Custom functions in rules in Firebase - Step-by-Step Execution

Process Flow - Custom functions in rules

Start: Request comes in

↓

Evaluate custom function

↓

Function returns true or false

↓

If true -> Allow access

↓

If false -> Deny access

↓

End

When a request comes, the rule calls a custom function that returns true or false to decide access.

Execution Sample

Firebase

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

allow read: if isOwner();

This rule uses a custom function to check if the user owns the data before allowing read.

Process Table

Step	Action	Function Called	Function Result	Rule Decision
1	Request received	isOwner()	Evaluating	Pending
2	Check if request.auth.uid equals resource.data.ownerId	isOwner()	true	Allow read
3	Access granted	None	N/A	Allow read
4	End	None	N/A	Request complete

💡 Function returned true, so access is allowed.

Status Tracker

Variable	Start	After Step 2	Final
request.auth.uid	user123	user123	user123
resource.data.ownerId	user123	user123	user123
isOwner() result	N/A	true	true

Key Moments - 2 Insights

Why does the function isOwner() return true or false?

Because it compares the user ID from the request with the owner ID in the data, returning true if they match (see execution_table step 2).

What happens if the function returns false?

The rule denies access immediately, stopping the request (not shown in this trace but would be the opposite of step 2).

Visual Quiz - 3 Questions

Look at the execution table, what is the function result at step 2?

A false

B true

C undefined

D error

💡 Hint

Check the 'Function Result' column in execution_table row for step 2.

At which step is the access decision made?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Rule Decision' column to see when access is allowed.

If request.auth.uid was different from resource.data.ownerId, how would the function result change?

A It would still be true

B It would cause an error

C It would be false

D It would be undefined

💡 Hint

Refer to variable_tracker and execution_table step 2 logic.

Concept Snapshot

Custom functions in Firebase rules let you reuse logic.
They return true or false to allow or deny access.
Called inside rules like allow read: if isOwner();
Helps keep rules clean and easy to manage.
Function runs each request to check conditions.

Full Transcript

When a request comes to Firebase, the security rules can call custom functions to decide if access is allowed. For example, a function named isOwner checks if the user ID making the request matches the owner ID of the data. If they match, the function returns true, and the rule allows the read. If not, access is denied. This process happens step-by-step: the request arrives, the function runs, returns true or false, and then the rule either grants or denies access. Using custom functions helps keep rules organized and easier to read.

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