How to download a json object as file?

Question

I am currently trying to let the user of my website download a json object as a json-file. With my following code i get the error message:

Error: Can't set headers after they are sent.
    at SendStream.headersAlreadySent (\node_modules\send\index.js:390:13)
    at SendStream.send (\node_modules\send\index.js:617:10)
    at onstat (\node_modules\send\index.js:729:10)
    at FSReqCallback.oncomplete (fs.js:168:5)

router.post('/about', ensureAuthenticated,
  function (req, res, next) {   
    console.log(req.user);

    var jsonVariable = JSON.stringify(req.user);
    var path_tmp = create_tmp_file(jsonVariable);
    
    res.download(path_tmp);
    
    res.redirect('/about');
    next();
  }
);

Is there a better way to download a json object directly with no need to save it in the filesystem?

score 4 · Answer 1 · edited Nov 16 '22 at 08:14

You can always inject some HTML into a page (or redirect to a page with some client-side JavaScript on it), and download it with the client. Just send the JSON string somehow to the new page you are redirecting to (it can even be a GET parameter), then download it with the following code (assuming the JSON string is in a variable called json):

var a = document.createElement("a")
a.href = URL.createObjectURL(
    new Blob([json], {type:"application/json"})
)
a.download = "myFile.json"
a.click()

score 3 · Accepted Answer · answered Jun 26 '20 at 09:21

var jsonVariable = JSON.stringify(req.user);
var path_tmp = create_tmp_file(jsonVariable);
res.download(path_tmp);

Send the data with res.json and use content-disposition to make it a download.

res.set('Content-Disposition', 'attachment; filename=example.json')
res.json(req.user);

res.redirect('/about');
next();

And don't do that (which is the cause of the error). You are responding with a download. You can't say "Here is the file you asked for" while, at the same time say, "The file you asked for isn't here, go to this URL instead".

Okay, thank you for your help! i edited my code and removed the redirect. — Joschua Xner, Jun 26 '20 at 09:34

How to download a json object as file?

2 Answers2