Firestore trigger functions in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this Firestore trigger function log?

Consider this Firestore trigger function that runs when a document is created in the 'users' collection:

exports.logNewUser = functions.firestore.document('users/{userId}').onCreate((snap, context) => {  const newUser = snap.data();  console.log('New user email:', newUser.email);  return null;});

What will be logged if a new user document with {email: '[e-mail]'} is added?

Firebase

exports.logNewUser = functions.firestore.document('users/{userId}').onCreate((snap, context) => {
  const newUser = snap.data();
  console.log('New user email:', newUser.email);
  return null;
});

A Error: snap.data is not a function

B New user email: undefined

C New user email: [e-mail]

D No output, function does not run on create

💡 Hint

Remember that snap.data() returns the document data when a document is created.

The onCreate trigger receives a snapshot of the new document. Calling snap.data() returns the document's fields. The console.log prints the email field from the new user document.

📝 Syntax

intermediate

Identify the syntax error in this Firestore trigger

Which option contains the correct syntax for a Firestore onUpdate trigger on the 'orders' collection?

A functions.firestore.document('orders/{orderId}').onUpdate((change, context) => { return null; })

B functions.firestore.document('orders/{orderId}').onUpdate((change, context) => { return null; }

C functions.firestore.document('orders/{orderId}').onUpdate(change, context) => { return null; }

D functions.firestore.document('orders/{orderId}').onUpdate((change, context) => return null; )

💡 Hint

Check the arrow function syntax and parentheses.

Option A correctly uses an arrow function with parentheses around parameters and braces around the function body. Other options have missing parentheses or braces causing syntax errors.

❓ optimization

advanced

Optimize this Firestore trigger to reduce reads

This Firestore onWrite trigger reads the same document twice:

exports.updateStats = functions.firestore.document('posts/{postId}').onWrite(async (change, context) => {  const postId = context.params.postId;  const doc1 = await admin.firestore().collection('posts').doc(postId).get();  const doc2 = await admin.firestore().collection('posts').doc(postId).get();  // ... use doc1 and doc2 ...  return null;});

Which option optimizes the code to avoid duplicate reads?

Firebase

exports.updateStats = functions.firestore.document('posts/{postId}').onWrite(async (change, context) => {
  const postId = context.params.postId;
  const doc1 = await admin.firestore().collection('posts').doc(postId).get();
  const doc2 = await admin.firestore().collection('posts').doc(postId).get();
  // ... use doc1 and doc2 ...
  return null;
});

A Use a single get() call and reuse the result: const doc = await ...get(); then use doc twice.

B Replace get() with onSnapshot() to listen for changes.

C Call get() twice but cache the second call result in a variable.

D Use two separate get() calls but add a delay between them.

💡 Hint

Reading the same document twice wastes reads and time.

Option A avoids duplicate reads by calling get() once and reusing the document snapshot. Other options either do not reduce reads or add unnecessary complexity.

🔧 Debug

advanced

Why does this Firestore trigger not run?

A developer wrote this Firestore trigger:

exports.onDeleteUser = functions.firestore.document('users/{userId}').onDelete((snap, context) => {  console.log('User deleted:', snap.id);  return null;});

But it never logs anything when a user document is deleted. What is the most likely reason?

Firebase

exports.onDeleteUser = functions.firestore.document('users/{userId}').onDelete((snap, context) => {
  console.log('User deleted:', snap.id);
  return null;
});

A snap.id is undefined in onDelete triggers.

B The function is not deployed or not enabled in Firebase console.

C onDelete triggers require a different event name.

D The function must return a promise, returning null causes it to not run.

💡 Hint

Check deployment and activation status of the function.

If the function is not deployed or disabled, it will not run. snap.id is valid in onDelete triggers. The event name onDelete is correct. Returning null is allowed.

🧠 Conceptual

expert

Which Firestore trigger type can access both before and after data?

You want to write a Firestore trigger that runs when a document is updated and needs to compare the document's data before and after the update.
Which trigger type should you use?

A onWrite trigger

B onDelete trigger

C onCreate trigger

D onUpdate trigger

💡 Hint

Only some triggers provide both previous and new data snapshots.

The onUpdate trigger provides a Change object with before and after snapshots to compare data. onWrite triggers also provide this but include create and delete events, so onUpdate is more specific. onCreate and onDelete only provide one snapshot.