Storing files on disk vs memory in Express - Practice Questions

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding file storage behavior in Express with multer

Consider an Express app using multer middleware to handle file uploads. If multer is configured with storage: multer.memoryStorage(), what happens to the uploaded file data?

Express

const multer = require('multer');
const upload = multer({ storage: multer.memoryStorage() });

app.post('/upload', upload.single('file'), (req, res) => {
  console.log(req.file.buffer);
  res.send('File received');
});

A The file is streamed directly to a cloud storage service without local storage.

B The file is saved directly to disk in a temporary folder and req.file.path contains the file path.

C The file is stored temporarily in RAM as a Buffer accessible via req.file.buffer.

D The file is discarded immediately and not accessible in the request handler.

💡 Hint

Think about where memoryStorage keeps the file data.

When using multer.memoryStorage(), multer stores the uploaded file data in RAM as a Buffer. This buffer is accessible in req.file.buffer. No file is saved on disk.

❓ component_behavior

intermediate

Behavior of multer.diskStorage in Express

When multer is configured with storage: multer.diskStorage(), what is true about the uploaded files?

Express

const multer = require('multer');
const storage = multer.diskStorage({
  destination: (req, file, cb) => cb(null, './uploads'),
  filename: (req, file, cb) => cb(null, file.originalname)
});
const upload = multer({ storage });

app.post('/upload', upload.single('file'), (req, res) => {
  console.log(req.file.path);
  res.send('File saved to disk');
});

A The file is converted to a base64 string and stored in req.file.base64.

B The file is stored in memory and req.file.buffer contains the data.

C The file is uploaded but not saved anywhere automatically.

D The file is saved to the specified disk folder and req.file.path contains the file path.

💡 Hint

Disk storage means saving files on your server's hard drive.

Using multer.diskStorage() saves the uploaded file physically on disk in the specified folder. The file path is available in req.file.path.

📝 Syntax

advanced

Identifying error in multer memoryStorage usage

Which option will cause a runtime error when trying to access the uploaded file's data in an Express route using multer with memoryStorage?

Express

const multer = require('multer');
const upload = multer({ storage: multer.memoryStorage() });

app.post('/upload', upload.single('file'), (req, res) => {
  // Access file data here
});

A console.log(req.file.path.length);

B console.log(req.file.buffer.toString('utf8'));

C console.log(req.file.size);

D console.log(req.file.mimetype);

💡 Hint

memoryStorage does not save files on disk, so some properties are missing.

When using memoryStorage, req.file.path is undefined because the file is not saved on disk. Accessing it causes a runtime error.

❓ state_output

advanced

Comparing memory usage between diskStorage and memoryStorage

In an Express app handling large file uploads, what is a key difference in server memory usage between multer's diskStorage and memoryStorage?

A Both use the same amount of RAM because multer buffers files in memory regardless of storage type.

B memoryStorage uses more RAM because files are kept in memory; diskStorage uses less RAM but uses disk space.

C diskStorage uses more RAM because it buffers files before saving; memoryStorage writes directly to disk.

D memoryStorage uses less RAM because it streams files directly to disk.

💡 Hint

Think about where the file data lives during upload.

memoryStorage keeps the entire file in RAM, increasing memory usage. diskStorage writes files to disk, reducing RAM usage but using disk space.

🔧 Debug

expert

Debugging file upload failure with multer diskStorage

An Express app uses multer.diskStorage to save uploaded files to './uploads'. However, uploads fail silently and no files appear in the folder. What is the most likely cause?

A The './uploads' folder does not exist or lacks write permissions.

B The Express app is missing the body-parser middleware.

C The file input field name in the HTML form does not match the multer field name.

D The multer middleware is missing the .single() or .array() method call.

💡 Hint

Check if the folder exists and can be written to by the server.

If the destination folder does not exist or the server lacks permission to write there, multer cannot save files, causing silent failures.