Updating state in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ state_output

intermediate

What is the output after clicking the button twice?

Consider this React component that updates state on button clicks.

import React, { useState } from 'react';

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

  function handleClick() {
    setCount(count + 1);
    setCount(count + 1);
  }

  return (
    <button onClick={handleClick}>Count: {count}</button>
  );
}

What will the button display after clicking it twice?

React

import React, { useState } from 'react';

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

  function handleClick() {
    setCount(count + 1);
    setCount(count + 1);
  }

  return (
    <button onClick={handleClick}>Count: {count}</button>
  );
}

A Count: 2 after first click, Count: 4 after second click

B Count: 1 after first click, Count: 1 after second click

C Count: 1 after first click, Count: 2 after second click

D Count: 0 after first click, Count: 0 after second click

💡 Hint

Remember that React batches state updates and the count variable does not update immediately inside the same function call.

Both setCount(count + 1) calls use the same count value because state updates are asynchronous and batched. So the count increases by 1 per click, not 2.

❓ component_behavior

intermediate

Which option correctly increments state twice in one event?

How can you update React state twice in one event handler so that the state increments by 2?

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

function handleClick() {
  // update state twice here
}

React

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

function handleClick() {
  // update state twice here
}

A

setCount(prev => count + 1);
setCount(prev => count + 1);

B

setCount(count + 1);
setCount(count + 1);

C setCount(count + 2);

D

setCount(prev => prev + 1);
setCount(prev => prev + 1);

💡 Hint

Use the updater function form of setCount to get the latest state value.

Using setCount(prev => prev + 1) twice correctly increments the state twice because each updater receives the latest state value.

🔧 Debug

advanced

Why does this state update not work as expected?

Look at this React component:

function Example() {
  const [text, setText] = useState('');

  function handleChange(e) {
    setText(e.target.value);
    console.log(text);
  }

  return <input value={text} onChange={handleChange} />;
}

Why does console.log(text) always log the previous value, not the current input?

React

function Example() {
  const [text, setText] = useState('');

  function handleChange(e) {
    setText(e.target.value);
    console.log(text);
  }

  return <input value={text} onChange={handleChange} />;
}

A Because console.log is called before setText is defined.

B Because state updates are asynchronous and the logged value is stale inside the event handler.

C Because the input value is not controlled properly and does not update.

D Because e.target.value is undefined in this context.

💡 Hint

Think about when React updates state and when the console.log runs.

React state updates do not happen immediately. The text variable inside the handler still holds the old value when console.log runs.

📝 Syntax

advanced

Which code correctly updates nested state immutably?

Given this React state:

const [user, setUser] = useState({ name: 'Alice', address: { city: 'NY' } });

Which option correctly updates the city to 'LA' without mutating the original state?

React

const [user, setUser] = useState({ name: 'Alice', address: { city: 'NY' } });

A setUser({ ...user, address: { city: 'LA' } });

B user.address.city = 'LA'; setUser(user);

C setUser({ name: 'Alice', address: { city: 'LA' } });

D setUser(prev => { prev.address.city = 'LA'; return prev; });

💡 Hint

Remember to create new objects for nested state to avoid mutation.

Option A creates a new user object and a new address object with the updated city, preserving immutability.

Options B and D mutate the existing state object, which is bad practice and can cause bugs.

Option A replaces the whole user object but hardcodes the name, which is less flexible.

🧠 Conceptual

expert

What happens if you call setState with the same value as current state?

In React, if you call setState (or setCount) with the same value as the current state, what will React do?

A React will skip re-rendering the component to optimize performance.

B React will always re-render the component regardless of state value.

C React will throw an error because state cannot be set to the same value.

D React will reset the state to its initial value.

💡 Hint

Think about React's optimization to avoid unnecessary work.

React compares the new state with the current state using Object.is. If they are the same, React skips re-rendering to improve performance.

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