Custom functions in rules in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a custom function named isOwner.

Firebase

function [1](userId) {
  return request.auth.uid == userId;
}

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

AcheckUser

BisOwner

CvalidateUser

DownerCheck

Attempts:

3 left

💡 Hint

Common Mistakes

Using a function name that does not match the rule call.

Misspelling the function name.

The function name must be isOwner to match the rule usage.

2fill in blank

medium

Complete the code to call the custom function isOwner inside the rule.

Firebase

allow read: if [1](resource.data.ownerId);

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

AisOwner

BcheckUser

CvalidateUser

DownerCheck

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a function name that was not defined.

Forgetting to pass the ownerId parameter.

The rule calls the custom function isOwner to check ownership.

3fill in blank

hard

Fix the error in the custom function to correctly check if the user is authenticated.

Firebase

function isOwner(userId) {
  return [1] != null && request.auth.uid == userId;
}

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

Arequest.auth.uid

Brequest.auth.token

Crequest.auth

Drequest.auth.id

Attempts:

3 left

💡 Hint

Common Mistakes

Checking only request.auth.uid without verifying auth exists.

Using incorrect property names like request.auth.id.

Checking request.auth != null ensures the user is signed in before comparing user IDs.

4fill in blank

hard

Fill both blanks to define a custom function that checks if the user is an admin or the owner.

Firebase

function [1](userId) {
  return request.auth != null && (request.auth.token.admin == true || request.auth.uid == [2]);
}

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

AisAdminOrOwner

BuserId

CownerId

DisOwner

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect function names that do not describe the check.

Using wrong variable names inside the function.

The function isAdminOrOwner checks if the user is an admin or matches the userId parameter.

5fill in blank

hard

Fill the blanks to write a rule that allows write if the user is admin or owner using the custom function.

Firebase

allow write: if [1](resource.data.[2]);

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

AisAdminOrOwner

BownerId

Cadmin

DisOwner

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong function name in the rule.

Using incorrect data field names.

The rule calls isAdminOrOwner(resource.data.ownerId).

Practice

1. What is the main benefit of using custom functions in Firebase security rules?

easy

A. They make rules easier to read and reuse common checks

B. They automatically fix security vulnerabilities

C. They increase the speed of database queries

D. They replace the need for authentication

Solution

1. Step 1: Understand the purpose of custom functions

   Custom functions in Firebase rules are designed to simplify and organize security logic by reusing common checks.

2. Step 2: Compare options to the purpose

   Only They make rules easier to read and reuse common checks correctly states that custom functions make rules easier to read and reuse common checks. Other options describe unrelated or incorrect benefits.

3. Final Answer:

   They make rules easier to read and reuse common checks -> Option A

4. Quick Check:

   Custom functions improve readability and reuse = A [OK]

Hint: Custom functions simplify and reuse checks in rules [OK]

Common Mistakes:

• Thinking custom functions speed up queries
• Believing they fix security automatically
• Confusing them with authentication features

2. Which of the following is the correct syntax to define a custom function named isOwner in Firebase rules?

easy

A. function isOwner() = request.auth.uid == resource.data.owner;

B. isOwner() => request.auth.uid == resource.data.owner;

C. function isOwner() { return request.auth.uid == resource.data.owner; }

D. isOwner() { return request.auth.uid == resource.data.owner; }

Solution

1. Step 1: Recall Firebase rules function syntax

   Firebase rules use the syntax: function name() { return expression; } to define custom functions.

2. Step 2: Identify the correct syntax

   function isOwner() { return request.auth.uid == resource.data.owner; } matches the correct syntax. isOwner() { return request.auth.uid == resource.data.owner; } omits the required function keyword. Options B and C use invalid syntax.

3. Final Answer:

   function isOwner() { return request.auth.uid == resource.data.owner; } -> Option C

4. Quick Check:

   Firebase custom function syntax uses function name() { return ... } = A [OK]

Hint: Firebase rules functions use function name() { return ... } syntax [OK]

Common Mistakes:

• Omitting the 'function' keyword
• Using arrow functions which are not supported
• Using '=' instead of function body block

3. Given the following Firebase rule snippet:

function isAdmin() { return request.auth.token.admin == true; }
allow write: if isAdmin();

What happens if request.auth.token.admin is false?

medium

A. Write access is denied

B. Write access is granted

C. Rule throws a syntax error

D. Write access is granted only if user is authenticated

Solution

1. Step 1: Understand the function logic

   The function isAdmin() returns true only if request.auth.token.admin is true.

2. Step 2: Evaluate the rule condition

   The write permission is allowed only if isAdmin() returns true. If request.auth.token.admin is false, the function returns false, so write is denied.

3. Final Answer:

   Write access is denied -> Option A

4. Quick Check:

   isAdmin false means no write access = A [OK]

Hint: Function returns false blocks access; true allows it [OK]

Common Mistakes:

• Assuming false admin still grants access
• Thinking rule throws error on false
• Confusing authentication with admin check

4. Identify the error in this Firebase rule custom function:

function isOwner() {
  request.auth.uid == resource.data.owner;
}
allow read: if isOwner();

medium

A. Incorrect function name syntax

B. Invalid comparison operator

C. Using 'allow' outside match block

D. Missing return statement inside the function

Solution

1. Step 1: Check function body for return

   The function compares values but does not return the result, so it returns null by default.

2. Step 2: Understand impact on rule

   Without a return, isOwner() returns null (falsey), so the rule will deny access unexpectedly.

3. Final Answer:

   Missing return statement inside the function -> Option D

4. Quick Check:

   Functions must return values explicitly = B [OK]

Hint: Always return a value in custom functions [OK]

Common Mistakes:

• Forgetting to add 'return' before expression
• Assuming last line auto-returns value
• Misplacing 'allow' statements

5. You want to create a reusable custom function canEdit() that allows editing if the user is either the owner or has an admin role. Which of the following is the correct way to write this function in Firebase rules?

hard

A. canEdit() { if (request.auth.uid == resource.data.owner) return true; else return false; }

B. function canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin == true; }

C. canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin == true; }

D. canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin; }

Solution

1. Step 1: Understand the logic needed

   The function should return true if the user is owner OR has admin role.

2. Step 2: Analyze each option

   function canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin == true; } correctly uses function keyword, logical OR (||) and explicit admin comparison. canEdit() { if (request.auth.uid == resource.data.owner) return true; else return false; } only checks owner. canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin == true; } and canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin; } omit required function keyword.

3. Final Answer:

   function canEdit() { return request.auth.uid == resource.data.owner || request.auth.token.admin == true; } -> Option B

4. Quick Check:

   function keyword + OR + admin == true = B [OK]

Hint: Use function keyword, || and explicit admin == true check [OK]

Common Mistakes:

• Using AND instead of OR for conditions
• Not explicitly comparing admin to true
• Ignoring admin role in logic