Nodejs - Express res.download giving Can't set headers after they are sent exception

Question

I want to make an api that will serve files of any extensions. Like this: http://localhost/download/[file].[extension]

Here is my code, but it is intermittently giving this message: Can't set headers after they are sent.

  var express = require('express');
  var app = express();

  app.get('/download/:fileName/:extension', function(req, res){
    var file = __dirname + '/' + req.params.fileName + '.' + req.params.extension;
    res.download(file, function(err){
        if (err) {
          res.sendStatus(404);
        }
      res.end();
    }); 
  });

  var server = app.listen(3000, function () {
    var host = server.address().address;
    var port = server.address().port;

    console.log('app listening at http://%s:%s', host, port);
  });

Answer 1

res.download has already sent a response (Not always true in the case of an error though)

You can fix this by doing

res.download(file, function(err){
  if(err) {
    // Check if headers have been sent
    if(res.headersSent) {
      // You may want to log something here or do something else
    } else {
      return res.sendStatus(SOME_ERR); // 404, maybe 500 depending on err
    }
  }
  // Don't need res.end() here since already sent
}

---

Other changes called out in the comments above:

• download uses sendFile , which you don't need res.end() after
• download 's documentation warns that you need to check res.headersSent when handling errors, as the headers may already be sent, which would mean you can't change the status