useState hook introduction in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the useState hook in React?

The useState hook lets you add state to functional components. It helps components remember information and update the UI when that information changes.

beginner

How do you initialize a state variable using useState?

You call <code>useState</code> with the initial value inside your component. It returns an array with the current state and a function to update it, like this:<br><code>const [count, setCount] = useState(0);</code>

beginner

What happens when you call the state update function returned by useState?

Calling the update function changes the state value and tells React to re-render the component. This updates the UI to show the new state.

beginner

Can you use multiple useState hooks in one component?

Yes! You can use as many useState hooks as you need to keep track of different pieces of state separately.

beginner

Why do we use array destructuring with useState?

Because useState returns an array with two items: the current state and the function to update it. Array destructuring lets us name these two items clearly in one line.

What does useState return?

A An array with the current state and a function to update it

B A single value representing the state

C A function that returns the state

D An object with state properties

useState returns an array with two elements: the current state value and a function to update that state.

How do you update the state when using useState?

A By directly changing the state variable

B By calling the update function returned by <code>useState</code>

C By reassigning the state variable

D By calling <code>setState</code> method

You must call the update function (like setCount) to change state and trigger a re-render.

Can you use useState inside class components?

A Yes, but only with extra setup

B Yes, it works the same as in functional components

C No, it only works in functional components

D No, because class components don't have state

useState is a hook designed for functional components only.

What is the initial value passed to useState used for?

A It sets the starting state value when the component first renders

B It updates the state every time the component renders

C It is ignored after the first render

D It defines the type of the state variable

The initial value sets the state when the component loads for the first time.

What is the correct way to declare a state variable called 'name' with initial value 'Alice'?

A const name = 'Alice';

B const name = useState('Alice');

C useState('Alice', name);

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

You use array destructuring to get the state and update function from useState.

Explain how the useState hook works in React functional components.

Think about how React remembers and updates information inside a function.

Describe how you would use multiple pieces of state in one React component using useState.

Imagine tracking different things like a counter and a text input separately.

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