Express "Can't set headers after they are sent" after deploying to Heroku - express

Question

after deploying my server code to Heroku I'm getting the error "Can't set headers after they are sent". I fetch my login data
fetch("url", {
method: "POST",
mode: "cors",
headers: {
"Accept": "application/json",
"Content-Type": "application/x-www-form-urlencoded"
//"Content-Type": "application/json"
},
body: requestBody
}).then((res, next) => {
if(res.ok){
res.json().then((json) => {
if(json.verifystate){
this.props.navigation.navigate('Home')
}
else{
this.setState({isAuthentic:false});
}
});
}else{
next();
}
})
And it calls my server post method
app.post('/users/auth', function(req, res) {
loginData(db, req.body.email, req.body.password, req.body.mphone, function(result){
if(result == 1){
res.send({"verifystate":1});
}else if(result == 2){
res.send({"verifystate":2});
}else{
res.send({"verifystate":3});
}
});
//console.log('json: '+JSON.stringify(data));
});
edit: Login function
module.exports = function loginData(db, email, myPlaintextPassword, mphone, callback){
var collectionUser = db.collection('users');
bcrypt.genSalt(saltRounds, function(err, salt) {
bcrypt.hash(myPlaintextPassword, salt, function(err, hash) {
var queryStr = {"emails.address": email};
collectionUser.findOne(queryStr, function(err, result) {
if(err)
{
return;
}else if(result != null){
bcrypt.compare(myPlaintextPassword, result.services.password.bcrypt, function(err, res) {
console.log("conpare result: ", res);
if(res){
callback("1");
}else{
callback("3");
}
});
if(!result.services.resume || result.services.resume.appLoginToken == null
|| !result.services.resume.appLoginToken.date || !result.services.resume.appLoginToken.base64Token){
var tokenStr = email + Math.random(1,100);
tokenStr = Base64.encode(tokenStr);
queryStr = {
"services.resume.appLoginToken.date": Date(),
"services.resume.appLoginToken.base64Token": tokenStr
};
var newDate = { "services.resume.appLoginToken.date": Date() }
var temp = {"emails.address": email};
collectionUser.update(temp,
{ $set: {"services.resume.appLoginToken.date": Date()} }, function(err, result) {
if(err)
{
return;
}else {
}
});
collectionUser.update(temp,
{ $set: {"services.resume.appLoginToken.base64Token": tokenStr} }, function(err, result) {
if(err)
{
return;
}else {
}
});
}else{
console.log("already have token");
}
}else{
callback("2")
}
});
});
});
};
And when it gets to res.send({"verifystate:"1}) it then throws the aforementioned error. Any help would be great thanks.

Answer 1

if(!res.headersSent){
res.send('hello world');
}
or
if(!res.headersSent){
res.json({message: 'hello world'});
}
you probably have a callback that fires more than once, often times this is with event emitters.
As a side note, I really recommend using 2 spaces for indentation with JS, because so much code is nested. And I also recommend putting else and .then() on a newline, more readable.

Answer 2

I did not read the whole code but you could try this:
module.exports = function loginData(db, email, myPlaintextPassword, mphone, callback) {
let collectionUser = db.collection('users');
bcrypt.genSalt(saltRounds, function (err, salt) {
bcrypt.hash(myPlaintextPassword, salt, function (err, hash) {
let queryStr = {"emails.address": email};
collectionUser.findOne(queryStr, function (err, result) {
if (err) {
//You should do something here or it would take you to an error , just an return won't work, you've to call a callback that send the error message
//callback(0)
} else if (result !== null) {
if (!result.services.resume || result.services.resume.appLoginToken === null
|| !result.services.resume.appLoginToken.date || !result.services.resume.appLoginToken.base64Token) {
let tokenStr = email + Math.random(1, 100), temp = {"emails.address": email};
tokenStr = Base64.encode(tokenStr);
queryStr = {
"services.resume.appLoginToken.date": new Date(),
"services.resume.appLoginToken.base64Token": tokenStr
};
collectionUser.update(temp,
{$set: {"services.resume.appLoginToken.date": new Date()}}, function (err, result) {
});
collectionUser.update(temp,
{$set: {"services.resume.appLoginToken.base64Token": tokenStr}}, function (err, result) {
});
} else {
console.log("already have token");
}
bcrypt.compare(myPlaintextPassword, result.services.password.bcrypt, function (err, res) {
console.log("conpare result: ", res);
if (res) {
callback(1);
} else {
callback(3);
}
});
} else {
callback(2);
}
});
});
})
};
And this:
app.post('/users/auth', function (req, res) {
loginData(db, req.body.email, req.body.password, req.body.mphone, function (result) {
res.send({verifystate: result});
});
});