Data validation rules in Firebase - Mini Project: Build & Apply

Firebase Data Validation Rules

📖 Scenario: You are building a simple Firebase Realtime Database to store user profiles. Each user has a unique ID, a name, and an age. You want to make sure that the data entered is correct and safe by using Firebase data validation rules.

🎯 Goal: Create Firebase Realtime Database validation rules that allow only user profiles with a name as a non-empty string and an age as a number between 1 and 120. Also, ensure that the user ID is a valid key.

📋 What You'll Learn

Create a users node in the database

Add a validation rule to check that name is a string and not empty

Add a validation rule to check that age is a number between 1 and 120

Ensure the user ID key is a valid Firebase key

💡 Why This Matters

🌍 Real World

Firebase data validation rules protect your database from bad or malicious data by enforcing correct data formats and values.

💼 Career

Understanding Firebase rules is essential for backend and full-stack developers working with Firebase to ensure data integrity and security.

1

Create the users node structure

Create a Firebase Realtime Database rules object with a users node that allows read and write access for now.

Firebase

{
  "rules": {
    "users": {
      // Your code here
    }
  }
}

Hint

Start by allowing all reads and writes on the users node.

2

Add validation for name field

Inside the users node, add a $userId wildcard and create a name child validation rule that checks if newData.isString() and newData.val().length > 0.

Firebase

{
  "rules": {
    "users": {
      "$userId": {
        // Your code here
      },
      ".read": true,
      ".write": true
    }
  }
}

Hint

Use $userId as a wildcard for user keys and validate the name field inside it.

3

Add validation for age field

Inside the $userId node, add an age child validation rule that checks if newData.isNumber() and newData.val() >= 1 and newData.val() <= 120.

Firebase

{
  "rules": {
    "users": {
      "$userId": {
        "name": {
          ".validate": "newData.isString() && newData.val().length > 0"
        },
        "age": {
          // Your code here
        },
        ".read": true,
        ".write": true
      }
    }
  }
}

Hint

Validate that age is a number between 1 and 120 inclusive.

4

Validate the $userId key format

Add a validation rule on the $userId wildcard to ensure the key matches the regex ^[a-zA-Z0-9_-]+$ using newData.exists() and newData.key.matches().

Firebase

{
  "rules": {
    "users": {
      "$userId": {
        ".validate": "",
        "name": {
          ".validate": "newData.isString() && newData.val().length > 0"
        },
        "age": {
          ".validate": "newData.isNumber() && newData.val() >= 1 && newData.val() <= 120"
        },
        ".read": true,
        ".write": true
      }
    }
  }
}

Hint

Use newData.key.matches() to check the user ID format.

Practice

1. What is the main purpose of Firebase data validation rules?

easy

A. To ensure data is correct and secure before saving

B. To speed up database queries

C. To create user interfaces automatically

D. To backup data to another server

Solution

1. Step 1: Understand what data validation means

   Data validation checks if data meets certain conditions before saving.

2. Step 2: Identify the role of Firebase rules

   Firebase rules check data type, format, and permissions to keep data safe and clean.

3. Final Answer:

   To ensure data is correct and secure before saving -> Option A

4. Quick Check:

   Data validation = correctness and security [OK]

Hint: Validation means checking data before saving [OK]

Common Mistakes:

• Thinking rules speed up queries
• Confusing validation with UI creation
• Assuming rules backup data

2. Which of the following is the correct syntax to allow only string data for a field name in Firebase rules?

easy

A. "name": { ".validate": "newData.isString()" }

B. "name": { ".validate": "newData.isNumber()" }

C. "name": { ".validate": "newData.exists()" }

D. "name": { ".validate": "newData.val() > 0" }

Solution

1. Step 1: Identify the method to check string type

   Firebase uses newData.isString() to check if data is a string.

2. Step 2: Match the syntax with the correct method

   "name": { ".validate": "newData.isString()" } uses newData.isString(), which is correct for string validation.

3. Final Answer:

   "name": { ".validate": "newData.isString()" } -> Option A

4. Quick Check:

   Check string type = newData.isString() [OK]

Hint: Use newData.isString() to validate strings [OK]

Common Mistakes:

• Using isNumber() for strings
• Using exists() which only checks presence
• Using val() > 0 which is for numbers

3. Given this Firebase rule snippet:

{
  "rules": {
    "users": {
      "$uid": {
        ".write": "$uid === auth.uid",
        ".validate": "newData.hasChildren(['name', 'age']) && newData.child('age').isNumber() && newData.child('age').val() >= 18"
      }
    }
  }
}

What happens if a user tries to write data with age set to 16?

medium

A. Write is denied because name is missing

B. Write is allowed because age is a number

C. Write is denied because age is less than 18

D. Write is allowed regardless of age

Solution

1. Step 1: Analyze the validation condition for age

   The rule requires age to be a number and at least 18.

2. Step 2: Check the given age value

   The user tries to write age = 16, which is less than 18, so validation fails.

3. Final Answer:

   Write is denied because age is less than 18 -> Option C

4. Quick Check:

   Age must be ≥ 18 = deny if 16 [OK]

Hint: Check numeric conditions carefully in validation [OK]

Common Mistakes:

• Ignoring the minimum age check
• Assuming any number is allowed
• Confusing write and validate rules

4. You wrote this Firebase rule to allow only emails ending with '@example.com':

{
  ".validate": "newData.isString() && newData.val().matches('.*@example.com$')"
}

But users with emails like 'user@exampleacom' are still allowed. What is the error?

medium

A. matches() does not work in Firebase rules

B. The regex pattern is incorrect; it should escape the dot

C. newData.isString() is missing

D. The rule should use newData.exists() instead

Solution

1. Step 1: Review the regex pattern used

   The pattern '.*@example.com$' treats '.' as any character, not a literal dot.

2. Step 2: Correct the regex to escape the dot

   It should be '.*@example\.com$' to match the literal '.com' ending.

3. Final Answer:

   The regex pattern is incorrect; it should escape the dot -> Option B

4. Quick Check:

   Escape dot in regex = \. [OK]

Hint: Escape dots in regex with double backslash \ [OK]

Common Mistakes:

• Not escaping '.' in regex
• Thinking matches() is unsupported
• Confusing isString() with exists()

5. You want to write a Firebase validation rule for a posts node where each post must have a title (string, max 100 chars) and tags (array of strings, max 5 tags). Which rule correctly enforces this?

hard

A. "posts": { "$postid": { ".validate": "newData.hasChildren(['title', 'tags']) && newData.child('title').isString() && newData.child('title').val().length() <= 100 && newData.child('tags').numChildren() <= 5 && newData.child('tags').val().every(tag => typeof tag === 'string')" } }

B. "posts": { "$postid": { ".validate": "newData.hasChildren(['title', 'tags']) && newData.child('title').isString() && newData.child('title').val().length() <= 100 && newData.child('tags').numChildren() <= 5 && newData.child('tags').val().forEach(tag => typeof tag === 'string')" } }

C. "posts": { "$postid": { ".validate": "newData.hasChildren(['title', 'tags']) && newData.child('title').isString() && newData.child('title').val().length() <= 100 && newData.child('tags').numChildren() <= 5 && newData.child('tags').val().every(tag => tag.isString())" } }

D. "posts": { "$postid": { ".validate": "newData.hasChildren(['title', 'tags']) && newData.child('title').isString() && newData.child('title').val().length() <= 100 && newData.child('tags').numChildren() <= 5" } }

Solution

1. Step 1: Check the available Firebase validation functions

   Firebase rules do not support JavaScript array methods like every() or typeof inside rules.

2. Step 2: Identify the valid checks

   We can check if tags is an array and its length, but cannot check each tag's type in rules.

3. Final Answer:

   "posts": { "$postid": { ".validate": "newData.hasChildren(['title', 'tags']) && newData.child('title').isString() && newData.child('title').val().length() <= 100 && newData.child('tags').numChildren() <= 5" } } -> Option D

4. Quick Check:

   Use only supported rule functions, no JS methods [OK]

Hint: Firebase rules don't support JS array methods like every() [OK]

Common Mistakes:

• Using JavaScript methods inside rules
• Trying to check each array element's type
• Assuming isArray() checks element types