Updating state in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update the state using the setter function.

React

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

function increment() {
  setCount([1]);
}

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

Acount * 2

Bcount - 1

C0

Dcount + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the old count without change.

Using subtraction instead of addition.

To increase the count by one, you add 1 to the current count and pass it to setCount.

2fill in blank

medium

Complete the code to update state based on the previous state value.

React

const [likes, setLikes] = useState(0);

function addLike() {
  setLikes([1] => [1] + 1);
}

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

Aprev

Bcount

Clikes

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using the state variable directly inside the setter function.

Forgetting to return the new state value.

When updating state based on previous state, use a function with a parameter representing the previous value, commonly named prev.

3fill in blank

hard

Fix the error in the state update to correctly toggle a boolean value.

React

const [isOn, setIsOn] = useState(false);

function toggle() {
  setIsOn([1] => ![1]);
}

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

Aprev

Bstate

CisOn

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the state variable directly inside the setter function.

Not negating the previous state value.

To toggle a boolean state, use the previous state parameter (commonly prev) and return its negation.

4fill in blank

hard

Fill both blanks to update an object state property correctly.

React

const [user, setUser] = useState({ name: 'Alice', age: 25 });

function updateName(newName) {
  setUser({ ...user, [1]: [2] });
}

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

Aname

BnewName

Cage

Duser

Attempts:

3 left

💡 Hint

Common Mistakes

Overwriting the entire user object without spreading.

Using the wrong property name.

To update the name property in the user object, spread the existing user and set name to newName.

5fill in blank

hard

Fill both blanks to update a nested state object immutably.

React

const [profile, setProfile] = useState({
  user: { name: 'Bob', details: { age: 30, city: 'NY' } }
});

function updateCity(newCity) {
  setProfile({
    ...profile,
    user: {
      ...profile.user,
      details: { ...profile.user.details, [1]: [2] }
    }
  });
}

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

Acity

BnewCity

Cage

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Not spreading nested objects causing loss of data.

Updating the wrong property.

To update the nested city property, spread the outer objects and set city to newCity. The age option is a distractor here and not used in the code.

Practice

1. What is the correct way to update a state variable count using the useState hook in React?

easy

A. Call the setter function like setCount(newValue) to update the state.

B. Directly assign a new value like count = newValue.

C. Use this.setState({ count: newValue }) inside the component.

D. Update the state by returning a new value from the component function.

Solution

1. Step 1: Understand how useState works

   The useState hook returns a pair: the current state and a function to update it.

2. Step 2: Use the setter function to update state

   To change the state, you must call the setter function with the new value, e.g., setCount(newValue).

3. Final Answer:

   Call the setter function like setCount(newValue) to update the state. -> Option A

4. Quick Check:

   Update state with setter function = A [OK]

Hint: Always use the setter function from useState to update state [OK]

Common Mistakes:

• Trying to assign state variable directly
• Using class component syntax in functional components
• Returning new value instead of calling setter

2. Which of the following is the correct syntax to initialize a state variable name with value "John" using useState?

easy

A. const name = useState('John');

B. const name = useState('John')[0];

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

D. const {name, setName} = useState('John');

Solution

1. Step 1: Recall useState returns an array

   The useState hook returns an array with two elements: the state variable and the setter function.

2. Step 2: Use array destructuring to get both

   Use array destructuring syntax: const [name, setName] = useState('John'); to get both the value and setter.

3. Final Answer:

   const [name, setName] = useState('John'); -> Option C

4. Quick Check:

   useState returns [state, setter] array = B [OK]

Hint: Use array destructuring to get state and setter from useState [OK]

Common Mistakes:

• Assigning useState result directly without destructuring
• Using object destructuring instead of array
• Only taking the first element without setter

3. What will be the output of the following React component when the button is clicked once?

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  const handleClick = () => {
    setCount(count + 1);
    setCount(count + 1);
  };
  return (
    <>
      
{count}
      Increment
    </>
  );
}

medium

A. 2

B. 0

C. Error

D. 1

Solution

1. Step 1: Understand state updates are asynchronous and batched

   Both setCount(count + 1) calls use the same count value (0) because state updates are batched.

2. Step 2: Result after both calls

   Both calls set count to 1, so after clicking, count becomes 1, not 2.

3. Final Answer:

   1 -> Option D

4. Quick Check:

   Multiple setState with same value = C [OK]

Hint: Multiple setState calls with same value batch to last one [OK]

Common Mistakes:

• Assuming count increments twice immediately
• Expecting count to be 2 after two calls
• Ignoring React's batching behavior

4. Identify the error in the following React component that tries to update state on button click:

import React, { useState } from 'react';

function Toggle() {
  const [isOn, setIsOn] = useState(false);
  function handleClick() {
    isOn = !isOn;
  }
  return {isOn ? 'ON' : 'OFF'};
}

medium

A. Button missing onClick handler.

B. Directly assigning to state variable instead of using setter.

C. Using incorrect initial state value.

D. Missing parentheses in function declaration.

Solution

1. Step 1: Check how state is updated

   The code tries to assign isOn = !isOn; directly, which is not allowed for state variables.

2. Step 2: Correct way to update state

   State must be updated by calling the setter function: setIsOn(!isOn);.

3. Final Answer:

   Directly assigning to state variable instead of using setter. -> Option B

4. Quick Check:

   Update state only via setter function = D [OK]

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

Common Mistakes:

• Assigning state variable directly
• Forgetting to call setter function
• Confusing state variable with setter

5. You want to update a state object user with a new age property without losing other properties. Which is the correct way to update state using setUser?

const [user, setUser] = useState({ name: 'Alice', age: 25 });

// Update age to 26

hard

A. setUser(prevUser => ({ ...prevUser, age: 26 }));

B. setUser({ age: 26 });

C. setUser(user.age = 26);

D. setUser(prevUser => { prevUser.age = 26 });

Solution

1. Step 1: Preserve existing properties when updating state object

   To update one property without losing others, copy previous state using spread syntax.

2. Step 2: Use functional update form to get latest state

   Use setUser(prevUser => ({ ...prevUser, age: 26 })) to merge old properties with new age.

3. Final Answer:

   setUser(prevUser => ({ ...prevUser, age: 26 })); -> Option A

4. Quick Check:

   Use spread and functional update to merge state = A [OK]

Hint: Use spread operator and functional setter to update object state [OK]

Common Mistakes:

• Replacing whole object losing other properties
• Mutating state directly instead of returning new object
• Not using functional update when depending on previous state