Testing rules with emulator in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the Firebase emulator for Firestore.

Firebase

firebase emulators:start --only [1]

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

Afirestore

Bfunctions

Cauth

Ddatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'functions' instead of 'firestore' to start the Firestore emulator.

Using 'auth' or 'database' which start other emulators.

The command firebase emulators:start --only firestore starts the Firestore emulator.

2fill in blank

medium

Complete the code to run security rules tests using the Firebase emulator.

Firebase

firebase emulators:exec --only [1] "npm test"

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

Afunctions

Bfirestore

Cauth

Ddatabase

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'functions' which runs cloud functions emulator instead.

Using 'auth' or 'database' which are unrelated to Firestore rules testing.

The firestore emulator is used to run security rules tests for Firestore.

3fill in blank

hard

Fix the error in the command to start the Firestore emulator with the correct port.

Firebase

firebase emulators:start --only firestore --[1] 8080

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

Aendpoint

Bhost

Caddress

Dport

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'host' or 'address' which specify network address, not port.

Using 'endpoint' which is not a valid flag.

The correct flag to specify the port is --port.

4fill in blank

hard

Fill both blanks to configure the Firestore emulator host and port in the Firebase config.

Firebase

firebase.firestore().settings({ host: '[1]', ssl: false, port: [2] });

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

Alocalhost

B127.0.0.1

C8080

D5000

Attempts:

3 left

💡 Hint

Common Mistakes

Using IP address instead of 'localhost' for host.

Using wrong port number like 5000.

Use localhost as the host and 8080 as the default Firestore emulator port.

5fill in blank

hard

Fill all three blanks to initialize the Firebase app with emulator settings for Firestore.

Firebase

const app = initializeApp({ apiKey: '[1]', projectId: '[2]' }); const db = getFirestore(app); connectFirestoreEmulator(db, '[3]', 8080);

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

Afake-api-key

Bdemo-project

Clocalhost

Dreal-api-key

Attempts:

3 left

💡 Hint

Common Mistakes

Using real API keys or project IDs in emulator setup.

Using wrong host instead of 'localhost' for emulator connection.

Use a fake API key and demo project ID for emulator testing, and connect to Firestore emulator on localhost.

Practice

1. What is the main purpose of using the Firebase emulator when testing Firestore security rules?

easy

A. To safely test rules without affecting live data

B. To deploy rules directly to production

C. To speed up Firestore queries in production

D. To backup Firestore data automatically

Solution

1. Step 1: Understand the role of the emulator

   The Firebase emulator creates a safe environment that mimics Firestore without touching live data.

2. Step 2: Identify the benefit of safe testing

   This allows developers to test security rules without risking data loss or exposure in production.

3. Final Answer:

   To safely test rules without affecting live data -> Option A

4. Quick Check:

   Emulator = Safe testing environment [OK]

Hint: Emulator means safe testing, no live data risk [OK]

Common Mistakes:

• Thinking emulator deploys rules to production
• Confusing emulator with data backup tool
• Assuming emulator speeds up production queries

2. Which command correctly starts only the Firestore emulator?

easy

A. firebase start firestore

B. firebase emulators:start --only firestore

C. firebase emulators:run firestore

D. firebase firestore:start

Solution

1. Step 1: Recall the correct Firebase CLI command

   The command to start emulators selectively uses 'firebase emulators:start --only' followed by the service name.

2. Step 2: Match the command to Firestore emulator

   For Firestore only, the command is 'firebase emulators:start --only firestore'.

3. Final Answer:

   firebase emulators:start --only firestore -> Option B

4. Quick Check:

   Correct CLI syntax = firebase emulators:start --only firestore [OK]

Hint: Use --only with service name to start specific emulator [OK]

Common Mistakes:

• Using 'firebase start firestore' which is invalid
• Confusing 'run' with 'start' in command
• Omitting '--only' flag

3. Given this Firestore security rule snippet tested in the emulator:

service cloud.firestore {
  match /databases/{database}/documents {
    match /messages/{msgId} {
      allow read: if request.auth != null;
      allow write: if false;
    }
  }
}

What will happen if an unauthenticated user tries to read a message?

medium

A. Read is denied

B. Write is allowed

C. Read is allowed

D. Write is denied

Solution

1. Step 1: Analyze the read rule condition

   The rule allows read only if request.auth is not null, meaning user must be authenticated.

2. Step 2: Check the user's authentication status

   The user is unauthenticated, so request.auth is null, failing the condition.

3. Final Answer:

   Read is denied -> Option A

4. Quick Check:

   Unauthenticated read = denied [OK]

Hint: Read allowed only if user is authenticated [OK]

Common Mistakes:

• Assuming read is allowed without authentication
• Confusing read and write permissions
• Ignoring the request.auth check

4. You wrote this rule in the emulator but all writes are failing:

allow write: if request.auth.uid == userId;

What is the likely error causing the failure?

medium

A. request.auth.uid is always null in emulator

B. Emulator does not support write rules

C. Write rules cannot use '==' operator

D. The variable 'userId' is not defined in the rule context

Solution

1. Step 1: Check the rule variable usage

   The rule compares request.auth.uid to userId, but userId is not defined or extracted from the document path or data.

2. Step 2: Understand rule variable scope

   Variables like userId must be defined in the path or request data; otherwise, the condition fails.

3. Final Answer:

   The variable 'userId' is not defined in the rule context -> Option D

4. Quick Check:

   Undefined variable causes rule failure [OK]

Hint: Ensure all variables in rules are properly defined [OK]

Common Mistakes:

• Assuming emulator nullifies request.auth.uid
• Thinking '==' is invalid in rules
• Believing emulator ignores write rules

5. You want to test that only users can write their own profile document in the emulator. The document path is /users/{userId}. Which rule correctly enforces this and can be tested in the emulator?

hard

A. allow write: if true;

B. allow write: if request.auth.uid != userId;

C. allow write: if request.auth.uid == userId;

D. allow write: if request.auth != null;

Solution

1. Step 1: Match userId from document path

   In rules, userId is a variable from the path /users/{userId} and can be used in conditions.

2. Step 2: Enforce write only if authenticated user matches userId

   The condition 'request.auth.uid == userId' ensures only the user can write their own profile.

3. Final Answer:

   allow write: if request.auth.uid == userId; -> Option C

4. Quick Check:

   User writes own doc = request.auth.uid == userId [OK]

Hint: Match auth uid to path userId for user-specific writes [OK]

Common Mistakes:

• Using '!=' instead of '==' causing wrong access
• Allowing any authenticated user to write all profiles
• Setting rule to always true, no security