Express breaking when getting concurent client calls - node.js

Question

Because of an error that I had in the client, my node / express API was getting the exact same PUT request twice simultaneously everytime that a form was sent.
In response, the server was always crashing with the following error:
Error: Can't set headers after they are sent
Now I fixed the client so server is not crashing anymore, but I'd like to fix the API properly. Anyone could kill my server by sending two PUT requests simultaneously.
So my question:
How to best handle the "can't set headers" error with Express?
What is the mistake in my server code that allowed for that error to happen in the first place?
Here is my server code:
server.js
router.route('/deals/:id').put(deal.update);
deal.js
var r = {};
exports.update = function(req, res) {
r = res;
var u = {
$set: {}
}
for(var x in req.body){
if (['name','stages'].indexOf(x)>=0)
u.$set[x] = req.body[x];
}
Pipeline // this is a mongoose object
.findOneAndUpdate({'_id.id':req.params.id},u,{new:true})
.exec(respond);
}
function respond(err, data) {
if (err) {
return r.send(err);
} else {
return r.json(data);
}
}
UPDATE:
The error came from how I declared a global r in order to have a single response callback outside for all of my methods.
I fixed it by having one respond callback for every method.
Still interested to know if I could get away with a bind(res) instead?

Answer 1

You missed a return, so that when there was an error, you called res.send twice. With the return, execution will stop after the first one.
I see what you are trying to do for your follow on question, but making a global variable r is definitely not a good way to go. The way I do patterns like this is to pass res to the follow on function - see below. There may be a better way, but this seems legitimate at least.
exports.update = function(req, res) {
var u = {
$set: {}
}
for(var x in req.body){
if (['name','stages'].indexOf(x)>=0)
u.$set[x] = req.body[x];
}
Pipeline // this is a mongoose object
.findOneAndUpdate({'_id.id':req.params.id},u,{new:true})
.exec(function(err, data) { respond(err, data, res) });
}
function respond(err, data, res) {
if (err) {
return res.send(err);
}
return res.json(data);
}