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);
});

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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);
});

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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);
});

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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);
});

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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.