Firestore trigger functions in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a Firestore trigger that runs when a document is created.

Firebase

exports.onUserCreate = functions.firestore.document('users/{userId}').[1]((snap, context) => {
  const newValue = snap.data();
  console.log('User created:', newValue);
});

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

AonDelete

BonUpdate

ConWrite

DonCreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using onUpdate instead of onCreate will trigger only on document changes, not creation.

Using onDelete triggers when a document is deleted, not created.

The onCreate trigger runs only when a new document is created in Firestore.

2fill in blank

medium

Complete the code to access the document ID inside a Firestore trigger function.

Firebase

exports.logDocId = functions.firestore.document('posts/{postId}').onWrite((change, context) => {
  const docId = context.params.[1];
  console.log('Document ID:', docId);
});

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

AdocId

BdocumentId

CpostId

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong parameter name that does not match the wildcard in the path.

Trying to get the ID from the change object instead of context.params.

The context.params object contains the wildcard values from the document path. Here, postId matches the wildcard in the path.

3fill in blank

hard

Fix the error in the Firestore trigger to correctly get the new data after an update.

Firebase

exports.onPostUpdate = functions.firestore.document('posts/{postId}').onUpdate((change, context) => {
  const newData = change.[1].data();
  console.log('Updated data:', newData);
});

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

Abefore

Bafter

Cprevious

Dnew

Attempts:

3 left

💡 Hint

Common Mistakes

Using change.before.data() returns the old data before the update.

Using change.new.data() is not a valid property.

In an onUpdate trigger, change.after contains the new document snapshot after the update.

4fill in blank

hard

Fill both blanks to create a Firestore trigger that runs on document deletion and logs the deleted data.

Firebase

exports.onDeletePost = functions.firestore.document('posts/{postId}').[1]((snap, [2]) => {
  const deletedData = snap.data();
  console.log('Deleted post:', deletedData);
});

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

AonDelete

Bcontext

Cevent

DonRemove

Attempts:

3 left

💡 Hint

Common Mistakes

Using onRemove instead of onDelete is incorrect for Firestore triggers.

Naming the second parameter something other than context or event is allowed but context is standard.

The onDelete trigger runs when a document is deleted. The second parameter is usually called context to access event info.

5fill in blank

hard

Fill all three blanks to create a Firestore trigger that runs on any write and logs old and new data.

Firebase

exports.onAnyWrite = functions.firestore.document('items/{itemId}').[1]((change, [2]) => {
  const oldData = change.[3].data();
  const newData = change.after.data();
  console.log('Old data:', oldData);
  console.log('New data:', newData);
});

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

AonWrite

Bcontext

Cbefore

DonUpdate

Attempts:

3 left

💡 Hint

Common Mistakes

Using onUpdate instead of onWrite limits the trigger to updates only.

Trying to get old data from change.after instead of change.before.

onWrite triggers on create, update, or delete. change.before is the old data snapshot. The second parameter is the event context.

Practice

1. What does a Firestore trigger function do in a Firebase project?

easy

A. Deletes Firestore data without any event

B. Manually updates Firestore data when called

C. Creates a new Firestore database

D. Automatically runs code when data changes in Firestore

Solution

1. Step 1: Understand Firestore triggers

   Firestore triggers are special functions that run automatically when data changes in Firestore.

2. Step 2: Identify the trigger's purpose

   They help keep your app reactive by running code on create, update, or delete events without manual calls.

3. Final Answer:

   Automatically runs code when data changes in Firestore -> Option D

4. Quick Check:

   Firestore triggers = automatic code run on data change [OK]

Hint: Triggers run code automatically on Firestore data changes [OK]

Common Mistakes:

• Thinking triggers run manually
• Confusing triggers with database creation
• Assuming triggers delete data without events

2. Which of the following is the correct syntax to create a Firestore trigger for document creation in Node.js Firebase Functions?

easy

A. exports.onCreate = functions.firestore.document('users').onCreate((snap, context) => { });

B. exports.onCreate = functions.firestore.document('users/{userId}').onCreate((snap, context) => { });

C. functions.firestore.onCreate('users/{userId}', (snap, context) => { });

D. functions.firestore.document('users/{userId}').create((snap, context) => { });

Solution

1. Step 1: Recall Firestore trigger syntax

   The correct syntax uses exports, functions.firestore.document(path).onCreate(handler).

2. Step 2: Match syntax to options

   exports.onCreate = functions.firestore.document('users/{userId}').onCreate((snap, context) => { }); matches this pattern exactly with document path and onCreate event.

3. Final Answer:

   exports.onCreate = functions.firestore.document('users/{userId}').onCreate((snap, context) => { }); -> Option B

4. Quick Check:

   Correct Firestore onCreate syntax = exports.onCreate = functions.firestore.document('users/{userId}').onCreate((snap, context) => { }); [OK]

Hint: Use exports with document(path).onCreate(handler) for create triggers [OK]

Common Mistakes:

• Missing exports keyword
• Using onCreate directly on firestore without document
• Wrong method name like create instead of onCreate

3. Given this Firestore trigger function, what will be logged when a document is updated?

exports.logUpdate = functions.firestore.document('posts/{postId}').onUpdate((change, context) => {
  const before = change.before.data();
  const after = change.after.data();
  console.log('Title before:', before.title);
  console.log('Title after:', after.title);
});

medium

A. Logs the title before and after the update

B. Logs only the new title after update

C. Logs only the old title before update

D. Throws an error because change.before is undefined

Solution

1. Step 1: Understand onUpdate trigger parameters

   The onUpdate trigger receives a change object with before and after snapshots of the document.

2. Step 2: Analyze the console logs

   The code logs before.title and after.title, so both old and new titles are printed.

3. Final Answer:

   Logs the title before and after the update -> Option A

4. Quick Check:

   onUpdate logs before and after data [OK]

Hint: onUpdate provides before and after data snapshots [OK]

Common Mistakes:

• Assuming only new data is available
• Thinking change.before is undefined
• Confusing onUpdate with onCreate

4. Identify the error in this Firestore trigger function code:

exports.deleteUser = functions.firestore.document('users/{userId}').onDelete((snap, context) => {
  const userData = snap.data;
  console.log('Deleted user:', userData.name);
});

medium

A. Missing async keyword for the function

B. onDelete trigger cannot be used on 'users/{userId}' path

C. snap.data should be snap.data() to get document data

D. context parameter is not allowed in onDelete triggers

Solution

1. Step 1: Check how to access document data

   Firestore document snapshot data is accessed by calling snap.data(), not snap.data property.

2. Step 2: Identify the error in code

   The code uses snap.data without parentheses, causing undefined userData and runtime error.

3. Final Answer:

   snap.data should be snap.data() to get document data -> Option C

4. Quick Check:

   Use snap.data() method to access document data [OK]

Hint: Use snap.data() method, not snap.data property [OK]

Common Mistakes:

• Using snap.data instead of snap.data()
• Thinking onDelete can't use context parameter
• Assuming async is always required

5. You want to create a Firestore trigger that updates a user's profile summary whenever their profile document changes. Which approach correctly ensures the trigger only runs when the 'bio' field changes, avoiding infinite loops?

hard

A. Use onUpdate trigger and compare change.before.data().bio with change.after.data().bio before updating

B. Use onWrite trigger and update the document unconditionally inside the trigger

C. Use onCreate trigger to update bio field after document creation

D. Use onUpdate trigger and update the document without checking any field changes

Solution

1. Step 1: Choose the right trigger for updates

   onUpdate triggers run on any document update, suitable for detecting changes.

2. Step 2: Prevent infinite loops by checking field changes

   Compare bio field before and after update; only proceed if bio changed to avoid recursive updates.

3. Final Answer:

   Use onUpdate trigger and compare change.before.data().bio with change.after.data().bio before updating -> Option A

4. Quick Check:

   Check field changes in onUpdate to avoid loops [OK]

Hint: Compare before and after fields in onUpdate to avoid loops [OK]

Common Mistakes:

• Using onWrite and updating unconditionally causing loops
• Using onCreate which triggers only once
• Not checking field changes before updating