useState hook introduction in React

Introduction

The useState hook lets you add and keep track of changing data in a React component. It helps your app remember things like clicks or typed text.

When you want to remember a number that changes, like a counter.

When you need to store text typed by a user in a form.

When you want to toggle something on or off, like showing or hiding a menu.

When you want to update the screen based on user actions.

When you want to keep track of simple data inside a component.

Syntax

React

const [state, setState] = useState(initialValue);

state is the current value you want to remember.

setState is a function you call to change that value and update the screen.

Examples

This creates a number state starting at 0, often used for counters.

React

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

This creates a text state starting empty, useful for input fields.

React

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

This creates a true/false state to show or hide something.

React

const [isVisible, setIsVisible] = useState(true);

Sample Program

This simple component shows a number and a button. Each time you click the button, the number goes up by one. The useState hook keeps track of the number and updates the screen.

React

import React, { useState } from 'react';

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

  return (
    <main>
      <h1>Count: {count}</h1>
      <button onClick={() => setCount(count + 1)} aria-label="Increase count">
        Increase
      </button>
    </main>
  );
}

export default Counter;

Important Notes

Always call useState at the top level of your component, not inside loops or conditions.

Calling the setter function (like setCount) tells React to update the screen with the new value.

You can use any type of value in useState: numbers, text, true/false, or even objects.

Summary

useState lets your React component remember and change data.

It returns the current value and a function to update it.

Updating state causes React to refresh what the user sees.

Practice

1. What does the useState hook do in a React component?

easy

A. It fetches data from an external API automatically.

B. It allows the component to remember and update data over time.

C. It styles the component with CSS.

D. It handles routing between pages.

Solution

1. Step 1: Understand the purpose of useState

   The useState hook is designed to let a React component keep track of data that can change.

2. Step 2: Identify what useState does

   It returns the current value and a function to update that value, enabling the component to remember and update data.

3. Final Answer:

   It allows the component to remember and update data over time. -> Option B

4. Quick Check:

   useState manages component data = A [OK]

Hint: useState is for storing and changing data inside components [OK]

Common Mistakes:

• Thinking useState fetches data from APIs
• Confusing useState with styling or routing
• Believing useState automatically updates the UI without calling the setter

2. Which of the following is the correct syntax to declare a state variable count with initial value 0 using useState?

easy

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

B. const count = useState(0);

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

D. const [count, setCount] = useState;

Solution

1. Step 1: Recall useState syntax

   The useState hook returns an array with two elements: the current state and a function to update it, so destructuring with square brackets is needed.

2. Step 2: Check each option

   const [count, setCount] = useState(0); correctly uses array destructuring and calls useState with initial value 0. Others either miss destructuring or call useState incorrectly.

3. Final Answer:

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

4. Quick Check:

   useState returns [value, setter] = C [OK]

Hint: useState returns [value, setter] array, use destructuring [OK]

Common Mistakes:

• Not using array destructuring
• Forgetting to call useState as a function
• Assigning useState to multiple variables without brackets

3. What will be displayed on the screen after clicking the button twice in this component?

import React, { useState } from 'react';

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

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

medium

A. Count: 0

B. Count: 1

C. Count: undefined

D. Count: 2

Solution

1. Step 1: Understand initial state and button action

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

2. Step 2: Calculate count after two clicks

   After first click, count becomes 1. After second click, count becomes 2.

3. Final Answer:

   Count: 2 -> Option D

4. Quick Check:

   Two increments from 0 = 2 [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. Identify the error in this React component using useState:

import React, { useState } from 'react';

function Example() {
  const [value, setValue] = useState;
  return (
    

      
{value}
       setValue('Hello')}>Set
    
  );
}

medium

A. State variable value is not declared.

B. The setter function setValue is missing.

C. useState is not called as a function.

D. The button's onClick handler is invalid.

Solution

1. Step 1: Check useState usage

   The code assigns useState without parentheses, so it is not called as a function, which is required.

2. Step 2: Understand consequences

   Without calling useState(), value and setValue are undefined, causing errors.

3. Final Answer:

   useState is not called as a function. -> Option C

4. Quick Check:

   useState must be called with () = B [OK]

Hint: Always call useState with parentheses: useState() [OK]

Common Mistakes:

• Forgetting parentheses after useState
• Assuming useState is a variable, not a function
• Misreading error as button or variable issue

5. You want to create a toggle button that switches text between "ON" and "OFF" using useState. Which code correctly implements this behavior?

hard

A. const [isOn, setIsOn] = useState(false);
setIsOn(!isOn)}>{isOn ? 'ON' : 'OFF'}

B. const [isOn, setIsOn] = useState('OFF');
setIsOn('ON')}>{isOn}

C. const [isOn, setIsOn] = useState(true);
setIsOn(true)}>{isOn ? 'ON' : 'OFF'}

D. const isOn = false;
isOn = !isOn}>{isOn ? 'ON' : 'OFF'}

Solution

1. Step 1: Understand toggle logic with useState

   To toggle, state must be boolean and updated to its opposite on click.

2. Step 2: Evaluate each option

   const [isOn, setIsOn] = useState(false); setIsOn(!isOn)}>{isOn ? 'ON' : 'OFF'} initializes state as false and toggles it with !isOn, displaying 'ON' or 'OFF' accordingly. Others either don't toggle or misuse state.

3. Final Answer:

   const [isOn, setIsOn] = useState(false); setIsOn(!isOn)}>{isOn ? 'ON' : 'OFF'} -> Option A

4. Quick Check:

   Toggle state by flipping boolean = A [OK]

Hint: Toggle boolean state by setting it to !currentValue [OK]

Common Mistakes:

• Using string state without toggling
• Setting state to same value repeatedly
• Trying to mutate state variable directly