Optimistic updates pattern in NextJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update the UI immediately after a user action.

NextJS

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

function addItem(newItem) {
  setItems([1]);
  // send update to server
}

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

A[...items, newItem]

Bitems.push(newItem)

Citems.concat()

Ditems.pop()

Attempts:

3 left

💡 Hint

Common Mistakes

Using push() which mutates the array and does not update state correctly.

Using pop() which removes items instead of adding.

We use [...items, newItem] to create a new array including the new item, updating state immediately.

2fill in blank

medium

Complete the code to rollback the UI update if the server request fails.

NextJS

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

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

A[newItem]

Bitems

C[]

DpreviousItems

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to reset to an empty array instead of previous state.

Using current items which already include the new item.

If the server request fails, we restore the previous state saved in previousItems.

3fill in blank

hard

Fix the error in the optimistic update pattern by completing the missing part.

NextJS

function handleDelete(id) {
  const previousItems = items;
  setItems(items.filter(item => item.id !== [1]));
  // send delete request
}

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

Aid

Bitem

Citems

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'item' which is the whole object, not the id.

Using 'items' which is the array, not a single id.

We compare each item's id to the given id to filter out the deleted item.

4fill in blank

hard

Fill both blanks to optimistically update and rollback a list after editing an item.

NextJS

async function editItem(updatedItem) {
  const previousItems = items;
  setItems(items.map(item => item.id === [1] ? [2] : item));
  try {
    await fetch('/api/edit', { method: 'POST', body: JSON.stringify(updatedItem) });
  } catch {
    setItems(previousItems);
  }
}

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

AupdatedItem.id

BupdatedItem

Citem.id

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variables for comparison or replacement.

Not replacing the item correctly in the map.

We check if item.id matches updatedItem.id and replace it with updatedItem for optimistic update.

5fill in blank

hard

Fill all three blanks to implement optimistic update with React Query's mutation.

NextJS

const mutation = useMutation({
  mutationFn: newItem => fetch('/api/add', { method: 'POST', body: JSON.stringify(newItem) }),
  onMutate: async newItem => {
    await queryClient.cancelQueries('items');
    const previousItems = queryClient.getQueryData('items');
    queryClient.setQueryData('items', old => [...old, [1]]);
    return { previousItems };
  },
  onError: (err, newItem, context) => {
    queryClient.setQueryData('items', [2]);
  },
  onSettled: () => {
    queryClient.invalidateQueries([3]);
  }
});

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

AnewItem

Bcontext.previousItems

C'items'

D'newItem'

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong keys or variables in queryClient methods.

Not returning previous state for rollback.

We add newItem optimistically, rollback to context.previousItems on error, and invalidate the 'items' query after mutation settles.

Practice

1. What is the main idea behind the optimistic updates pattern in Next.js?

easy

A. Update the UI immediately before server confirmation to improve user experience

B. Wait for server response before updating the UI to ensure accuracy

C. Reload the entire page after every data change to keep UI fresh

D. Disable user input until the server confirms the update

Solution

1. Step 1: Understand the purpose of optimistic updates

   Optimistic updates aim to make the app feel faster by showing changes immediately.

2. Step 2: Compare UI update timing

   Instead of waiting for the server, the UI updates first, then confirms or reverts based on server response.

3. Final Answer:

   Update the UI immediately before server confirmation to improve user experience -> Option A

4. Quick Check:

   Optimistic updates = Immediate UI update [OK]

Hint: UI updates first, server confirms later [OK]

Common Mistakes:

• Waiting for server before UI update
• Reloading entire page unnecessarily
• Disabling user input during update

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

easy

A. Use useEffect to update state only after server confirms

B. Send API request first, then call setState after response

C. Call setState to update UI, then send API request, revert on error

D. Reload the page after API call to update UI

Solution

1. Step 1: Identify optimistic update flow

   Optimistic update means updating UI state immediately with setState.

2. Step 2: Confirm API call and handle errors

   Send API request after UI update, and revert state if the request fails.

3. Final Answer:

   Call setState to update UI, then send API request, revert on error -> Option C

4. Quick Check:

   Update state first, then API call [OK]

Hint: Update state first, then call API [OK]

Common Mistakes:

• Waiting for API before updating state
• Using useEffect incorrectly for optimistic update
• Reloading page instead of updating state

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

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

async function handleLike() {
  setLikes(likes + 1);
  try {
    await fetch('/api/like', { method: 'POST' });
  } catch {
    setLikes(likes); // revert on error
  }
}

What will the UI show if the API call fails?

medium

A. The likes count stays incremented by 1

B. The likes count increments by 2

C. The likes count resets to zero

D. The likes count reverts to the original value

Solution

1. Step 1: Analyze optimistic update behavior

   The UI increments likes immediately by 1 using setLikes(likes + 1).

2. Step 2: Check error handling

   If the API call fails, setLikes(likes) resets likes to the original value before increment.

3. Final Answer:

   The likes count reverts to the original value -> Option D

4. Quick Check:

   API error triggers revert to old likes [OK]

Hint: Revert state on API failure [OK]

Common Mistakes:

• Assuming UI stays incremented after failure
• Resetting likes to zero incorrectly
• Incrementing likes twice by mistake

4. You wrote this optimistic update code in Next.js but the UI never reverts on API failure:

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

async function increment() {
  setCount(count + 1);
  try {
    await fetch('/api/increment', { method: 'POST' });
  } catch {
    setCount(count - 1);
  }
}

What is the bug causing the revert to fail?

medium

A. Using stale count value inside catch block

B. Not awaiting the setCount call

C. Missing return statement after setCount

D. API call method should be GET, not POST

Solution

1. Step 1: Understand state closure issue

   The count variable inside catch is the old value before increment.

2. Step 2: Explain why revert fails

   Subtracting 1 from stale count does not revert to original because count was already incremented in UI.

3. Final Answer:

   Using stale count value inside catch block -> Option A

4. Quick Check:

   State closure causes revert failure [OK]

Hint: Avoid stale state in async error handlers [OK]

Common Mistakes:

• Expecting setState to be async
• Ignoring stale closure of state variable
• Wrong HTTP method for API call

5. You want to implement optimistic updates for a comment submission in Next.js. Which approach best handles UI update, server confirmation, and error rollback?

hard

A. Reload the page after comment submission to show new comment

B. Add comment to UI state immediately, send API request, remove comment if API fails

C. Add comment to UI state only after API confirms success

D. Send API request first, then add comment to UI state after success

Solution

1. Step 1: Apply optimistic update pattern

   Update UI immediately by adding comment to state to improve responsiveness.

2. Step 2: Handle server confirmation and errors

   Send API request; if it fails, remove the comment from UI to keep data consistent.

3. Final Answer:

   Add comment to UI state immediately, send API request, remove comment if API fails -> Option B

4. Quick Check:

   UI update first, revert on error [OK]

Hint: Add UI item first, remove on failure [OK]

Common Mistakes:

• Waiting for API before UI update
• Adding comment only after success
• Reloading page instead of updating state