ExpressJS 4 execute function after res.send() - node.js

Question

I am new to nodeJS and asynchronous programming. I am using express as the base for my app, there is really only one route that serves a page and accepts an upload from a form. I would like to make a POST request to an external service after the the file has been uploaded. Attempting to execute any code after res.send(200) however results in an error message of Error: Can't set headers after they are sent.
I am using the request package to make the post to the external service.
var express = require('express');
var router = express.Router();
var util = require("util");
var request = require("request");
/* POST uploads. */
router.post('/', function(req, res, next) {
console.log("LOG:" + util.inspect(req.files));
res.send('respond with a resource');
postFile(req.files.file.path);
});
var postFile = function(path) {
var opts = {
method: 'POST',
uri: 'https://example.com/api/files.upload',
formData: {
token: "xxx",
file: fs.createReadStream(req.files.file.path)
}
}
// console.log("LOG:\n" + util.inspect(opts));
request.post(opts, function(error, response, body) {
if (error) {
console.log("ERROR LOG: " + util.inspect(error));
} else {
console.log("RESPONSE LOG:" + util.inspect(response));
}
});
}
module.exports = router;
The postFile function works fine on it's own and even adding a console.log directly after the res.send results in the same error. How can I continue to execute code on the server after the response has been sent to the client?
Output log from node:
POST /uploads 200 85.201 ms - 23
Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:335:11)
at ServerResponse.header (/Users/jason/dev/test/file-share/node_modules/express/lib/response.js:700:10)
at ServerResponse.send (/Users/jason/dev/test/file-share/node_modules/express/lib/response.js:154:12)
at fn (/Users/jason/dev/test/file-share/node_modules/express/lib/response.js:934:10)
at View.exports.renderFile [as engine] (/Users/jason/dev/test/file-share/node_modules/jade/lib/index.js:374:12)
at View.render (/Users/jason/dev/test/file-share/node_modules/express/lib/view.js:93:8)
at EventEmitter.app.render (/Users/jason/dev/test/file-share/node_modules/express/lib/application.js:566:10)
at ServerResponse.res.render (/Users/jason/dev/test/file-share/node_modules/express/lib/response.js:938:7)
at /Users/jason/dev/test/file-share/app.js:58:7
at Layer.handle_error (/Users/jason/dev/test/file-share/node_modules/express/lib/router/layer.js:58:5)

Answer 1

There's nothing wrong in the code below, you can execute code after responding the request. You just can't send headers again.
router.post('/', function(req, res, next) {
console.log("LOG:" + util.inspect(req.files));
res.send('respond with a resource');
postFile(req.files.file.path);
});
Using the request to POST or GET something will not respond to your request, unless you want to.

Answer 2

Your postFile() function is referring to req.files.file.path in this line:
file: fs.createReadStream(req.files.file.path)
but the req object is not being passed to it. This should generate some sort of exception.
It appears you should be using just:
file: fs.createReadStream(path)
since you are passing the path to postFile(path).