Express: Can't set headers after they are sent - node.js

Question

Following is my server file. I am making 2 calls, one post and one get. It works fine at times. But gives an error of : Can't set headers after they are sent. Does this have anything to do with my client side code?
server.js
var express = require('express')
var mongoose = require('mongoose')
var path = require('path')
var bodyParser = require("body-parser")
var cors = require("cors")
var app = express()
var port = process.env.PORT || 3000
var Url = require("./data/url-schema");
//Express request pipeline
app.use(express.static(path.join(__dirname,"../client")))
app.use(bodyParser.json())
app.use(cors());
/*
Your server must be ready to handle real URLs. When the app first loads at / it will probably work, but as the user navigates around and then hits refresh at /dashboard your web server will get a request to /dashboard. You will need it to handle that URL and include your JavaScript application in the response.
*/
app.get('*', function (request, response, next){
response.sendFile(path.resolve(__dirname, '../client', 'index.html'))
next()
})
app.get('/:code', function(req, res) {
console.log("reg", req.params.code)
Url.findOne({code:req.params.code}, function(err, data){
console.log("data", data)
if(data)
res.redirect(302, data.longUrl)
else
res.end()
})
})
app.post('/addUrl', function (req, res, next) {
console.log("on create");
Url.findOne({longUrl:req.body.longUrl}, function(err, data) {
if (err)
res.send(err);
else if(data) {
console.log("already exists",data)
res.send("http://localhost:3000/"+data.code);
} else {
var url = new Url({
code : Utility.randomString(6,"abcdefghijklm"),
longUrl : req.body.longUrl
});
console.log("in last else data created",url)
url.save(function (err, data) {
console.log(data)
if (err)
res.send(err);
else
res.send("http://localhost:3000/"+data.code);
});
}
});
})
app.listen(port, function () {
console.log('Example app listening on port 3000!')
});
// Connect to our mongo database
mongoose.connect('mongodb://localhost/shortUrl');
I get the Following error
error
_http_outgoing.js:335
throw new Error('Can\'t set headers after they are sent.');
^
Error: Can't set headers after they are sent.
at ServerResponse.OutgoingMessage.setHeader (_http_outgoing.js:335:11)
at ServerResponse.header (/opt/lampp/htdocs/url-shortener/node_modules/express/lib/response.js:718:10)
at ServerResponse.location (/opt/lampp/htdocs/url-shortener/node_modules/express/lib/response.js:835:8)
at ServerResponse.redirect (/opt/lampp/htdocs/url-shortener/node_modules/express/lib/response.js:874:8)
at Query.<anonymous> (/opt/lampp/htdocs/url-shortener/server/server.js:30:8)
at /opt/lampp/htdocs/url-shortener/node_modules/mongoose/node_modules/kareem/index.js:177:19
at /opt/lampp/htdocs/url-shortener/node_modules/mongoose/node_modules/kareem/index.js:109:16
at process._tickCallback (node.js:355:11)

Answer 1

From the execution order, in * route handler, the body is being assigned to the response and then in /:code, the response code 302 is being added, where Location header is also added, hence the error. Any header must be added before the body to the response.
To solve this problem, simply change the order of the two GET statements.

Answer 2

Finally found the solution:
var express = require('express')
var mongoose = require('mongoose')
var path = require('path')
var bodyParser = require("body-parser")
var app = express()
var port = process.env.PORT || 3000
var Url = require("./data/url-schema")
var Utility = require("./utility")
//Express request pipeline
app.use(express.static(path.join(__dirname,"../client")))
app.use(bodyParser.json())
/*
Your server must be ready to handle real URLs. When the app first loads at / it will probably work, but as the user navigates around and then hits refresh at /dashboard your web server will get a request to /dashboard. You will need it to handle that URL and include your JavaScript application in the response.
*/
app.get('/dashboard', function (request, response, next){
response.sendFile(path.resolve(__dirname, '../client', 'index.html'))
next()
})
app.get('/about', function (request, response, next){
response.sendFile(path.resolve(__dirname, '../client', 'index.html'))
next()
})
app.get('/:code', function(req, res) {
Url.findOne({code:req.params.code}, function(err, data){
if(data){
res.redirect(302, data.longUrl)
}
})
})
app.post('/addUrl', function (req, res, next) {
Url.findOne({longUrl:req.body.longUrl}, function(err, data) {
if (err){
res.send(err)
}
else if(data) {
res.send("http://localhost:3000/"+data.code);
} else {
var newCode = getCode()
checkCode(newCode)
.then(function(data){
var url = new Url({
code : data,
longUrl : req.body.longUrl
});
url.save(function (err, data) {
if (err)
res.send(err);
else
res.send("http://localhost:3000/"+data.code);
});
})
}
});
})
app.listen(port, function () {
console.log('Example app listening on port 3000!')
});
// Connect to our mongo database
mongoose.connect('mongodb://localhost/shortUrl');
//Generate a random code
function getCode() {
return Utility.randomString(6,"abcdefghijklmnopqrstuvwxyz")
}
//Check if the code is unique
function checkCode(code) {
return new Promise(function (resolve, reject){
Url.findOne({code:code}, function(err, data) {
if(err === null){
resolve(code)
}else if(data){
saveUrlCode(getCode())
}
})
})
}
My earlier route which was :
app.get('*', function (request, response, next){
response.sendFile(path.resolve(__dirname, '../client', 'index.html'))
next()
})
The get route was getting executed twice on account of the above call and the
app.get(":/code") call.
So I had to handle the routes properly which I have done by handling the dashboard and about routes separately instead of using the "*" route.