Data validation rules in Firebase - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to allow read access only if the user is authenticated.

Firebase

allow read: if request.auth [1] null;

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

A<

B==

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' causes access only for unauthenticated users.

Using comparison operators like '<' or '>' is invalid here.

The rule request.auth != null checks if the user is signed in.

2fill in blank

medium

Complete the code to allow write access only if the user's ID matches the document ID.

Firebase

allow write: if request.auth.uid [1] resource.id;

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

A==

B!=

C<=

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!=' allows users to write data that is not theirs.

Using '<=' or '>=' is not meaningful for string IDs.

The rule request.auth.uid == resource.id ensures users can only write their own data.

3fill in blank

hard

Fix the error in the rule to allow writes only if the new data has a 'score' field that is a number.

Firebase

allow write: if request.resource.data.score [1] int;

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

Ais

Bin

Ctypeof

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' compares the value to the string 'number', which is incorrect.

Using 'typeof' is not valid syntax in Firebase rules.

The is operator checks the data type of a field in Firebase rules.

4fill in blank

hard

Fill both blanks to allow writes only if the new data has a 'name' field that is a string and is not empty.

Firebase

allow write: if request.resource.data.name [1] string && request.resource.data.name.[2] > 0;

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

Ais

Blength

Csize

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'is' for type checking.

Using 'length' or 'count' which are not valid string properties in Firebase rules.

The is operator checks the type, and size gets the string length to ensure it's not empty.

5fill in blank

hard

Fill all three blanks to allow writes only if the user is authenticated, the 'age' field is a number, and the age is at least 18.

Firebase

allow write: if request.auth [1] null && request.resource.data.age [2] int && request.resource.data.age [3] 18;

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

A!=

Bis

C>=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' for authentication check.

Using '<' instead of '>=' for age comparison.

This rule checks that the user is signed in, the age is a number, and the age is 18 or older.

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