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.