Optimistic state updates in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a state variable for the count using React's useState.

NextJS

const [count, [1]] = useState(0);

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

AupdateCount

BcountSet

CsetCount

DchangeCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a setter name that doesn't start with 'set'.

Mixing the order of the array destructuring.

The setter function for a state variable named count is conventionally named setCount.

2fill in blank

medium

Complete the code to optimistically update the count state before the server confirms.

NextJS

function increment() {
  setCount(count [1] 1);
  // call server update here
}

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

A*

B+

C-

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using subtraction instead of addition.

Using multiplication or division which changes the value incorrectly.

To increase the count by one, use the plus operator +.

3fill in blank

hard

Fix the error in the async function to update the count optimistically and revert if the server fails.

NextJS

async function updateCount() {
  const oldCount = count;
  setCount(count + 1);
  try {
    await fetch('/api/update', { method: 'POST' });
  } catch (error) {
    setCount([1]);
  }
}

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

AoldCount

Bcount

C0

Dcount + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Reverting to the current count which is already incremented.

Setting count to zero which resets incorrectly.

If the server update fails, revert the count to the old value stored in oldCount.

4fill in blank

hard

Fill both blanks to create an optimistic update with a rollback on failure.

NextJS

async function toggle() {
  const previous = [1];
  setCount(count [2] 1);
  try {
    await fetch('/api/toggle', { method: 'POST' });
  } catch {
    setCount(previous);
  }
}

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

Acount

BsetCount

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Saving the setter function instead of the count value.

Using subtraction instead of addition for increment.

Save the current count in previous before updating. Then add 1 to count for optimistic update.

5fill in blank

hard

Fill all three blanks to implement an optimistic update with rollback and server confirmation.

NextJS

async function addItem(item) {
  const previousItems = [1];
  setItems([...items, [2]]);
  try {
    await fetch('/api/add', {
      method: 'POST',
      body: JSON.stringify({ item: [3] })
    });
  } catch {
    setItems(previousItems);
  }
}

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

Aitems

Bitem

DsetItems

Attempts:

3 left

💡 Hint

Common Mistakes

Saving the setter function instead of the items list.

Using wrong variable names for the new item.

Not sending the item in the request body.

Save the current items list in previousItems. Add the new item optimistically to the list. Send the item in the server request body.

Practice

1. What is the main purpose of optimistic state updates in Next.js applications?

easy

A. To delay UI updates until the server responds

B. To prevent any UI changes until the user refreshes the page

C. To update the UI immediately before the server confirms the change

D. To disable user interactions during server requests

Solution

1. Step 1: Understand optimistic updates concept

   Optimistic updates update the UI immediately to make the app feel faster.

2. Step 2: Compare options with concept

   Only To update the UI immediately before the server confirms the change describes updating UI before server confirmation, matching optimistic updates.

3. Final Answer:

   To update the UI immediately before the server confirms the change -> Option C

4. Quick Check:

   Optimistic update = immediate UI change [OK]

Hint: Optimistic means acting before confirmation [OK]

Common Mistakes:

• Thinking UI waits for server response
• Confusing optimistic with pessimistic updates
• Assuming UI disables interactions

2. Which of the following is the correct way to implement an optimistic update using React's useState in Next.js?

easy

A. Use setTimeout to delay state update by 2 seconds

B. Update state after server response inside then()

C. Do not update state at all until server confirms

D. Update state immediately before sending server request

Solution

1. Step 1: Recall optimistic update timing

   Optimistic updates change state immediately before server response.

2. Step 2: Match options to timing

   Update state immediately before sending server request updates state immediately before request, fitting optimistic update pattern.

3. Final Answer:

   Update state immediately before sending server request -> Option D

4. Quick Check:

   Immediate state update = optimistic update [OK]

Hint: Update state before server reply for optimism [OK]

Common Mistakes:

• Waiting for server response before updating state
• Delaying state update unnecessarily
• Not updating state at all

3. Consider this Next.js React component snippet using optimistic updates:

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

async function increment() {
  setCount(count + 1); // optimistic update
  try {
    await fetch('/api/increment', { method: 'POST' });
  } catch {
    setCount(count); // revert on failure
  }
}

What will be the displayed count if the server request fails?

medium

A. The count reverts to the original value before increment

B. The count becomes undefined

C. The count resets to 0 regardless

D. The count remains incremented by 1

Solution

1. Step 1: Analyze optimistic update and revert logic

   Count increments immediately, but on fetch failure, count resets to original value.

2. Step 2: Understand revert on failure

   The catch block sets count back to original, undoing optimistic increment.

3. Final Answer:

   The count reverts to the original value before increment -> Option A

4. Quick Check:

   Failed fetch triggers revert = original count [OK]

Hint: Revert state on error to original value [OK]

Common Mistakes:

• Assuming optimistic update stays after failure
• Thinking count resets to zero always
• Ignoring catch block revert

4. Given this code snippet for optimistic updates in Next.js:

const [items, setItems] = useState([]);

async function addItem(newItem) {
  setItems([...items, newItem]);
  try {
    await fetch('/api/add', { method: 'POST', body: JSON.stringify(newItem) });
  } catch {
    // What is missing here?
  }
}

What is the best fix to handle errors properly?

medium

A. Remove the optimistic update and update only after fetch

B. Add code to revert items state to previous value in catch

C. Ignore errors since optimistic update is enough

D. Add a console.log in catch without state change

Solution

1. Step 1: Identify missing error handling

   Optimistic update changes state immediately, but no revert on failure.

2. Step 2: Add revert logic in catch block

   To keep UI consistent, revert items to previous state if fetch fails.

3. Final Answer:

   Add code to revert items state to previous value in catch -> Option B

4. Quick Check:

   Revert state on error = correct error handling [OK]

Hint: Revert state in catch to fix errors [OK]

Common Mistakes:

• Not reverting state on failure
• Removing optimistic update unnecessarily
• Only logging errors without UI fix

5. You want to implement optimistic updates for a list of tasks in Next.js. The user can add a task, and the UI should update immediately. However, if the server rejects the addition, the UI must revert. Which approach correctly handles this scenario?

hard

A. Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update

B. Wait for server confirmation before updating tasks state to avoid inconsistencies

C. Update tasks state immediately and do nothing on failure, assuming server will eventually succeed

D. Clear the tasks list on server failure to force user refresh

Solution

1. Step 1: Understand optimistic update with revert

   Optimistic update means immediate UI change, but must revert on failure to keep UI correct.

2. Step 2: Evaluate options for correct revert handling

   Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update saves previous state and reverts on failure, matching best practice.

3. Final Answer:

   Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update -> Option A

4. Quick Check:

   Optimistic update + revert = Update the tasks state immediately, then on server failure, reset tasks to previous state saved before update [OK]

Hint: Save old state to revert if server fails [OK]

Common Mistakes:

• Not reverting UI on failure
• Waiting for server before UI update
• Clearing list unnecessarily on error