Add a button that, when clicked, increases the number by 1. The number should update on the screen each time you click the button.

📋 What You'll Learn

Use React functional component

Use the useState hook to create a state variable called count

Initialize count to 0

Add a button with an onClick event that increases count by 1

Display the current value of count in a <div>

💡 Why This Matters

🌍 Real World

Tracking user interactions like clicks, form inputs, or toggles in a web app.

💼 Career

Understanding state is essential for building interactive React applications used in many web development jobs.

1

Set up the React component and initial state

Create a React functional component called Counter. Inside it, use the useState hook to create a state variable named count and a setter function named setCount. Initialize count to 0.

React

import React, { useState } from 'react';

function Counter() {
  // Create state variable count and setCount, initialize count to 0
  
}

Hint

Remember to import useState from React. Use const [count, setCount] = useState(0); inside the component.

2

Add a button to increase the count

Inside the Counter component, return a <button> element. Add an onClick event to the button that calls setCount(count + 1) to increase the count by 1 when clicked.

React

import React, { useState } from 'react';

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

  // Return a button with onClick that increases count by 1
  return (
    
  );
}

Hint

Use onClick={() => setCount(count + 1)} inside the button tag.

3

Display the current count value

Modify the returned JSX to include a <div> that shows the current value of count above the button.

React

import React, { useState } from 'react';

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

  return (
    
      {/* Show count here */}
       setCount(count + 1)}>Increase
    
  );
}

Hint

Use a <div> with {count} inside the returned JSX.

4

Complete the component export

Add export default Counter; at the end of the file to make the component usable in other parts of the app.

React

import React, { useState } from 'react';

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

  return (
    
      {count}
       setCount(count + 1)}>Increase
    
  );
}

// Export the Counter component

Hint

Write export default Counter; at the end of the file.

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