Getting a single document in Firebase - Mini Project: Build & Apply

Getting a Single Document from Firebase Firestore

📖 Scenario: You are building a simple app that needs to fetch user profile information stored in Firebase Firestore. You want to get the data of a single user by their unique ID.

🎯 Goal: Learn how to write code that retrieves a single document from a Firestore collection using the document ID.

📋 What You'll Learn

Create a Firestore collection reference

Set a variable with the document ID to fetch

Write the code to get the document snapshot

Access the document data safely

💡 Why This Matters

🌍 Real World

Fetching a single user profile or settings document from Firestore is common in apps that personalize content or manage user data.

💼 Career

Understanding how to retrieve single documents from Firestore is essential for frontend and backend developers working with Firebase to build real-time, scalable applications.

1

Create a Firestore collection reference

Write a line of code to create a Firestore collection reference called usersRef that points to the collection named users using firebase.firestore().collection('users').

Firebase

// Create a Firestore collection reference called usersRef
// Your code here

Hint

Use firebase.firestore().collection('users') to get the collection reference.

2

Set the document ID to fetch

Create a constant variable called userId and set it to the string 'user123', which is the ID of the user document you want to get.

Firebase

const usersRef = firebase.firestore().collection('users');
// Create a constant userId with value 'user123'
// Your code here

Hint

Use const userId = 'user123'; to store the document ID.

3

Write the code to get the document snapshot

Write an asynchronous function called getUserData that uses usersRef.doc(userId).get() to get the document snapshot and stores it in a variable called doc.

Firebase

const usersRef = firebase.firestore().collection('users');
const userId = 'user123';

// Write async function getUserData to get document snapshot
// Your code here

Hint

Use await usersRef.doc(userId).get() inside an async function to get the document.

4

Access the document data safely

Inside the getUserData function, add code to check if doc.exists is true. If yes, create a variable called userData and set it to doc.data(). This will hold the user's data.

Firebase

const usersRef = firebase.firestore().collection('users');
const userId = 'user123';

async function getUserData() {
    const doc = await usersRef.doc(userId).get();
    // Check if document exists and get data
    // Your code here
}

Hint

Use if (doc.exists) to check if the document was found, then use doc.data() to get its contents.

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