NodeJS - Cannot set headers after they are sent - Multiple Calls - angularjs

Question

I'm trying to write an app that find a city in a MongoDB collection and uses the latitude and longitude it returns to find all zip codes within a certain distance. It seems to work, but the problem is that I'm getting an error that I can't set headers after they've already been sent. However, I've separated the to routes into different requests I don't understand why I'm still getting this error. What is the best way to make multiple calls to the API?
Here is my router in Node/Express:
// route to get city
app.get('/cities/:zip', function(req, res) {
// use mongoose to get the city in the database
console.log(req.params.zip);
var query = City.find({"zip" : req.params.zip});
query.exec(function(err, city) {
if (err)
res.send(err);
res.json(city);
});
});
// route to find cities within 50 miles
app.get('/matches/:latMin/:latMax/:lonMin/:lonMax', function(req, res) {
console.log(req.params.latMin + req.params.latMax + req.params.lonMin + req.params.lonMax);
var matches = City.find({latitude: {$gt: req.param.latMin, $lt:req.params.latMax }, longitude : {$gt :req.param.lonMin, $lt : req.param.lonMax}});
matches.exec(function(err, match){
if(err)
res.send(err);
console.log(match);
res.json(match);
});
});
app.get('*', function(req, res) {
res.sendfile('./public/views/index.html'); // load our public/index.html file
});
Here is my Angular Controller
$scope.update = function (zip) {
City.get({zip : zip}).success(function(response){
$scope.weather = response
}).then(function(response){
$scope.weather = response.data;
})
if(zip.length = 5){
$http.jsonp('http://api.openweathermap.org/data/2.5/weather?zip='+ zip +',us&callback=JSON_CALLBACK&units=imperial').success(function(data){
$scope.data=data;
});
var box = getBoundingBox([$scope.weather[0].latitude, $scope.weather[0].longitude], 50);
City.matches(box[1], box[3], box[0], box[2]).success(function(response){
$scope.matches = response
}).then(function(response){
$scope.matches = response.data;
console.log($scope.matches);
})
}

Answer 1

res.send does not return; the call continues to res.json. And please use braces. Please. Maybe they don't look cool or whatever. Just use them.
if (err) { handleError(res, err); return; }
res.status(200).json(city);
Further down, keeping things DRY:
function handleError(res, err) {
res.status(500).json(err);
}