Using Node/Express to stream file to user for download

Question

I want to use a Node/Express server to stream a file to the client as an attachment. I would like to make an async request from the client to a /download endpoint and then serve an object received via API proxy to the client as a downloadable file (similar to the way res.attachment(filename); res.send(body); behaves).

For example:

fetch(new Request('/download'))
    .then(() => console.log('download complete'))

app.get('/download', (req, res, next) => {
    // Request to external API
    request(config, (error, response, body) => {
        const jsonToSend = JSON.parse(body);
        res.download(jsonToSend, 'filename.json');
    })
});

This will not work because res.download() only accepts a path to a file. I want to send the response from an object in memory. How is this possible with existing Node/Express APIs?

Setting the appropriate headers does not trigger a download, either:

    res.setHeader('Content-disposition', 'attachment; filename=filename.json');
    res.setHeader('Content-type', 'application/json');
    res.send({some: 'json'});

score 5 · Answer 1 · edited May 23 '17 at 12:07

5

This worked for me. I use the content type octet-stream to force the download. Tested on chrome the json was downloaded as 'data.json'
You can't make the download with ajax according to: Handle file download from ajax post

You can use a href / window.location / location.assign. This browser is going to detect the mime type application/octet-stream and won't change the actual page only trigger the download so you can wrap it a ajax success call.

//client
const endpoint = '/download';

fetch(endpoint, {
  method: 'POST',
  credentials: 'include',
  headers: {
    'Accept': 'application/json',
    'Content-Type': 'application/json'
  }
  })
  .then(res => res.json())
  .then(res => {
     //look like the json is good to download
     location.assign(endpoint);
   })
   .catch(e => {
     //json is invalid and other e
   });

//server
const http = require('http');

http.createServer(function (req, res) {
    const json = JSON.stringify({
      test: 'test'
    });
    const buf = Buffer.from(json);
    res.writeHead(200, {
      'Content-Type': 'application/octet-stream',
      'Content-disposition': 'attachment; filename=data.json'
    });
    res.write(buf);
    res.end();
}).listen(8888);

edited May 23 '17 at 12:07

Community

1
1

answered Nov 19 '16 at 00:40

mrdotb

612
6
15

Looks like you're reading from the `data.json` file, though. How can I perform the same behavior and send a plain object `{"some": "json"}`? – Himmel Nov 19 '16 at 00:43
What headers did you include from the client? – Himmel Nov 19 '16 at 00:53
I edit the code it make download a serialized JSON but not as Stream. – mrdotb Nov 19 '16 at 00:55
How are you making the request from the client? – Himmel Nov 19 '16 at 00:58
curl and chrome but it should work with ajax too I'm going to check. – mrdotb Nov 19 '16 at 01:00
Hmm look like you can't launch the download with an ajax call. But you can trigger it using `location.assign('/download');` or `window.location = 'http://localhost:8888/download';` or with a a href link without reloading the page. – mrdotb Nov 19 '16 at 01:16
It's not possible at all? – Himmel Nov 19 '16 at 01:41
it's possible check the link on the answer I add it. – mrdotb Nov 19 '16 at 01:55
That solution works but isn't an ajax call unfortunately, it's just a way to trigger a download. – Himmel Nov 19 '16 at 02:07
Hmm look like it's not possible to start a dowload using ajax except in the success cb using one of the hacks. I edit my answer again. – mrdotb Nov 19 '16 at 12:45

score 0 · Answer 2 · edited May 23 '17 at 11:59

0

You can set the header to force the download, then use res.send

see those links

edited May 23 '17 at 11:59

Community

1
1

answered Nov 18 '16 at 23:33

user3

740
3
15

I saw it, you're asking the same question as in the first link I provided – user3 Nov 19 '16 at 01:23
That link does not provide me with enough information to fix the problem I'm having. – Himmel Nov 19 '16 at 01:39

Using Node/Express to stream file to user for download

2 Answers2