Multiple file uploads in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What middleware is commonly used in Express to handle multiple file uploads?

Multer is the popular middleware used in Express to handle multiple file uploads easily.

beginner

How do you specify multiple files upload field in Multer?

Use the upload.array('fieldname', maxCount) method where 'fieldname' is the name of the input field and maxCount is the max number of files allowed.

beginner

What property of the request object holds the array of uploaded files when using upload.array()?

The req.files property contains an array of uploaded file objects when using upload.array().

intermediate

Why is it important to set limits on file size and number of files in multiple uploads?

Setting limits prevents server overload, protects from denial of service attacks, and controls storage usage.

intermediate

How can you handle errors during multiple file uploads in Express with Multer?

You can use error-handling middleware or check for errors in the route callback to respond properly if upload fails or files exceed limits.

Which Multer method is used to upload multiple files from a single input field?

A upload.array()

B upload.single()

C upload.fields()

D upload.none()

upload.array() handles multiple files from one input field. upload.single() is for one file. upload.fields() handles multiple fields.

Where are the uploaded files stored temporarily by Multer before you move or process them?

A In <code>req.body</code>

B In <code>req.file</code>

C In <code>req.files</code> array

D In <code>req.params</code>

For multiple files, Multer stores them in req.files as an array of file objects.

What is a good practice to avoid server overload when allowing multiple file uploads?

A Disable file uploads

B Set limits on file size and number of files

C Allow unlimited files and sizes

D Only accept text files

Setting limits helps protect the server from overload and abuse.

Which property contains the original name of the uploaded file in Multer's file object?

A originalname

B filename

C path

D size

The originalname property holds the file name from the user's computer.

How do you handle errors from Multer middleware in Express routes?

A Restart the server

B Use a try-catch block around the upload middleware

C Ignore errors and continue

D Use error-handling middleware or check error in callback

Errors from Multer can be caught in error-handling middleware or by checking the error argument in the route callback.

Explain how to set up multiple file uploads in an Express app using Multer.

Think about middleware setup and how files are accessed in the request.

Describe why setting limits on file size and number of files is important in multiple file uploads.

Consider server safety and resource management.

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