res.download for file downloads in Express

Introduction

res.download lets your server send files to users so they can save them on their device. It makes downloading files easy and automatic.

When you want users to download a report or invoice from your website.

When providing a file like a PDF, image, or document for users to save.

When you need to let users download software or updates.

When sharing exported data files like CSV or JSON from your app.

When sending any file that should be saved instead of shown in the browser.

Syntax

Express

res.download(path, [filename], [options], [callback])

path is the file location on your server.

filename is optional and sets the name users see when saving.

Examples

Sends the file 'report.pdf' for download with its original name.

Express

res.download('/files/report.pdf');

Sends the file but users see it named 'monthly-report.pdf'.

Express

res.download('/files/report.pdf', 'monthly-report.pdf');

Downloads 'image.png' and logs an error if something goes wrong.

Express

res.download('/files/image.png', (err) => {
  if (err) console.log('Download error:', err);
});

Sample Program

This Express server has a route '/download' that sends the file 'sample.txt' from the 'public' folder. The file will be downloaded as 'example.txt'. If the file is missing, it sends an error message.

Express

import express from 'express';
const app = express();

app.get('/download', (req, res) => {
  const filePath = './public/sample.txt';
  res.download(filePath, 'example.txt', (err) => {
    if (err) {
      res.status(500).send('Sorry, file not found.');
    }
  });
});

app.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Important Notes

Make sure the file path is correct and accessible by your server.

Use the callback to handle errors like missing files or permission issues.

res.download sets headers so browsers know to download, not display, the file.

Summary

res.download sends files to users for easy downloading.

You can rename the file for the user with an optional filename.

Always handle errors to improve user experience.

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