Assuming I have the following in a function:
exports.addnames = function(req, res) {
var names = ["Kelley", "Amy", "Mark"];
for(var i = 0; i < names.length; i++) {
(function (name_now) {
Person.findOne({ name: name_now},
function(err, doc) {
if(!err && !doc) {
var personDoc = new PersonDoc();
personDoc.name = name_now;
console.log(personDoc.name);
personDoc.save(function(err) {});
} else if(!err) {
console.log("Person is in the system");
} else {
console.log("ERROR: " + err);
}
}
);
)(names[i]);
}
My issue is after I save the names, I want to return the results:
Person.find({}, function(err, doc) {
res.json(200, doc);
})
Though I have a callback for names, it appears that the last block of code (Persons.find({})) gets executed before the calls to save all the names is complete... thusly when the user goes to the url in the browser, "doc" is empty... Is there some way I can ensure that the Persons.find({}) is called after the for loop completes?
The easiest way to do things like this is to use an async library like the aptly named async which can be found at https://github.com/caolan/async.
If you have a list of names that you want to save and then return when complete, it would look like:
// save each of the names asynchronously
async.forEach(names, function(name, done) {
Person.findOne({name: name},
function(err, doc) {
// return immediately if there was an error
if(err) return done(err);
// save the person if it doesn't already exist
if(!doc) {
var personDoc = new PersonDoc();
personDoc.name = name;
console.log(personDoc.name);
// the async call is complete after the save completes
return personDoc.save(done);
}
// or if the name is already there, just return successfully
console.log("Person is in the system");
done();
}
);
},
// this function is called after all of the names have been saved
// or as soon as an error occurs
function(err) {
if(err) return console.log('ERROR: ' + err);
Person.find({}, function(err, doc) {
res.json(200, doc);
})
});
Related
This is my first time asking a question on stackoverflow. Sorry if I made posting mistakes.
I am trying to exit a function after sending a response to prevent continuing through the function.
node -v = v12.6.0
express = ^4.17.1
mongoose = ^5.6.6
// handle adding a new book request submission
addNewBook: function (req, res) {
var response = null;
var name = req.body.name.toLowerCase();
var url = req.body.url.toLowerCase();
var category = req.body.category.toLowerCase();
var tags = req.body.tags.toLowerCase();
// checking if category already exist. if not, make a new category
Category.find({label: category}).exec(function(err, data) {
if(err) {
response = res.status(400).send({message:'Error finding category.'});
} else if(data.length === 0) {
var newCategory = new Category({label: category, description: '', keywords: ''});
newCategory.save(function(err, data){
if(err) {
response = res.status(400).send({message:'Error saving new category.'});
}
})
}
});
// checking if book name already exist
Book.find({name: name}).exec(function(err, data){
if(err) {
response = res.status(400).send({message:'Error validating Book existence'});
} else if(data.length > 0) {
response = res.status(200).send({message:'book name already exist'});
} else {
req.body.name = name;
req.body.url = url;
req.body.category = category;
req.body.tags = tags;
// make a new book document
var newBook = new Book(req.body);
newBook.save(function (err, data) {
if (err) {
response = res.status(400).send({message: 'Error saving new Book.'});
} else {
response = res.json(data);
}
})
}
});
return response;
},
Function continues to executes other part of the function code after a return.
I am also getting "Cannot set headers after they are sent to the client" error on node. Im guessing, preventing the function to continue after sending a response will fix this as well?
Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
There are two problems with the flow of your logic. First is that return only returns a function. It does not return the function that calls a function or the function that defines a function.
Basically, your code is:
Category.find({label: category}).exec(function(err, data) {
if(err) {
// ...
return;
} else if(/* ... */) {
// ...
newCategory.save(function(err, data){
if(err) {
// ...
return;
}
})
}
});
moreStuffDownHere();
// ...
Let's rewrite that to not use anonymous functions to make it clear what's really happening
function findCallback (err, data) {
if(err) {
// ...
return; // it's obvious that this returns form findCallback()
// and NOT yourFunction()
} else if(/* ... */) {
// ...
newCategory.save(saveCallback);
}
}
function saveCallback (err, data) {
if(err) {
// ...
return;
}
}
function yourFunction () {
Category.find({label: category}).exec(findCallback);
moreStuffDownHere();
}
So you can now see that you are not calling return anywhere in yourFunction().
The second problem is that Category.find().exec() is asynchronous. This means it returns immediately and let any code below it run before calling findCallback(). To solve the async issue just move moreStuffDownHere() inside findCallback().
Therefore, the simplest change to get your program flow working is to move moreStuffDownHere:
Category.find({label: category}).exec(function(err, data) {
if(err) {
res.status(400).send({message: 'Error finding category.'});
return;
} else if(data.length === 0) {
var newCategory = new Category({label: category, description: '', keywords: ''});
newCategory.save(function(err, data){
if(err) {
res.status(400).send({message: 'Error saving new category.'});
return;
}
// More stuff down here, that now will only execute if there are no errors
})
}
});
Improve program flow
One issue I have with the solution above is that now moreStuffDownHere is hardcoded inside the save callback. One way around it is to refactor the entire operation and make it your own internal API:
function addNewCategory (category, callback) {
// callback will be passed status depending on success or failure
Category.find({label: category}).exec(function(err, data) {
if(err) {
// ...
callback('FIND_FAILURE');
return;
} else if(/* ... */) {
// ...
newCategory.save(function(err, data){
if(err) {
// ...
callback('SAVE_FAILURE');
return;
}
callback('OK');
})
}
});
}
Now inside yourFunction() you can check the result of the entire operation and decide to return or continue:
function yourFunction() {
// ...
addNewCategory(category, function (status) {
switch (status) {
case 'FIND_FAILURE':
res.status(400).send({message: 'Error finding category.'});
return;
case 'SAVE_FAILURE':
res.status(400).send({message: 'Error saving new category.'});
return;
}
// More stuff down here ...
});
}
Improvement 2 - Promises
It's possible to make the program flow much easier to read by using Promises along with async/await. For that you need to wrap the operation in a promise. We can use the addNewCategory function we wrote above as an example:
function addNewCategory (category) {
// returns a Promise of true/false
return new Promise(function (resolve, reject) {
Category.find({label: category}).exec(function(err, data) {
if(err) {
// ...
resolve('FIND_FAILURE'); // you can also use reject if you want
// to use a try/catch flow
return;
} else if(/* ... */) {
// ...
newCategory.save(function(err, data){
if(err) {
// ...
resolve('SAVE_FAILURE');
return;
}
resolve('OK');
})
}
});
});
}
Now the code is slightly easier to follow because it allows you to keep moreStuffDownHere where you originally have it without moving it inside another function:
async function yourFunction() {
// ...
var status = await addNewCategory(category);
switch (status) {
case 'FIND_FAILURE':
res.status(400).send({message: 'Error finding category.'});
return;
case 'SAVE_FAILURE':
res.status(400).send({message: 'Error saving new category.'});
return;
}
// More stuff down here ...
}
Note: Express accepts functions marked as async as routes/middlewares. You just need to call res.send() or next() as usual
The error is as a result of your condition. Hence, both code blocks are run resulting in the response being sent twice. To fix this change your code to this below.
Category.find({label: category}).exec(function(err, data) {
if(err) {
res.status(400).send({message: 'Error finding category.'});
} else if(data.length>0) {
//there is no data with that label - Hence, create one
var newCategory = new Category({label: category, description: '', keywords: ''});
newCategory.save(function(err, data){
if(err) {
//if error
res.status(400).send({message: 'Error saving new category.'});
}else{
//if item saves
res.status(200).send({message: 'Item saved'});
}
})
}else{
//there is a data with that label availble - do something else
res.status(200).send(data)
}
});
The error you report happens when there are code paths that can send a response more than once. You get one and only one response per request. So, calling res.send() more than once is one way that you get that error.
Preventing this when you have a number of asynchronous operations requires a bit more work as you have to make sure that all your code is properly sequenced and all error paths are properly terminated (so further processing doesn't happen). In general, this code is a lot easier to write using promise-based interfaces for your asynchronous operations, but since you aren't using the promise interface on your database, I'll show how you can do it with your existing callback interface. In generally, it involves a lot of nesting inside of asynchronous callbacks and very careful if/else and return around conditionals and errors.
Your code is subject to this error because you are running Category.find() and Book.find() in parallel. You don't wait for the Category.find() code to finish before doing the book operations. If the category code causes an error, you will send that error response, but still continue with the book code which will then send its response. Instead, you need to make sure that if there's an error with the category stuff that you don't run the book code at all. For the plain callback interface on your database, that means nesting the book code inside a callback from the category code. To make this simpler to write, I put the category code into it's own function that has one completion callback that we can use to know when its all done.
Here's one way to do it:
// utility function to create category if needed, requires callback
// to communicate results
function createCategoryIfNeeded(category, fn) {
// checking if category already exist. if not, make a new category
Category.find({label: category}).exec(function(err, data) {
if(err) {
fn({message:'Error finding category.'});
} else if(data.length === 0) {
let newCategory = new Category({label: category, description: '', keywords: ''});
newCategory.save(function(err, data){
if (err) {
fn({message:'Error saving new category.'});
} else {
// category created
fn(null, true);
}
})
} else {
// category already exists
fn(null, false);
}
});
}
// handle adding a new book request submission
addNewBook: function (req, res) {
var name = req.body.name.toLowerCase();
var url = req.body.url.toLowerCase();
var category = req.body.category.toLowerCase();
var tags = req.body.tags.toLowerCase();
createCategoryIfNeeded(category, function(err, created) {
if (err) {
res.status(400).send(err);
} else {
// checking if book name already exist
Book.find({name: name}).exec(function(err, data){
if(err) {
res.status(400).send({message:'Error validating Book existence'});
} else if(data.length > 0) {
res.status(200).send({message:'book name already exist'});
} else {
req.body.name = name;
req.body.url = url;
req.body.category = category;
req.body.tags = tags;
// make a new book document
var newBook = new Book(req.body);
newBook.save(function (err, data) {
if (err) {
res.status(400).send({message: 'Error saving new Book.'});
} else {
res.json(data);
}
});
}
});
}
});
},
The error meassage says that, res can be send once it has been send. So returning it along with the response.
Category.find({label: category}).exec(function(err, data) {
if(err) {
return res.status(400).send({message: 'Error finding category.'});
} else if(!data) {
var newCategory = new Category({label: category, description: '', keywords: ''});
newCategory.save(function(err, data){
if(err) {
return res.status(400).send({message: 'Error saving new category.'});
}
})
}
});
Probably this is a promise implementation but would like to check with experts before doing so.
Need to do:
Read entire file line-by-line into MongoDB collection A.
Upon completion of step 1, Insert/Update/Delete documents from collection B based on state in collection A. If document not present in A delete from B.
Problem: Even before completion of step 1 above, step 2 starts execution and starts deleting records from B.
Tried so far: Async.series does not work. Below given is my code.
MongoClient.connect(config.mongodb.uri, function (err, db) {
if (err) {
logger.error('Unable to connect to the mongoDB server. Error:', err);
reject(err);
} else {
let startTime = new Date();
async.series([
function(callback) {
console.log('First in series');
db.collection('eligibilityStage').drop({}, function (err, oldObject) {
debugger;
var lr = new LineByLineReader(config.eligibiltyFile.fileRemoteLocation + '/' + latestEligibilityfileName);
lr.on('error', function (err) {
console.log(err);
});
var lineCount;
lr.on('line', function (line) { //** --> Jumps from here to second function in series, line#43**
if (line.length == config.eligibiltyFile.detailRecordlineWidth) {
var document = require('fixy').parse({
map: mapData, options: {
skiplines: null, fullwidth: config.eligibiltyFile.detailRecordlineWidth
}
}, line);
db.collection('eligibilityStage').insertOne(document[0], function (err, records) {
lineCount++;
if (err) {
console.log(err);
}
});
}
});
lr.on('end', function () {
console.log('File is closed, read lines:'+lineCount);
console.log('File is closed, rowcount:'+db.eigibilityStage.Count());
});
callback(null, 'loadStage');
});
},
function(callback) {
// Deletes
console.log('Series 2 function, read lines:'+lineCount);
console.log('Series 2 function, rowcount:'+db.eigibilityStage.Count());
callback(null, 'processStage');
}
],
function(err, results){
});
}
})
Am I doing it wrong? Or is this a standard problem to be solved using promise?
I have an array, and for each row I need to do findIfExist and save into mongodb. The code is here:
router.post('/namespaceUpload', function(req, res,next) {
var data=req.body;
var totalRows=data.allRows.length;
var conceptObject ={};
var existingConcept;
for (var i=0;i<totalRows;i++){
async.series([
conceptPrepare,
conceptFind,
conceptSave,
], function (err, result) {
console.log('kraj');
res.json('Ok');
});
}
function conceptPrepare(callback){
conceptObject.name= data.allRows[i].name;
conceptObject.user= data.userId;
callback();
}
function conceptFind(callback){
namespaces.find({name: conceptObject.name}, function(err, result) {
if (err)
next(err);
else {
if (result.length==0){
console.log('0');
existingConcept='';
} else {
console.log(result.length);
existingConcept=result[0];
}
}
callback();
});
}
function conceptSave(callback){
var namespace = new namespaces();
if (existingConcept==''){
namespace.name=conceptObject.name;
namespace.description=conceptObject.description;
namespace.lastUpdate.user=conceptObject.user;
namespace.save(function(err) {
if (err)
return next(err);
callback();
})
}
}
So I Used async.series, but only last record is written in database as much times as many array members i have. Also, I get an error " Can't set headers after they are sent." Any idea?
You're getting the Can't set headers after they are sent error message because you 're not allowed to return smtg eg : res.send,res.render more than one time but in your for loop, it goes totalRows times
try to return one value at the end of the loop
I am mew to node js, I have something like this,
get_contacts(data, function(contacts) {
if (contacts.length) {
var count = contacts.length;
for (var i = 0; i < count; i++) {
result = {
id: contacts[i].id,
name: contacts[i].name,
sent1: get_sent(data.userId, contacts[i].id, function(resp) {
result.sent = resp.count;
}),
}
result1[i] = result;
}
output = {
contacts: result1,
}
} else {
output = {
error: "No Contacts.",
}
}
res.writeHead(200, {'content-type': 'text/html'});
res.end(JSON.stringify(output));
});
get_contacts is a callback function which will return contact list.result1 & result are objects. Now value for sent should come from a function get_sent, and get sent is like this
function get_sent(userId, contactId, callback) {
pool.getConnection(function(err, connection) {
connection.query("my query here", function(err, rows) {
connection.release();
if (!err) {
callback(rows);
} else {
console.log(err)
}
});
});
}
But im not getting any value since nodejs. since nodejs is async it is not waiting for the function to return value. I know, im doing it in wrong way. Please help
You need to use a callback. In simple words is a function that you'll execute after something happens. You should read more about that. You should get a book about javascript but you can start reading here for example.
About your case, you could solve it like this
//Asumming that you object `result` is global.
result = {
id: contacts[i].id,
name: contacts[i].name,
sent: -1 //Some default value
}
//Just to put the code into a function, you have to put it where you need
function constructObject (){
get_sent(uId, cId, function(err, total){
if(err){
console.log("Something was wrong.", err);
}
result.sent = total;
//Here you have your object completed
console.log(result);
});
}
//You need to use a callback
function get_sent(uId, cId, callback) {
pool.getConnection(function(err, connection) {
//Note that I add an alias here
connection.query("SELECT count(*) as total FROM table_name", function(err, rows) {
connection.release();
if (!err) {
//I am returning the result of the query and a null error
callback(err, rows[0].total);
} else {
console.log(err);
//I am returning an error
callback(err);
}
});
});
}
//For example you could call this function here
constructObject();
And it depends of what are you doing exactly but Maybe you need a callback on your constructObject too.
I'm trying to design a webpage. I have a function that I call to get all info needed for an individual's home page. A snippet of the code is:
exports.getHomePageData = function(userId, cb) {
var pageData = {};
pageData.userFullName = dbUtil.findNameByUserId(userId, function(err){
if (err) cb(err);
});
pageData.classes = dbUtil.findUserClassesByUserId(userId, function(err){
if (err) cb(err);
});
cb(pageData);
}
The problem I'm having is that the cb(pageData) is being called before I even finish setting the elements.
I've seen that people use the async library to solve this, but I was wondering if there was any other way for me to do it without needing more modules.
One possible approach:
exports.getHomePageData = function(userId, cb) {
var pageData = {},
filler = function() {
if ('userFullName' in pageData
&& 'classes' in pageData)
cb(null, pageData);
};
dbUtil.findNameByUserId(userId, function(err, name) {
if (err) {
cb(err);
return;
}
pageData.userFullName = name;
filler();
});
dbUtil.findUserClassesByUserId(userId, function(err, classes) {
if (err) {
cb(err);
return;
}
pageData.classes = classes;
filler();
});
}
It looks like dbUtil.findUserClassesByUserId and dbUtil.findNameByUserId are asynchronous methods; that usually indicates that they do not return a value, and instead use the callback to give you the data.
Both functions are most likely expecting a signature like follows:
function(err, data) {
// if err is set, an error occurred, otherwise data is set with the result
}
Thus, your function should look like this instead:
exports.getHomePageData = function(userId, cb) {
dbUtil.findNameByUserId(userId, function(err, userFullName){
if (err) {
cb(err);
return;
}
dbUtil.findUserClassesByUserId(userId, function(err, classes){
if (err) {
cb(err);
return;
}
var pageData = {
userFullName: userFullName,
classes: classes
};
cb(pageData);
});
});
}