Deleting files in Firebase - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ service_behavior

intermediate

What happens when you delete a file in Firebase Storage?

You delete a file from Firebase Storage using the SDK. What is the immediate effect on the file's availability?

Firebase

const storageRef = firebase.storage().ref('images/photo.jpg');
storageRef.delete().then(() => {
  console.log('File deleted');
});

A The file is immediately removed and cannot be accessed anymore.

B The file is marked for deletion but remains accessible for 24 hours.

C The file is moved to a recycle bin and can be restored within 7 days.

D The file remains accessible until the Firebase project is deleted.

💡 Hint

Think about how cloud storage handles file deletion requests.

When you delete a file in Firebase Storage, it is immediately removed and cannot be accessed anymore. There is no recycle bin or delay.

❓ security

intermediate

Which Firebase Storage rule prevents unauthorized file deletion?

Given these Firebase Storage security rules, which one correctly prevents users from deleting files they do not own?

Firebase

rules_version = '2';
service firebase.storage {
  match /b/{bucket}/o {
    match /user_files/{userId}/{allPaths=**} {
      allow read, write: if request.auth != null && request.auth.uid == userId;
    }
  }
}

A allow write: if request.auth != null && request.auth.uid == userId;

B allow delete: if request.auth.uid == userId;

C allow write: if request.auth != null;

D allow delete: if request.auth != null;

💡 Hint

Consider which rule covers both upload and deletion permissions.

The 'write' permission includes delete operations. The rule allowing write only if the user is authenticated and owns the file prevents unauthorized deletions.

❓ Configuration

advanced

What is the correct way to delete multiple files in Firebase Storage efficiently?

You need to delete 100 files stored under 'user_uploads/'. Which approach is best to delete all files efficiently?

A Call delete() on each file reference sequentially in a for loop.

B Use Promise.all() to call delete() on all file references concurrently.

C Overwrite each file with an empty file to simulate deletion.

D Delete the entire bucket to remove all files at once.

💡 Hint

Think about how to handle many asynchronous operations efficiently.

Using Promise.all() runs all delete operations concurrently, making the process faster than sequential deletes. Deleting the entire bucket is not allowed, and overwriting files does not delete them.

❓ Architecture

advanced

How to design a system to securely delete user files after account deletion?

You want to ensure all user files are deleted from Firebase Storage when a user deletes their account. What is the best architectural approach?

A Set Firebase Storage rules to automatically delete files when the user account is deleted.

B Require users to manually delete their files before deleting their account.

C Trigger a Cloud Function on user deletion to delete all files under their folder.

D Schedule a daily batch job to delete files of deleted users.

💡 Hint

Consider automation and event-driven design.

Using a Cloud Function triggered by user deletion events allows automatic and immediate cleanup of user files. Storage rules cannot delete files automatically, and manual deletion is error-prone.

🧠 Conceptual

expert

What error occurs if you try to delete a non-existent file in Firebase Storage?

You attempt to delete a file that does not exist in Firebase Storage using the SDK. What error will the SDK return?

Firebase

const storageRef = firebase.storage().ref('nonexistent/file.txt');
storageRef.delete().catch(error => {
  console.log(error.code);
});

A 'storage/canceled'

B 'storage/quota-exceeded'

C 'storage/unauthorized'

D 'storage/object-not-found'

💡 Hint

Think about the error code for missing files in Firebase Storage.

When deleting a file that does not exist, Firebase Storage returns the 'storage/object-not-found' error code.

Practice

1. What does the deleteObject function do in Firebase Storage?

easy

A. It creates a new folder in Firebase Storage.

B. It uploads a new file to Firebase Storage.

C. It reads the contents of a file in Firebase Storage.

D. It deletes a file from Firebase Storage.

Solution

1. Step 1: Understand the purpose of deleteObject

   The deleteObject function is designed to remove files from Firebase Storage.

2. Step 2: Compare with other functions

   Uploading files uses uploadBytes, reading uses getDownloadURL, and folders are virtual, not created by deleteObject.

3. Final Answer:

   It deletes a file from Firebase Storage. -> Option D

4. Quick Check:

   deleteObject removes files = A [OK]

Hint: Remember: deleteObject means remove file [OK]

Common Mistakes:

• Confusing deleteObject with upload functions
• Thinking deleteObject creates folders
• Assuming deleteObject reads file content

2. Which of the following is the correct syntax to delete a file named image.png from Firebase Storage using JavaScript?

easy

A. storage.delete('image.png');

B. await deleteFile('image.png');

C. await deleteObject(ref(storage, 'image.png'));

D. deleteObject('image.png');

Solution

1. Step 1: Identify correct function and parameters

   The deleteObject function requires a reference to the file, created by ref(storage, 'filename').

2. Step 2: Check syntax correctness

   await deleteObject(ref(storage, 'image.png')); correctly uses await deleteObject(ref(storage, 'image.png'));. Other options use incorrect functions or missing references.

3. Final Answer:

   await deleteObject(ref(storage, 'image.png')); -> Option C

4. Quick Check:

   Use ref() with deleteObject = C [OK]

Hint: Use ref() to locate file before deleteObject [OK]

Common Mistakes:

• Calling deleteObject without ref()
• Using non-existent deleteFile function
• Omitting await for async deleteObject

3. Consider this code snippet:

const fileRef = ref(storage, 'docs/report.pdf');
try {
  await deleteObject(fileRef);
  console.log('File deleted');
} catch (error) {
  console.log('Error:', error.message);
}

What will be printed if the file docs/report.pdf does not exist?

medium

A. 'File deleted'

B. 'Error: File not found'

C. No output

D. SyntaxError

Solution

1. Step 1: Understand deleteObject behavior on missing file

   If the file does not exist, deleteObject throws an error caught by the catch block.

2. Step 2: Check catch block output

   The catch block logs the error message, which typically indicates the file was not found.

3. Final Answer:

   'Error: File not found' -> Option B

4. Quick Check:

   Missing file causes error message = B [OK]

Hint: Missing file triggers catch error message [OK]

Common Mistakes:

• Assuming deleteObject silently succeeds if file missing
• Expecting no output on error
• Confusing error message with syntax error

4. Identify the error in this code snippet that tries to delete a file:

const fileRef = ref('storage', 'photos/pic.jpg');
deleteObject(fileRef).then(() => {
  console.log('Deleted');
}).catch(error => {
  console.error(error);
});

medium

A. The ref function is called with wrong arguments.

B. deleteObject should not use promises.

C. Missing await keyword before deleteObject.

D. The file path should not include folders.

Solution

1. Step 1: Check ref function usage

   The ref function requires the storage instance as the first argument, not the string 'storage'.

2. Step 2: Validate other parts

   Using promises with then and catch is valid. Await is optional here. File paths can include folders.

3. Final Answer:

   The ref function is called with wrong arguments. -> Option A

4. Quick Check:

   ref() needs storage instance, not string = A [OK]

Hint: ref() needs storage object, not string 'storage' [OK]

Common Mistakes:

• Passing string 'storage' instead of storage object
• Thinking await is mandatory with promises
• Believing file paths cannot have folders

5. You want to delete multiple files in Firebase Storage: file1.txt, file2.txt, and file3.txt. Which approach correctly deletes all files and logs 'All deleted' only after all deletions succeed?

hard

A. Use Promise.all with deleteObject calls for each file reference.

B. Call deleteObject sequentially without waiting for promises.

C. Use a for loop with deleteObject but no await or Promise.all.

D. Delete only the first file; others delete automatically.

Solution

1. Step 1: Understand asynchronous deletion

   Each deleteObject returns a promise. To wait for all deletions, use Promise.all.

2. Step 2: Check other options

   Sequential calls without waiting may not complete before logging. For loop without await or Promise.all won't wait. Files do not delete automatically.

3. Final Answer:

   Use Promise.all with deleteObject calls for each file reference. -> Option A

4. Quick Check:

   Promise.all waits for all deletes = D [OK]

Hint: Use Promise.all to wait for multiple deletes [OK]

Common Mistakes:

• Not waiting for all delete promises
• Assuming deleting one file deletes others
• Ignoring asynchronous nature of deleteObject