Nested objects and arrays in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a nested object to Firestore.

Firebase

db.collection('users').doc('user1').set({ profile: [1] })

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

Anull

B['Alice', 30]

C'Alice'

D{ name: 'Alice', age: 30 }

Attempts:

3 left

💡 Hint

Common Mistakes

Using an array instead of an object.

Using a string instead of an object.

The nested object must be an object with keys and values, not an array or string.

2fill in blank

medium

Complete the code to add an array of strings inside a Firestore document.

Firebase

db.collection('tasks').doc('task1').set({ tags: [1] })

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

A['urgent', 'home']

Burgent, home

C'urgent, home'

D{ 'urgent', 'home' }

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces {} which create objects, not arrays.

Using a string instead of an array.

Arrays in Firestore are represented as JavaScript arrays with square brackets [].

3fill in blank

hard

Fix the error in the nested object assignment to Firestore.

Firebase

db.collection('orders').doc('order1').set({ details: [1] })

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

A{ item: 'book', qty: 2 }

Bitem: 'book', qty: 2

C'{ item: book, qty: 2 }'

D[{ item: 'book', qty: 2 }]

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string instead of an object.

Using an array when an object is expected.

The nested object must be an object literal, not a string or incomplete syntax.

4fill in blank

hard

Fill both blanks to create a nested object with an array inside Firestore.

Firebase

db.collection('projects').doc('proj1').set({ info: [1], tags: [2] })

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

A{ name: 'Build Shed', status: 'open' }

B['open', 'urgent']

C{ name: 'Build Shed', status: 'closed' }

D['done', 'low']

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping object and array types.

Using strings instead of arrays or objects.

The 'info' field is an object, and 'tags' is an array of strings.

5fill in blank

hard

Fill all three blanks to create a Firestore document with nested objects and arrays.

Firebase

db.collection('events').doc('event1').set({ organizer: [1], attendees: [2], details: [3] })

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

A{ name: 'John Doe', contact: '[e-mail]' }

B['Alice', 'Bob', 'Charlie']

C{ date: '2024-07-01', location: 'Park' }

D['2024-07-01', 'Park']

Attempts:

3 left

💡 Hint

Common Mistakes

Using arrays where objects are needed.

Using strings instead of arrays or objects.

The 'organizer' and 'details' fields are objects, while 'attendees' is an array of strings.

Practice

1. What is the main purpose of using nested objects in Firebase documents?

easy

A. To create separate databases within Firebase

B. To group related data inside one document for better organization

C. To store multiple documents in a collection

D. To encrypt data automatically

Solution

1. Step 1: Understand nested objects concept

   Nested objects allow grouping related data inside a single document, making it easier to manage.

2. Step 2: Compare with other options

   Options A, B, and D describe different Firebase features unrelated to nested objects.

3. Final Answer:

   To group related data inside one document for better organization -> Option B

4. Quick Check:

   Nested objects = group related data [OK]

Hint: Nested objects group related data inside one document [OK]

Common Mistakes:

• Confusing nested objects with collections
• Thinking nested objects create separate databases
• Assuming nested objects encrypt data

2. Which of the following is the correct way to define an array inside a Firebase document?

easy

A. "tags": ["firebase", "cloud"]

B. "tags": {"0": "firebase", "1": "cloud"}

C. "tags": "firebase, cloud"

D. "tags": {"firebase", "cloud"}

Solution

1. Step 1: Identify correct array syntax in Firebase

   Arrays in Firebase are defined using square brackets with comma-separated values.

2. Step 2: Evaluate each option

   "tags": ["firebase", "cloud"] uses square brackets correctly. Options A and D use curly braces which define objects, not arrays. "tags": "firebase, cloud" is a string, not an array.

3. Final Answer:

   "tags": ["firebase", "cloud"] -> Option A

4. Quick Check:

   Arrays use square brackets [] [OK]

Hint: Arrays use square brackets [] in Firebase [OK]

Common Mistakes:

• Using curly braces {} instead of square brackets []
• Writing arrays as comma-separated strings
• Confusing objects with arrays

3. Given this Firebase document snippet:

{"user": {"name": "Anna", "roles": ["admin", "editor"]}}

What will be the value of user.roles[1]?

medium

A. "admin"

B. undefined

C. "editor"

D. "user"

Solution

1. Step 1: Understand array indexing in Firebase data

   Arrays start at index 0, so roles[0] is "admin" and roles[1] is "editor".

2. Step 2: Identify the value at index 1

   The second element (index 1) in the roles array is "editor".

3. Final Answer:

   "editor" -> Option C

4. Quick Check:

   Index 1 in roles array = "editor" [OK]

Hint: Array indexes start at 0; roles[1] is second item [OK]

Common Mistakes:

• Assuming array indexes start at 1
• Confusing object keys with array indexes
• Thinking roles[1] is undefined

4. You have this Firebase document:

{"profile": {"contacts": {"email": "[e-mail]", "phones": ["123-456", "789-012"]}}}

Which code snippet correctly accesses the second phone number?

medium

A. profile[contacts].phones[1]

B. profile.contacts.phones[2]

C. profile.contacts[phones][1]

D. profile.contacts.phones[1]

Solution

1. Step 1: Understand correct dot and bracket notation

   Access nested objects with dots and arrays with brackets. Index 1 is second item.

2. Step 2: Evaluate each option

   profile.contacts.phones[1] uses correct dot notation and index 1. profile.contacts.phones[2] uses index 2 which is out of range. Options C and D misuse bracket notation for keys.

3. Final Answer:

   profile.contacts.phones[1] -> Option D

4. Quick Check:

   Use dot for keys, brackets for array index [OK]

Hint: Use dot notation for keys, brackets for array index [OK]

Common Mistakes:

• Using wrong array index (2 instead of 1)
• Using brackets incorrectly for object keys
• Mixing quotes and brackets wrongly

5. You want to store a list of users where each user has a name and a list of favorite colors. Which Firebase structure correctly uses nested objects and arrays?

hard

A. {"users": [{"name": "Tom", "favorites": ["red", "blue"]}, {"name": "Sue", "favorites": ["green"]}]}

B. {"users": {"Tom": "red, blue", "Sue": "green"}}

C. {"users": [{"name": "Tom", "favorites": "red, blue"}, {"name": "Sue", "favorites": "green"}]}

D. {"users": ["Tom", "Sue"], "favorites": ["red", "blue", "green"]}

Solution

1. Step 1: Identify correct nested structure for users and their favorite colors

   Each user should be an object with a name and an array of favorite colors.

2. Step 2: Evaluate each option

   {"users": [{"name": "Tom", "favorites": ["red", "blue"]}, {"name": "Sue", "favorites": ["green"]}]} correctly nests user objects with arrays for favorites. {"users": {"Tom": "red, blue", "Sue": "green"}} uses a string instead of array. {"users": [{"name": "Tom", "favorites": "red, blue"}, {"name": "Sue", "favorites": "green"}]} uses strings for favorites, not arrays. {"users": ["Tom", "Sue"], "favorites": ["red", "blue", "green"]} separates users and favorites incorrectly.

3. Final Answer:

   {"users": [{"name": "Tom", "favorites": ["red", "blue"]}, {"name": "Sue", "favorites": ["green"]}]} -> Option A

4. Quick Check:

   Nested objects with arrays for lists = {"users": [{"name": "Tom", "favorites": ["red", "blue"]}, {"name": "Sue", "favorites": ["green"]}]} [OK]

Hint: Use objects for users, arrays for lists inside them [OK]

Common Mistakes:

• Using strings instead of arrays for lists
• Separating users and favorites incorrectly
• Not nesting favorites inside each user object