Custom functions in rules in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom function in Firebase security rules?

A custom function is a reusable block of code inside Firebase security rules that helps simplify and organize rule logic by grouping repeated checks or calculations.

beginner

How do you define a custom function in Firebase rules?

You define a custom function using the <code>function</code> keyword followed by the function name and parameters inside parentheses and curly braces. For example: <br><code>function isOwner(userId) { return userId == request.auth.uid; }</code>

beginner

Why use custom functions in Firebase rules?

Custom functions make rules easier to read, maintain, and avoid repeating the same logic multiple times. They help keep rules clean and reduce errors.

intermediate

Can custom functions in Firebase rules access request data?

Yes, custom functions can access request and resource objects to check authentication, data values, or other request details needed for security decisions.

intermediate

Example: What does this custom function do?<br><code>function isAdmin() { return request.auth.token.admin == true; }</code>

This function checks if the authenticated user has an admin flag set to true in their authentication token, meaning they have admin privileges.

What keyword is used to define a custom function in Firebase security rules?

A def

B function

C func

D lambda

Firebase security rules use the keyword function to define custom functions.

Can custom functions in Firebase rules help reduce repeated code?

A Yes, they make rules cleaner and reusable

B No, they increase code duplication

C Only if you use external libraries

D Only for read operations

Custom functions allow you to reuse logic, reducing repetition and making rules easier to maintain.

Which object can custom functions access to check the user's identity?

A storage.bucket

B resource.data

C database.ref

D request.auth

The request.auth object contains authentication info about the user making the request.

What is a good reason to use custom functions in Firebase rules?

A To store user data

B To slow down rule evaluation

C To make rules easier to read and maintain

D To bypass security checks

Custom functions improve readability and maintainability of security rules.

Which of these is a valid custom function call in Firebase rules?

A isOwner(resource.data.userId)

B call isOwner(resource.data.userId)

C invoke isOwner(resource.data.userId)

D run isOwner(resource.data.userId)

Custom functions are called by their name followed by parentheses with arguments, like isOwner(resource.data.userId).

Explain what custom functions are in Firebase security rules and why they are useful.

Think about how you avoid repeating yourself in code.

Describe how you would write a custom function to check if the current user is the owner of a data item.

Focus on comparing user IDs.

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