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Returning Callback in Javascript

Taking inspiration from this answer https://stackoverflow.com/a/21571589/9036255 I decided to copy this into my own code. I've found that it doesn't give any response.
function getCallback(callbackTrash) {
var Books = mongoose.model('Book');
Books.find({'username': name[usernames]}, function (err, result) {
callbackTrash(result);
});
}
getCallback(function(result) {
books.push(result);
});
books is just an array. When I console log it later on, it returns "[]"
If I change books.push to console.log, it correctly logs all of the results.
My question is how to get the result to be pushed into the books array?
I have no experience with asynchronous things. What's a practical answer without a steep learning curve?
Here's my greater context:
exports.timeline = function(req, res) {
var Followers = mongoose.model('Follow');
Followers.find({'follower': req.user.username}, function(err, followerResult) {
var name = [req.user.username];
var books = [];
function addName(username) {
name.push(username);
}
for(var user in followerResult) {
addName(followerResult[user]['followed']);
}
for(var usernames in name) {
function getCallback(callbackTrash) {
var Books = mongoose.model('Book');
Books.find({'username': name[usernames]}, function (err, loadOfBollocks) {
callbackTrash(loadOfBollocks);
});
}
getCallback(function(result) {
books.push(result);
});
}
console.log(books);
res.send(books);
});
}

I think that your code is correct. But:
where did you initialize "books" ?
Where did you console log it ?
EDIT
Ok i see now. You put you console and res.send in the wrong place.
Those lines are executed before your loop (for) process is finished That's why you have an empty array for "books".
Maybe you need to executed these codes only when you have finished to loop in the "name" array. Here is the code:
exports.timeline = function(req, res) {
var Followers = mongoose.model('Follow');
Followers.find({'follower': req.user.username}, function(err, followerResult) {
var name = [req.user.username];
var books = [];
function addName(username) {
name.push(username);
}
for(var user in followerResult) {
addName(followerResult[user]['followed']);
}
let count = 0;
for(var usernames in name) {
function getCallback(callbackTrash) {
var Books = mongoose.model('Book');
Books.find({'username': name[usernames]}, function (err, loadOfBollocks) {
count++;
callbackTrash(loadOfBollocks);
if(count == name.length){
console.log(books);
res.send(books);
}
});
}
getCallback(function(result) {
books.push(result);
});
}
});
}
Hope it helps !

How to wait to render page with express, while API grabs data?

I am trying to load data from the twitter api, getting user information and save that in a temporary array. That array will then be loaded on the page for viewing. The array is getting loaded by the API call, but it doesn't display.
I think I need to use an asynchronous thing like React or Angular, not sure. Would love some input!
function getUserIds (userId) {
T.get('statuses/retweeters/ids', { id: userId }, function (err, data, response) {
for(var i = 0; i < data.ids.length; i++){
ids.push(data.ids[i]);
}
getUserInfo();
});
}
function getUserInfo() {
for(var i = 0; i < ids.length; i++) {
T.get('users/lookup', { user_id: ids[i] }, function (err, data, response) {
names.push(data[0].screen_name);
pics.push(data[0].profile_image_url_https);
console.log(names);
});
}
res.render('display', {names: names, pics:pics});
}

The issue is that you are running ids.length async calls and those will finish some time in the future. You have to render your page only when they are all done. But, your for loop is synchronous so you are calling res.render() before any of them have finished. In addition, your T.get() calls may finish in any order (if that matters).
I would normally use promises for coordinating multiple asynchronous operations since it is a very, very good tool for that. But, if you aren't using promises, here's a simple technique to test when you have all your results back:
function getUserInfo() {
var names = [];
var pics = [];
for(var i = 0; i < ids.length; i++) {
T.get('users/lookup', { user_id: ids[i] }, function (err, data, response) {
if (err) {
// decide what to display if you get an API error
names.push("unknown due to API error");
} else {
names.push(data[0].screen_name);
pics.push(data[0].profile_image_url_https);
console.log(names);
}
if (names.length === ids.length) {
res.render('display', {names: names, pics:pics});
}
});
}
}
As I said above, this does not necessarily collect the results in order. If you need them in order, then you could do something like this:
function getUserInfo() {
var names = new Array(ids.length);
var pics = new Array(ids.length);
var doneCntr = 0;
ids.forEach(function(id, i) {
T.get('users/lookup', { user_id: id }, function (err, data, response) {
if (err) {
// decide what to display if you get an API error
names[i] = "unknown due to API error";
} else {
names[i] = data[0].screen_name;
pics[i] = data[0].profile_image_url_https;
}
++doneCntr;
if (doneCntr === ids.length) {
res.render('display', {names: names, pics: pics});
}
});
});
}
My preferred solution would to be to use Promise.all() and use a promisified version of T.get().

Log only shows one of four rows of data

I am writing a small Node js application for automatic vehicle location system.
Here is the code for where I am getting trouble.
markerData contains 4 rows but only in the log I can see the last row.
for (var i = 0, len = markerData.length; i < len; i++) {
var thisMarker = markerData[i];
sql.connect(config, function (err) {
var request = new sql.Request();
request.input('myval', sql.Int, thisMarker.id);
request.query('SELECT d.id, d.name, d.lastupdate, p.latitude, p.longitude, p.speed, p.course FROM dbo.devices AS d INNER JOIN dbo.positions AS p ON d.positionid = p.id AND d.id = p.deviceid WHERE (d.id = #myval)', function (err, recordset2) {
if (typeof recordset2 != 'undefined') {
thisMarker.position.lat = recordset2[0].latitude;
thisMarker.position.long = recordset2[0].longitude;
console.log(recordset2[0].id);
}
});
});
}
Please help me to solve the issue.

As var is not a block level variable in terms of scope, when `sql' module takes time to connect to the database asynchronously, the synchronous loop may change the value of the variable that's why you have the last row printed since the variable holds the reference to the last object at the time of successful connection.
Instead of _.each, I would recommend to use async module with async.each since you have few asynchronous operation to get rid of a synchronous loop.
You can check for samples here,
http://justinklemm.com/node-js-async-tutorial/
Here is your updated code with async.each
-> Install async module with npm install async --save
-> Then add the below reference in the required place,
// Reference
var async = require('async');
-> Modified code:
sql.connect(config, function (err) {
if(err) {
console.log('Connection error: ');
console.log(err);
} else {
async.each(markerData, function(thisMarker, callback) {
var request = new sql.Request();
request.input('myval', sql.Int, thisMarker.id);
request.query('SELECT d.id, d.name, d.lastupdate, p.latitude, p.longitude, p.speed, p.course FROM dbo.devices AS d INNER JOIN dbo.positions AS p ON d.positionid = p.id AND d.id = p.deviceid WHERE (d.id = #myval)', function (err, recordset2) {
if(err) {
console.log(err);
callback();
} else {
if (typeof recordset2 != 'undefined') {
thisMarker.position.lat = recordset2[0].latitude;
thisMarker.position.long = recordset2[0].longitude;
console.log(recordset2[0].id);
} else {
console.log('Recordset empty for id: ' + thisMarker.id);
}
callback();
}
});
}, function(err){
if(err) {
console.log(err);
}
});
}
});

I'm not entirely sure how your library works, but presumably recordset2 is an array of records. recordset2[0] is therefore the first record. If you want the next one you should probably try recordset2[1] and so on and so forth.
Arrays: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
You'll probably need to loop through all the elements in the array at some point. use a for loop for that:
for (var i = 0; i < recordset2.length; i++ {
console.log(recordset2[i])
}
That will print out everything your query returns.

node.js return data from find collection in loop

I am trying to return data from this function. Console.log(documents) successfully shows the data in console. But this works only in body of the function. I can't return this data to the template. What should I do? Should I use some async package for node.js, or can be accomplished somehow like this?
Thank you.
var projects = req.user.projects;
var docs = [];
db.collection('documents', function(err, collection) {
for (i = 0; i < projects.length; i++) {
collection.find({'_projectDn': projects[i].dn},function(err, cursor) {
cursor.each(function(err, documents) {
if(documents != null){
console.log(documents);
//or docs += documents;
}
});
});
}
});
console.log(documents); // undefined
res.render('projects.handlebars', {
user : req.user,
documents: docs
});

Those db functions are async, which means that when you try to log it, the function hasn't finished yet. You can log it using a callback, for example:
function getDocuments(callback) {
db.collection('documents', function(err, collection) {
for (i = 0; i < projects.length; i++) {
collection.find({
'_projectDn': projects[i].dn
}, function(err, cursor) {
cursor.each(function(err, documents) {
if (documents !== null) {
console.log(documents);
callback(documents);// run the function given in the callback argument
}
});
});
}
});
}
//use the function passing another function as argument
getDocuments(function(documents) {
console.log('Documents: ' + documents);
});

Javascript async in nested for loop MongoDB

I have an asynchronous function inside a for loop nested in another for loop.
// recipesArray is an array of arrays of objects
// recipeObject is an array of objects
// currentRecipe is an object
connectToDb(function(){
// LOOP 1
for (var i=0, l=recipesArray.length; i < l; i++) {
// recipeObject is an
var recipeObject = recipesArray[i];
// LOOP 2
for (var x=0, y=recipeObject.length; x < y; x++) {
var currentRecipe = recipeObject[x];
// this is an asynchronous function
checkRecipe(currentRecipe, function (theRecipe) {
if (theRecipe === undefined) {
console.log('RECIPE NOT FOUND');
} else {
console.log('RECIPE FOUND', theRecipe);
}
});
}
}
});
I need to add data to the recipesArray based on the results of the checkRecipe function.
I've been trying different things...
- do i try to keep track of i and x...
- do i try to have multiple callbacks...
- do i even need to do all of that, or is there some other way....
I also tried using the async library for node(which actually has been very helpful with other situations), but the forEach doesn't take objects(only an array).
Stuck.
Any suggestions would be greatly appreciated.

Assuming checkRecipe() can be run in parallel with no limits, here's how you might use async.each():
connectToDb(function() {
async.each(recipesArray, function(subArray, callback) {
async.each(subArray, function(currentRecipe, callback2) {
checkRecipe(currentRecipe, function(theRecipe) {
if (theRecipe === undefined)
return callback2(new Error('Recipe not found'));
callback2();
});
}, callback);
}, function(err) {
if (err)
return console.error('Error: ' + err);
// success, all recipes found
});
});