res.download for file downloads in Express - Step-by-Step Execution

Concept Flow - res.download for file downloads

Client sends request

↓

Server receives request

↓

res.download called with file path

↓

Express sets headers for download

↓

File is streamed to client

↓

Client browser prompts to save file

↓

Download completes or errors handled

This flow shows how Express handles a file download request by setting headers and streaming the file to the client.

Execution Sample

Express

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

This code sends the file 'report.pdf' to the client as a download when they visit '/file'.

Execution Table

Step	Action	Input/Parameter	Result/Effect
1	Client requests '/file'	GET /file	Request received by server
2	Express calls route handler	req, res	Handler function runs
3	res.download called	'./files/report.pdf'	Sets headers for download
4	Headers set	Content-Disposition: attachment	Browser knows to download
5	File streamed	report.pdf content	Data sent to client
6	Client browser prompts	Download dialog	User chooses save location
7	Download completes	File fully received	Process ends successfully

💡 Download completes or errors handled if file missing or interrupted

Variable Tracker

Variable	Start	After res.download call	After streaming	Final
req	Request object	Request object	Request object	Request object
res	Response object	Headers set	Streaming file	Response finished
filePath	undefined	'./files/report.pdf'	'./files/report.pdf'	'./files/report.pdf'

Key Moments - 3 Insights

Why does the browser show a download prompt instead of opening the file?

Because res.download sets the 'Content-Disposition' header to 'attachment', telling the browser to download instead of display. See execution_table step 4.

What happens if the file path is wrong or file is missing?

Express will send an error response or call the error handler. The download won't start. This is implied after step 7 in execution_table.

Does res.download block the server while sending the file?

No, Express streams the file asynchronously, so the server can handle other requests during download (step 5).

Visual Quiz - 3 Questions

Look at the execution_table, at which step does Express tell the browser to download the file?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Check the 'Headers set' row in execution_table where Content-Disposition is applied.

According to variable_tracker, what is the state of 'res' after streaming starts?

A Streaming file

B Response finished

C Headers set

D Request object

💡 Hint

Look at the 'After streaming' column for 'res' in variable_tracker.

If the file path is incorrect, what will likely happen in the execution flow?

A Client immediately downloads an empty file

B Browser shows download prompt with wrong file name

C Server sends an error response instead of streaming

D Download completes successfully

💡 Hint

Refer to key_moments about file missing and error handling after step 7.

Concept Snapshot

res.download(filePath) sends a file as a download to the client.
It sets 'Content-Disposition: attachment' header.
Express streams the file asynchronously.
Browser shows a save dialog.
Errors occur if file missing.
Use in route handlers to serve files.

Full Transcript

When a client requests a file download, Express receives the request and runs the route handler. Calling res.download with the file path sets headers telling the browser to download the file instead of displaying it. Express streams the file content to the client asynchronously. The browser then shows a save dialog for the user to choose where to save the file. If the file path is wrong or the file is missing, Express sends an error response instead of streaming. This process allows efficient file downloads without blocking the server.

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