res.download for file downloads in Express - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does res.download() do in Express?

res.download() sends a file to the client and prompts them to download it. It sets the right headers so the browser knows it's a file to save.

beginner

What are the main parameters of res.download()?

The main parameters are: path (file path), filename (optional new name for the file), and callback (optional function called after download or on error).

intermediate

How does res.download() differ from res.sendFile()?

res.download() forces the browser to download the file, while res.sendFile() just displays or opens the file in the browser if possible.

intermediate

What happens if the file path in res.download() is incorrect or file is missing?

The callback function receives an error, and Express can handle it (like sending a 404 response). Without a callback, Express sends a 404 response.

beginner

Why is it important to use absolute paths with res.download()?

Absolute paths avoid confusion about where the file is located, preventing errors and security issues by ensuring the correct file is sent.

What does res.download() do in Express?

A Sends a file and prompts the user to download it

B Sends JSON data to the client

C Renders a view template

D Redirects the client to another URL

res.download() sends a file and triggers the browser's download prompt.

Which parameter is NOT part of res.download()?

A File path

B HTTP status code

C Callback function

D Filename for download

res.download() does not take an HTTP status code parameter.

If you want the browser to open a file instead of downloading, which method should you use?

A res.download()

B res.redirect()

C res.sendFile()

D res.json()

res.sendFile() sends the file to display or open in the browser.

What should you do to handle errors when using res.download()?

A Set a timeout for the download

B Ignore errors, Express handles them automatically

C Use try-catch around <code>res.download()</code>

D Use a callback function to catch errors

Passing a callback lets you handle errors like missing files gracefully.

Why prefer absolute paths with res.download()?

A To avoid file path confusion and security issues

B To improve download speed

C To change the file name automatically

D To compress the file before sending

Absolute paths ensure the correct file is sent and reduce security risks.

Explain how to use res.download() to send a file for download in Express.

Think about what parameters you pass and what happens in the browser.

Describe the difference between res.download() and res.sendFile() in Express.

Consider how the browser treats the file in each case.

Practice

1. What does res.download() do in an Express app?

easy

A. Uploads a file from the user to the server

B. Sends a file to the user for download

C. Deletes a file on the server

D. Reads a file and logs its content

Solution

1. Step 1: Understand the purpose of res.download

   res.download() is used to send files to users so they can download them easily.

2. Step 2: Compare with other file operations

   Uploading, deleting, or reading files are different operations and not what res.download() does.

3. Final Answer:

   Sends a file to the user for download -> Option B

4. Quick Check:

   res.download() = Sends file [OK]

Hint: Remember: download means sending file to user [OK]

Common Mistakes:

• Confusing download with upload
• Thinking it deletes files
• Assuming it reads file content

2. Which is the correct syntax to send a file named report.pdf for download using res.download?

easy

A. res.download('report.pdf')

B. res.download(report.pdf)

C. res.download('report.pdf', 'download')

D. res.download('report.pdf', {filename: 'download.pdf'})

Solution

1. Step 1: Check the basic usage of res.download

   The first argument is the file path as a string, so res.download('report.pdf') is correct.

2. Step 2: Analyze other options

   res.download(report.pdf) misses quotes making it invalid syntax. res.download('report.pdf', 'download') adds a second argument incorrectly as a string. res.download('report.pdf', {filename: 'download.pdf'}) uses an object which is not valid for filename.

3. Final Answer:

   res.download('report.pdf') -> Option A

4. Quick Check:

   File path string only = res.download('report.pdf') [OK]

Hint: File path must be a string in quotes [OK]

Common Mistakes:

• Omitting quotes around filename
• Passing wrong argument types
• Using object instead of string for filename

3. What will happen when this code runs?

app.get('/file', (req, res) => {
  res.download('files/data.txt', 'info.txt');
});

medium

A. The file 'files/data.txt' is sent and saved as 'info.txt' by the user

B. The file 'info.txt' is sent from the server

C. An error occurs because the second argument is invalid

D. The file 'data.txt' is sent without renaming

Solution

1. Step 1: Understand res.download parameters

   The first argument is the path on the server, the second is the filename for the user.

2. Step 2: Apply to given code

   The server sends 'files/data.txt' but the user will see it saved as 'info.txt'.

3. Final Answer:

   The file 'files/data.txt' is sent and saved as 'info.txt' by the user -> Option A

4. Quick Check:

   res.download(path, filename) renames file [OK]

Hint: Second argument renames file for user [OK]

Common Mistakes:

• Thinking second argument is server file path
• Assuming no renaming happens
• Believing second argument causes error

4. Identify the error in this code snippet:

app.get('/download', (req, res) => {
  res.download();
});

medium

A. res.download requires a callback as first argument

B. res.download cannot be used inside app.get

C. No error, code works fine

D. Missing file path argument in res.download

Solution

1. Step 1: Check res.download required arguments

   res.download() requires at least the file path as the first argument.

2. Step 2: Analyze given code

   The code calls res.download() with no arguments, causing an error.

3. Final Answer:

   Missing file path argument in res.download -> Option D

4. Quick Check:

   res.download needs file path [OK]

Hint: Always provide file path to res.download [OK]

Common Mistakes:

• Calling res.download without arguments
• Thinking res.download needs callback first
• Assuming no error if arguments missing

5. You want to send a file report.pdf for download but also handle errors if the file is missing. Which code correctly does this?

hard

A. res.download('report.pdf', (err) => { if (err) throw err; });

B. res.download('report.pdf').catch(() => res.status(404).send('File not found'));

C. res.download('report.pdf', err => { if (err) res.status(404).send('File not found'); });

D. res.download('report.pdf', 'report.pdf', err => { if (err) res.send('Error'); });

Solution

1. Step 1: Understand error handling in res.download

   res.download accepts an optional callback as the last argument to handle errors.

2. Step 2: Check each option

   res.download('report.pdf', err => { if (err) res.status(404).send('File not found'); }); uses the callback correctly to send 404 if error occurs. res.download('report.pdf').catch(() => res.status(404).send('File not found')); wrongly uses .catch which is not supported. res.download('report.pdf', 'report.pdf', err => { if (err) res.send('Error'); }); has an extra filename argument which is unnecessary here. res.download('report.pdf', (err) => { if (err) throw err; }); throws error instead of handling gracefully.

3. Final Answer:

   res.download('report.pdf', err => { if (err) res.status(404).send('File not found'); }); -> Option C

4. Quick Check:

   Use callback to handle errors in res.download [OK]

Hint: Use callback function to catch errors in res.download [OK]

Common Mistakes:

• Using promises or .catch with res.download
• Not handling errors at all
• Passing wrong number of arguments