File metadata in Firebase - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is file metadata in Firebase Storage?

File metadata is information about a file stored in Firebase Storage, such as its size, content type, creation date, and custom tags. It helps describe and manage the file without accessing the file content.

beginner

Name two common default metadata properties stored with a file in Firebase Storage.

Two common default metadata properties are contentType (the file's MIME type like image/png) and size (the file size in bytes).

intermediate

How can you add custom metadata to a file in Firebase Storage?

You can add custom metadata by including a customMetadata object when uploading or updating the file. For example, you can add tags or descriptions as key-value pairs.

beginner

What Firebase Storage method would you use to retrieve metadata of a stored file?

You use the getMetadata() method on a file reference to retrieve its metadata.

beginner

Why is managing file metadata useful in cloud storage?

Managing file metadata helps organize files, control access, track file details, and improve app performance by avoiding downloading files just to get info.

Which property in Firebase Storage metadata tells you the file type?

A contentType

B size

C creationTime

D customMetadata

The contentType property specifies the MIME type of the file, like image/jpeg or text/plain.

How do you update custom metadata for an existing file in Firebase Storage?

A Use getMetadata() method

B Delete and re-upload the file

C Use the updateMetadata() method

D Modify the file content directly

The updateMetadata() method allows you to change metadata without re-uploading the file.

What does the size property in file metadata represent?

A File size in bytes

B Number of downloads

C File creation date

D File owner ID

The size property shows the file size in bytes.

Which of these is NOT a default metadata property in Firebase Storage?

A contentType

B downloadCount

C size

D customMetadata

downloadCount is not a default metadata property in Firebase Storage.

Why might you use custom metadata in Firebase Storage?

A To delete the file

B To change the file content

C To increase file size

D To store extra info like tags or descriptions

Custom metadata lets you add extra information about the file without changing its content.

Explain what file metadata is in Firebase Storage and why it is important.

Think about how metadata helps describe a file without opening it.

Describe how you would add and update custom metadata for a file in Firebase Storage.

Consider the difference between uploading a new file and updating metadata on an existing file.

Practice

1. What is the main purpose of file metadata in Firebase Storage?

easy

A. To store extra information about files like type and custom tags

B. To store the actual file content

C. To manage user authentication

D. To create database records

Solution

1. Step 1: Understand what metadata means

   Metadata is data about data, so it holds extra info about files, not the files themselves.

2. Step 2: Identify the role of metadata in Firebase Storage

   Firebase Storage uses metadata to store info like file type and custom tags to help manage files better.

3. Final Answer:

   To store extra information about files like type and custom tags -> Option A

4. Quick Check:

   File metadata = extra info about files [OK]

Hint: Metadata holds info about files, not the files themselves [OK]

Common Mistakes:

• Confusing metadata with file content
• Thinking metadata manages users
• Assuming metadata creates database records

2. Which of the following is the correct way to set custom metadata for a file in Firebase Storage using JavaScript?

easy

A. storageRef.updateMetadata({ customMetadata: { owner: 'user1' } })

B. storageRef.setMetadata({ owner: 'user1' })

C. storageRef.metadata = { owner: 'user1' }

D. storageRef.uploadMetadata({ customMetadata: { owner: 'user1' } })

Solution

1. Step 1: Recall Firebase Storage method for updating metadata

   The correct method to update metadata is updateMetadata(), not setMetadata or uploadMetadata.

2. Step 2: Check the structure for custom metadata

   Custom metadata must be inside a customMetadata object within the metadata parameter.

3. Final Answer:

   storageRef.updateMetadata({ customMetadata: { owner: 'user1' } }) -> Option A

4. Quick Check:

   Use updateMetadata() with customMetadata object [OK]

Hint: Use updateMetadata() with customMetadata key [OK]

Common Mistakes:

• Using setMetadata() which does not exist
• Assigning metadata directly to storageRef
• Using uploadMetadata() which is invalid

3. Given the following code snippet, what will be logged to the console?

const metadata = await storageRef.getMetadata();
console.log(metadata.contentType);

medium

A. The file's download URL

B. The file size in bytes

C. The MIME type of the file, e.g., 'image/png'

D. An error because getMetadata() returns undefined

Solution

1. Step 1: Understand what getMetadata() returns

   getMetadata() returns an object with file metadata including contentType, size, and more.

2. Step 2: Identify what metadata.contentType holds

   contentType holds the MIME type of the file, such as 'image/png' or 'application/pdf'.

3. Final Answer:

   The MIME type of the file, e.g., 'image/png' -> Option C

4. Quick Check:

   metadata.contentType = MIME type [OK]

Hint: contentType in metadata shows file type [OK]

Common Mistakes:

• Thinking contentType is file size
• Expecting download URL from metadata
• Assuming getMetadata() returns undefined

4. You try to update file metadata with this code but get an error:

await storageRef.updateMetadata({ owner: 'user1' });

What is the likely cause?

medium

A. updateMetadata() cannot update metadata

B. storageRef is not defined

C. owner is not a valid metadata field

D. Missing the customMetadata wrapper around owner

Solution

1. Step 1: Check the structure required for custom metadata

   Custom metadata must be inside a customMetadata object, not directly as a field.

2. Step 2: Identify the error cause

   Passing { owner: 'user1' } directly causes an error because owner is not a top-level metadata field.

3. Final Answer:

   Missing the customMetadata wrapper around owner -> Option D

4. Quick Check:

   Custom metadata needs customMetadata key [OK]

Hint: Wrap custom fields inside customMetadata object [OK]

Common Mistakes:

• Passing custom fields directly without customMetadata
• Thinking updateMetadata() can't update metadata
• Ignoring storageRef definition errors

5. You want to tag files uploaded by users with their user ID and file category. Which is the best way to store this info in Firebase Storage metadata?

hard

A. Add user ID and category as top-level metadata fields

B. Use customMetadata with keys 'userId' and 'category' inside updateMetadata()

C. Save user ID and category in Firestore only, not in metadata

D. Store user ID and category in the file name

Solution

1. Step 1: Understand how to store extra info in Firebase Storage

   Firebase Storage supports customMetadata to store extra info like tags.

2. Step 2: Evaluate options for tagging files

   Using customMetadata keys 'userId' and 'category' inside updateMetadata() is the best practice for tagging files.

3. Step 3: Why other options are less ideal

   File names can get messy, Firestore is separate, and top-level metadata fields do not support custom keys.

4. Final Answer:

   Use customMetadata with keys 'userId' and 'category' inside updateMetadata() -> Option B

5. Quick Check:

   Tag files using customMetadata keys [OK]

Hint: Use customMetadata keys for tagging files [OK]

Common Mistakes:

• Putting tags in file names instead of metadata
• Relying only on Firestore for file tags
• Trying to add custom fields as top-level metadata