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.

Practice

1. What does the arrayUnion operation do in Firebase Firestore?

easy

A. Removes all elements from an array field

B. Adds unique elements to an array field without duplicates

C. Replaces the entire array with a new one

D. Sorts the array elements in ascending order

Solution

1. Step 1: Understand arrayUnion purpose

   The arrayUnion operation adds elements to an array field only if they are not already present, ensuring no duplicates.

2. Step 2: Compare with other options

   The other options describe removing all elements, replacing the array, or sorting, which are not what arrayUnion does.

3. Final Answer:

   Adds unique elements to an array field without duplicates -> Option B

4. Quick Check:

   arrayUnion adds unique items without duplicates [OK]

Hint: arrayUnion only adds new unique items to arrays [OK]

Common Mistakes:

• Thinking arrayUnion removes items
• Confusing arrayUnion with replacing arrays
• Assuming arrayUnion sorts the array

2. Which of the following is the correct syntax to add an element 'blue' to an array field 'colors' using arrayUnion in Firestore?

easy

A. docRef.update({ colors: arrayUnion('blue') })

B. docRef.update({ colors: arrayRemove('blue') })

C. docRef.set({ colors: arrayUnion('blue') })

D. docRef.add({ colors: arrayUnion('blue') })

Solution

1. Step 1: Identify correct method for updating fields

   To update an existing document's field, update() is used, not set() or add().

2. Step 2: Use arrayUnion to add element

   The syntax arrayUnion('blue') correctly adds 'blue' to the array without duplicates.

3. Final Answer:

   docRef.update({ colors: arrayUnion('blue') }) -> Option A

4. Quick Check:

   Use update() with arrayUnion() to add items [OK]

Hint: Use update() with arrayUnion() to add array items [OK]

Common Mistakes:

• Using arrayRemove instead of arrayUnion
• Using set() which overwrites the whole document
• Using add() which is for collections, not fields

3. Given the Firestore document field tags: ['news', 'tech'], what will be the value of tags after this update?

docRef.update({ tags: arrayUnion('tech', 'science') })

medium

A. ['news', 'tech', 'science']

B. ['news', 'science']

C. ['tech', 'science']

D. ['news', 'tech', 'tech', 'science']

Solution

1. Step 1: Understand arrayUnion behavior with existing elements

   arrayUnion adds only new unique elements. 'tech' already exists, so it won't be duplicated.

2. Step 2: Add new element 'science'

   'science' is new, so it will be added to the array.

3. Final Answer:

   ['news', 'tech', 'science'] -> Option A

4. Quick Check:

   arrayUnion adds only new unique items [OK]

Hint: arrayUnion skips duplicates, adds only new items [OK]

Common Mistakes:

• Expecting duplicates to be added
• Thinking arrayUnion removes items
• Confusing arrayUnion with arrayRemove

4. You want to remove the string 'urgent' from the array field 'labels' in a Firestore document. Which code snippet contains an error?

medium

A. docRef.update({ labels: arrayRemove('urgent', 'important') })

B. docRef.update({ labels: arrayRemove(['urgent']) })

C. docRef.update({ labels: arrayRemove() })

D. docRef.update({ labels: arrayRemove('urgent') })

Solution

1. Step 1: Check valid usage of arrayRemove

   arrayRemove requires at least one argument specifying which elements to remove.

2. Step 2: Analyze each option

   docRef.update({ labels: arrayRemove() }) calls arrayRemove with no arguments, which is invalid and causes an error.
   docRef.update({ labels: arrayRemove(['urgent']) }) attempts to remove the array ['urgent'] (won't match/remove string 'urgent').
   docRef.update({ labels: arrayRemove('urgent', 'important') }) removes both if present.
   docRef.update({ labels: arrayRemove('urgent') }) correctly removes 'urgent'.

3. Final Answer:

   docRef.update({ labels: arrayRemove() }) -> Option C

4. Quick Check:

   arrayRemove needs at least one element argument [OK]

Hint: arrayRemove must have at least one element argument [OK]

Common Mistakes:

• Calling arrayRemove with no arguments
• Passing an array inside arrayRemove instead of elements
• Confusing arrayRemove with arrayUnion

5. You have a Firestore document with an array field participants: ['alice', 'bob', 'carol']. You want to update it so that 'bob' is removed and 'dave' is added, ensuring no duplicates. Which update code achieves this?

hard

A. docRef.update({ participants: arrayRemove('bob'), participants: arrayUnion('dave') })

B. docRef.update({ participants: arrayRemove('bob', 'dave') })

C. docRef.update({ participants: arrayUnion('dave') })

D. docRef.update({ participants: arrayRemove('bob') }); docRef.update({ participants: arrayUnion('dave') })

Solution

1. Step 1: Understand Firestore update limitations

   You cannot apply both arrayRemove and arrayUnion to the same field in a single update object (duplicate keys overwrite, only last applies).

2. Step 2: Analyze options

   docRef.update({ participants: arrayRemove('bob'), participants: arrayUnion('dave') }): duplicate keys, only arrayUnion('dave') applies (adds 'dave', keeps 'bob').
   docRef.update({ participants: arrayRemove('bob', 'dave') }): removes 'bob' ('dave' ignored since absent), no add.
   docRef.update({ participants: arrayUnion('dave') }): adds 'dave', no remove.
   docRef.update({ participants: arrayRemove('bob') }); docRef.update({ participants: arrayUnion('dave') }): first removes 'bob', then adds 'dave' -> ['alice', 'carol', 'dave'].

3. Final Answer:

   docRef.update({ participants: arrayRemove('bob') }); docRef.update({ participants: arrayUnion('dave') }) -> Option D

4. Quick Check:

   Repeated keys overwrite; combine operations carefully [OK]

Hint: Firestore update keys must be unique; combine operations carefully [OK]

Common Mistakes:

• Using duplicate keys in update object
• Expecting multiple updates in one call with repeated keys
• Not using transactions for complex updates