Multiple file uploads in Express - Performance & Optimization

Performance: Multiple file uploads

MEDIUM IMPACT

This affects the server response time and client-side rendering speed during file upload processes.

Uploading multiple files from a client to an Express server

Express

app.post('/upload', async (req, res) => {
  const files = req.files;
  await Promise.all(files.map(file => fs.promises.writeFile(`uploads/${file.name}`, file.data)));
  res.send('Files uploaded');
});

Asynchronous parallel file writes prevent blocking, improving server responsiveness.

📈 Performance GainNon-blocking parallel writes reduce INP and server response delay.

Uploading multiple files from a client to an Express server

Express

app.post('/upload', (req, res) => {
  const files = req.files;
  files.forEach(file => {
    // Synchronously save each file one by one
    fs.writeFileSync(`uploads/${file.name}`, file.data);
  });
  res.send('Files uploaded');
});

Synchronous file writes block the event loop, causing slow response and poor input responsiveness.

📉 Performance CostBlocks server event loop during each file write, increasing INP and response time.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Synchronous file writes in upload handler	N/A	N/A	Blocks server response causing delayed client paint	[X] Bad
Asynchronous parallel file writes with Promise.all	N/A	N/A	Non-blocking server response enables faster client paint	[OK] Good

Rendering Pipeline

Multiple file uploads impact the server's ability to respond quickly, which affects the client's interaction to next paint (INP). Blocking operations delay server responses, causing slower UI updates.

→Server Processing

→Network Transfer

→Client Rendering

⚠️ BottleneckServer Processing due to synchronous/blocking file operations

Core Web Vital Affected

INP

This affects the server response time and client-side rendering speed during file upload processes.

Optimization Tips

1Avoid synchronous file operations in upload handlers to prevent blocking.

2Use asynchronous parallel file writes to improve server responsiveness.

3Monitor server response times in DevTools Network tab to detect blocking.

Performance Quiz - 3 Questions

What is the main performance problem with synchronous file writes during multiple file uploads in Express?

A They cause layout shifts on the client side.

B They block the server event loop, delaying responses.

C They increase the bundle size of the client code.

D They reduce the number of files that can be uploaded.

DevTools: Network

How to check: Open DevTools, go to Network tab, upload multiple files, and observe the request duration and timing waterfall.

What to look for: Look for long blocking times or delayed server response indicating synchronous blocking; shorter, parallel requests indicate good performance.

Practice

1. In Express, which method from multer is used to handle multiple file uploads at once?

easy

A. upload.array()

B. upload.single()

C. upload.fields()

D. upload.none()

Solution

1. Step 1: Understand multer upload methods

   Multer provides different methods: single() for one file, array() for multiple files, and fields() for multiple fields.

2. Step 2: Identify method for multiple files

   To upload multiple files under the same field name, upload.array() is used.

3. Final Answer:

   upload.array() -> Option A

4. Quick Check:

   Multiple files = upload.array() [OK]

Hint: Multiple files use upload.array() method in multer [OK]

Common Mistakes:

• Using upload.single() for multiple files
• Confusing upload.fields() with upload.array()
• Not specifying max count in upload.array()

2. Which of the following is the correct syntax to accept up to 5 files named 'photos' using multer in Express?

easy

A. upload.array('photos', 5)

B. upload.single('photos', 5)

C. upload.fields([{ name: 'photos', maxCount: 5 }])

D. upload.none('photos', 5)

Solution

1. Step 1: Recall multer syntax for multiple files

   The method upload.array(fieldname, maxCount) accepts multiple files with the same field name and limits the count.

2. Step 2: Match syntax to question

   To accept up to 5 files named 'photos', upload.array('photos', 5) is correct.

3. Final Answer:

   upload.array('photos', 5) -> Option A

4. Quick Check:

   Max 5 files = upload.array('photos', 5) [OK]

Hint: Use upload.array('fieldname', maxCount) for multiple files [OK]

Common Mistakes:

• Using upload.single() with max count
• Confusing upload.fields() syntax
• Using upload.none() incorrectly

3. Given this Express route using multer:

app.post('/upload', upload.array('docs', 3), (req, res) => {
  res.send(req.files.length);
});

What will be the response if a user uploads 2 files named 'docs'?

medium

A. 3

B. Error

C. 1

D. 2

Solution

1. Step 1: Understand upload.array behavior

   The middleware upload.array('docs', 3) allows up to 3 files. If user uploads 2 files, both are accepted.

2. Step 2: Check response code

   The handler sends req.files.length, which is the number of uploaded files, here 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Uploaded files count = 2 [OK]

Hint: req.files.length shows how many files were uploaded [OK]

Common Mistakes:

• Assuming maxCount is always returned
• Confusing req.file with req.files
• Expecting error if fewer than maxCount files uploaded

4. What is wrong with this Express route for multiple file uploads?

app.post('/upload', upload.array('files'), (req, res) => {
  console.log(req.file);
  res.send('Uploaded');
});

medium

A. upload.array() cannot be used for multiple files

B. upload.array() requires a maxCount parameter

C. Should use req.files instead of req.file to access multiple files

D. res.send() must send JSON, not string

Solution

1. Step 1: Check multer usage for multiple files

   When using upload.array(), uploaded files are stored in req.files (an array), not req.file.

2. Step 2: Identify error in code

   The code logs req.file, which will be undefined or incorrect for multiple files.

3. Final Answer:

   Should use req.files instead of req.file to access multiple files -> Option C

4. Quick Check:

   Multiple files = req.files [OK]

Hint: Use req.files (plural) for multiple files, not req.file [OK]

Common Mistakes:

• Forgetting req.files vs req.file difference
• Assuming maxCount is mandatory
• Thinking upload.array() is invalid

5. You want to accept multiple files from two different fields: 'images' (max 3 files) and 'documents' (max 2 files). Which multer setup correctly handles this?

hard

A. upload.array('images', 3), upload.array('documents', 2)

B. upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }])

C. upload.single('images'), upload.single('documents')

D. upload.none()

Solution

1. Step 1: Understand multer methods for multiple fields

   To handle multiple fields with multiple files each, use upload.fields() with an array of objects specifying field names and max counts.

2. Step 2: Analyze options

   upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }]) correctly uses upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }]). upload.array('images', 3), upload.array('documents', 2) is invalid syntax as you cannot chain two upload.array() calls in one route.

3. Final Answer:

   upload.fields([{ name: 'images', maxCount: 3 }, { name: 'documents', maxCount: 2 }]) -> Option B

4. Quick Check:

   Multiple fields = upload.fields() [OK]

Hint: Use upload.fields() for multiple fields with multiple files [OK]

Common Mistakes:

• Trying to chain multiple upload.array() calls
• Using upload.single() for multiple files
• Using upload.none() which disables file uploads