Getting a single document in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get a single document from Firestore.

Firebase

const docRef = doc(db, 'users', [1]);
const docSnap = await getDoc(docRef);

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

A'db'

B'getDoc'

C'users'

D'userId123'

Attempts:

3 left

💡 Hint

Common Mistakes

Using the collection name instead of the document ID.

Passing the database object instead of the document ID.

You need to specify the document ID to get a single document. Here, 'userId123' is the document ID.

2fill in blank

medium

Complete the code to check if the document exists.

Firebase

if ([1].exists()) {
  console.log('Document data:', docSnap.data());
} else {
  console.log('No such document!');
}

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

AdocRef

BgetDoc

CdocSnap

Ddb

Attempts:

3 left

💡 Hint

Common Mistakes

Calling exists() on the document reference instead of the snapshot.

Using the database object instead of the snapshot.

The exists() method is called on the document snapshot object, which is docSnap.

3fill in blank

hard

Fix the error in the code to correctly import the Firestore functions.

Firebase

import { [1] } from 'firebase/firestore';

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

AgetDoc

BdocGet

CfetchDoc

DgetDocument

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function names like getDocument or fetchDoc.

Misspelling the function name.

The correct function to import for getting a document is getDoc.

4fill in blank

hard

Fill both blanks to create a document reference and get the document snapshot.

Firebase

const docRef = [1](db, 'products', [2]);
const docSnap = await getDoc(docRef);

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

Adoc

BgetDoc

C'productId456'

D'products'

Attempts:

3 left

💡 Hint

Common Mistakes

Using collection name instead of document ID in the second blank.

Using getDoc instead of doc for the first blank.

Use doc to create a document reference and provide the document ID as the third argument.

5fill in blank

hard

Fill all three blanks to check if a document exists and log its data.

Firebase

if ([1].[2]()) {
  console.log('Data:', [3].data());
} else {
  console.log('No document found');
}

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

AdocSnap

BdocRef

Dexists

Attempts:

3 left

💡 Hint

Common Mistakes

Calling exists() on the document reference.

Logging data from the reference instead of the snapshot.

You check if the document snapshot docSnap exists by calling exists(), then log its data.

Practice

1. What does the getDoc() function do in Firebase when working with a single document?

easy

A. It fetches the data of a specific document from the database.

B. It creates a new document in the database.

C. It deletes a document from the database.

D. It updates the fields of a document.

Solution

1. Step 1: Understand the purpose of getDoc()

   The getDoc() function is used to retrieve data from a specific document in Firebase Firestore.

2. Step 2: Differentiate from other functions

   Functions like setDoc() create or update documents, while deleteDoc() removes them. getDoc() only reads data.

3. Final Answer:

   It fetches the data of a specific document from the database. -> Option A

4. Quick Check:

   getDoc() reads document data [OK]

Hint: Remember: getDoc() reads, setDoc() writes [OK]

Common Mistakes:

• Confusing getDoc() with setDoc() or deleteDoc()
• Thinking getDoc() creates documents
• Assuming getDoc() updates data

2. Which of the following is the correct way to get a single document from a Firestore collection named users with ID user123 in JavaScript?

easy

A. const docRef = getDoc(db, 'users', 'user123');

B. const docSnap = await getDoc('users/user123');

C. const docRef = doc(db, 'users', 'user123'); const docSnap = await getDoc(docRef);

D. const docSnap = doc(db, 'users', 'user123');

Solution

1. Step 1: Use doc() to reference the document

   The doc() function takes the database, collection name, and document ID to create a reference.

2. Step 2: Use getDoc() with the document reference

   Pass the document reference to getDoc() to fetch the document snapshot asynchronously.

3. Final Answer:

   const docRef = doc(db, 'users', 'user123'); const docSnap = await getDoc(docRef); -> Option C

4. Quick Check:

   doc() creates ref, getDoc() fetches [OK]

Hint: Always create doc ref first, then getDoc(ref) [OK]

Common Mistakes:

• Passing string path directly to getDoc()
• Using getDoc() to create doc reference
• Calling doc() without database argument

3. Given the following code snippet, what will be logged if the document with ID doc1 exists and has a field name with value Alice?

const docRef = doc(db, 'users', 'doc1');
const docSnap = await getDoc(docRef);
if (docSnap.exists()) {
  console.log(docSnap.data().name);
} else {
  console.log('No such document!');
}

medium

A. No such document!

B. Alice

C. undefined

D. Error: docSnap.data() is not a function

Solution

1. Step 1: Check if document exists

   The code uses docSnap.exists() to verify the document presence, which is true here.

2. Step 2: Access the name field from document data

   Since the document exists, docSnap.data() returns the data object, and name is Alice.

3. Final Answer:

   Alice -> Option B

4. Quick Check:

   Document exists, print name field [OK]

Hint: Check exists() before accessing data() [OK]

Common Mistakes:

• Not checking exists() before data()
• Expecting data() to return undefined if no doc
• Confusing data() with get()

4. What is wrong with the following code snippet for getting a document from Firestore?

const docSnap = await getDoc('users/user123');
if (docSnap.exists()) {
  console.log(docSnap.data());
}

medium

A. You cannot use console.log with docSnap.data().

B. Missing await keyword before getDoc().

C. docSnap.exists() is not a valid method.

D. getDoc() requires a document reference, not a string path.

Solution

1. Step 1: Understand getDoc() argument requirements

   The getDoc() function expects a document reference created by doc(), not a string path.

2. Step 2: Identify the error in argument type

   Passing a string like 'users/user123' directly causes an error because it's not a valid document reference.

3. Final Answer:

   getDoc() requires a document reference, not a string path. -> Option D

4. Quick Check:

   getDoc() needs doc() ref [OK]

Hint: Always use doc() to create reference before getDoc() [OK]

Common Mistakes:

• Passing string path directly to getDoc()
• Forgetting to create doc reference
• Assuming getDoc() accepts string paths

5. You want to fetch a user's profile document from Firestore and safely handle the case when the document does not exist. Which code snippet correctly implements this behavior?

hard

A. const docRef = doc(db, 'profiles', userId);
const docSnap = await getDoc(docRef);
if (docSnap.exists()) {
return docSnap.data();
} else {
return null;
}

B. const docSnap = await getDoc(doc(db, 'profiles', userId));
return docSnap.data();

C. const docRef = doc(db, 'profiles', userId);
const docSnap = getDoc(docRef);
if (docSnap.exists()) {
return docSnap.data();
} else {
return {};
}

D. const docSnap = await getDoc('profiles/' + userId);
if (docSnap.exists()) {
return docSnap.data();
} else {
throw new Error('Document missing');
}

Solution

1. Step 1: Create a document reference correctly

   Use doc(db, 'profiles', userId) to get the document reference.

2. Step 2: Await the asynchronous getDoc() call and check existence

   Use await getDoc(docRef) and check docSnap.exists() before accessing data.

3. Step 3: Return data if exists, else return null

   This safely handles missing documents by returning null instead of causing errors.

4. Final Answer:

   Code snippet that awaits getDoc, checks exists(), returns data or null. -> Option A

5. Quick Check:

   Check exists() before data() to avoid errors [OK]

Hint: Always check exists() before using data() [OK]

Common Mistakes:

• Not awaiting getDoc() causing unresolved promises
• Accessing data without checking exists()
• Passing string path directly to getDoc()
• Throwing error instead of handling missing doc gracefully