Array update operations (arrayUnion, arrayRemove) in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What is the final array after using arrayUnion?

You have a Firestore document with a field tags containing ["cloud", "storage"]. You run an update with arrayUnion("storage", "database"). What will be the new value of tags?

A ["cloud", "database"]

B ["database"]

C ["cloud", "storage", "database"]

D ["cloud", "storage", "storage", "database"]

💡 Hint

arrayUnion adds elements only if they are not already present.

The arrayUnion operation adds elements to the array only if they do not already exist. Since "storage" is already present, it is not duplicated. "database" is added.

❓ service_behavior

intermediate

What happens when arrayRemove removes a non-existing element?

A Firestore document has a field features with value ["auth", "hosting"]. You run an update with arrayRemove("functions"). What will be the value of features after the update?

A ["auth", "hosting"]

B []

C ["functions"]

D ["auth"]

💡 Hint

arrayRemove only removes elements if they exist.

If the element to remove does not exist in the array, the array remains unchanged.

❓ Configuration

advanced

Which update operation correctly adds multiple unique elements to an array field?

You want to update a Firestore document's permissions array to add "read" and "write" without duplicates. Which update code snippet is correct?

A update({ permissions: arrayRemove("read", "write") })

B update({ permissions: arrayUnion(["read", "write"]) })

C update({ permissions: arrayRemove(["read", "write"]) })

D update({ permissions: arrayUnion("read", "write") })

💡 Hint

arrayUnion accepts multiple arguments, not an array argument.

arrayUnion takes multiple elements as separate arguments to add them uniquely. Passing an array as a single argument adds the array as one element, not its contents.

❓ security

advanced

What is a security risk when using arrayUnion with user input?

If you use arrayUnion to add user-provided tags to a Firestore array field without validation, what risk might occur?

A arrayUnion will cause a syntax error if tags are invalid.

B Users can add malicious or inappropriate tags that persist in the database.

C Users cannot add duplicate tags due to arrayUnion restrictions.

D arrayUnion will delete existing tags accidentally.

💡 Hint

Think about what happens if you trust user input blindly.

Using arrayUnion with unchecked user input can allow users to add unwanted or harmful data to your database, which can affect data quality and security.

❓ Architecture

expert

How to efficiently maintain a unique list of active users in Firestore using array operations?

You want to keep a Firestore document field activeUsers updated with unique user IDs as users become active or inactive. Which approach best ensures uniqueness and efficient updates?

A Use arrayUnion to add user IDs when active and arrayRemove to remove when inactive.

B Replace the entire activeUsers array with a new array on every update.

C Use arrayRemove to add user IDs and arrayUnion to remove them.

D Store activeUsers as a map with user IDs as keys instead of an array.

💡 Hint

Consider how arrayUnion and arrayRemove work for adding/removing unique elements.

Using arrayUnion and arrayRemove ensures that user IDs are added or removed uniquely without duplicates or overwriting the entire array, making updates efficient and safe.