Custom functions in rules in Firebase - Mini Project: Build & Apply

Custom Functions in Firebase Security Rules

📖 Scenario: You are building a simple Firebase app where users can read and write their own profile data. To keep the data safe, you want to use Firebase security rules with custom functions to check if a user is allowed to read or write their own data.

🎯 Goal: Build Firebase security rules that use a custom function isOwner() to check if the user is accessing their own profile document. The rules should allow read and write only if the user is the owner.

📋 What You'll Learn

Create a custom function called isOwner() that checks if request.auth.uid matches the document ID

Use the isOwner() function in the allow read, write rules

Apply the rules to the profiles/{userId} document path

Ensure the rules deny access if the user is not authenticated or not the owner

💡 Why This Matters

🌍 Real World

Firebase security rules protect your app's data by controlling who can read or write it. Custom functions help keep rules clean and reusable.

💼 Career

Understanding Firebase security rules and custom functions is essential for backend and full-stack developers working with Firebase to build secure apps.

1

Create the base rules structure

Write the Firebase security rules starting with rules_version = '2'; and service cloud.firestore block. Inside, create a match block for /databases/{database}/documents/profiles/{userId}.

Firebase

rules_version = '2';
service cloud.firestore {
  # Your code here
}

Hint

Start by defining the rules version and the Firestore service. Then add a match block for the profiles collection documents.

2

Add the custom function isOwner()

Inside the match block, define a custom function called isOwner() that returns request.auth != null && request.auth.uid == userId.

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents/profiles/{userId} {
    // Define isOwner function here
    // Your code here
  }
}

Hint

Define a function inside the match block that checks if the user is authenticated and their UID matches the document ID.

3

Use isOwner() in read and write rules

Add allow read, write: if isOwner(); inside the match block to allow access only if the user is the owner.

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents/profiles/{userId} {
    function isOwner() {
      return request.auth != null && request.auth.uid == userId;
    }
    // Add allow read, write rule here
    // Your code here
  }
}

Hint

Use the custom function in the allow statement to restrict access.

4

Complete the rules with proper closing braces

Ensure the rules end with the correct closing braces for the match and service blocks.

Firebase

rules_version = '2';
service cloud.firestore {
  match /databases/{database}/documents/profiles/{userId} {
    function isOwner() {
      return request.auth != null && request.auth.uid == userId;
    }
    allow read, write: if isOwner();
  }
  # Close service block
  # Your code here
}

Hint

Make sure all blocks are properly closed with braces.

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