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Using push for inserting JS object into an array wont work at certain context

In the below code, users.push used within ‘db.each’ wont work. However, if I move ‘users.push’ outside then it seems to work.
How can I push the new objects from db.each into the users array?
let db = new sqlite3.Database('./db/main.db', (err) => {
if (err) console.error(err.message);
console.log('Connected to the main database.');
});
var users = [];
db.serialize(() => {
db.each(`SELECT email, name FROM users`, (err, row) => {
if (err) console.error(err.message);
let user = {
email: row.email,
name: row.name
}
users.push(user);
});
});
console.log(JSON.stringify(users));
db.close();
I am using express and sqlite3 node packages.

It's because db.serializeand db.each are asynchronous functions (and return immediately, thus executing console.log before the db callbacks are executed).
Here should be a working example :
db.serialize(() => {
db.each(`SELECT email,
name
FROM users`, (err, row) => {
if (err) {
console.error(err.message);
}
let user = {
email : row.email,
name : row.name
}
users.push(user);
console.log(JSON.stringify(users));
db.close();
});
});

First error: asynchronicity not handled properly
As Antoine Chalifour pointed out, you call console.log(JSON.stringify(users)); before users gets modified in the asynchronous callback. Refer to his answer for fix and explanations.
Second error: errors not handled
You wrote if (err) { console.error(err.message); } then go on with the rest of the function. That is bad, because an error might happen and you'd just continue with your program. You should instead write something like:
if (err) {
console.error(err);
return;
}
or:
if (err) throw err;

Nodejs express and promises not doing what I expect

I am trying to build a login API using NodeJS, but my code is not doing what I expect it to. I am very new to js, promises and all so please simplify any answer if possible.
From what I can see in the output of my code, the first promise part does not wait until the function findUsers(...) is finished.
I have a routes file where I want to run a few functions sequentially:
Find if user exist in database
if(1 is true) Hash and salt the inputted password
... etc
The routes file now contains:
var loginM = require('../models/login');
var loginC = require('../controllers/login');
var Promise = require('promise');
module.exports = function(app) {
app.post('/login/', function(req, res, next) {
var promise = new Promise(function (resolve, reject) {
var rows = loginM.findUser(req.body, res);
if (rows.length > 0) {
console.log("Success");
resolve(rows);
} else {
console.log("Failed");
reject(reason);
}
});
promise.then(function(data) {
return new Promise(function (resolve, reject) {
loginC.doSomething(data);
if (success) {
console.log("Success 2");
resolve(data);
} else {
console.log("Failed 2");
reject(reason);
}
});
}, function (reason) {
console.log("error handler second");
});
});
}
And the findUser function contains pooling and a query and is in a models file:
var connection = require('../dbConnection');
var loginC = require('../controllers/login');
function Login() {
var me = this;
var pool = connection.getPool();
me.findUser = function(params, res) {
var username = params.username;
pool.getConnection(function (err, connection) {
console.log("Connection ");
if (err) {
console.log("ERROR 1 ");
res.send({"code": 100, "status": "Error in connection database"});
return;
}
connection.query('select Id, Name, Password from Users ' +
'where Users.Name = ?', [username], function (err, rows) {
connection.release();
if (!err) {
return rows;
} else {
return false;
}
});
//connection.on('error', function (err) {
// res.send({"code": 100, "status": "Error in connection database"});
// return;
//});
});
}
}
module.exports = new Login();
The output i get is:
Server listening on port 3000
Something is happening
error handler second
Connection
So what I want to know about this code is twofold:
Why is the first promise not waiting for findUser to return before proceeding with the if/else and what do I need to change for this to happen?
Why is error handler second outputed but not Failed?
I feel like there is something I am totally misunderstanding about promises.
I am grateful for any answer. Thanks.

Issues with the code
Ok, there are a lot of issues here so first things first.
connection.query('...', function (err, rows) {
connection.release();
if (!err) {
return rows;
} else {
return false;
}
});
This will not work because you are returning data to the caller, which is the database query that calls your callback with err and rows and doesn't care about the return value of your callback.
What you need to do is to call some other function or method when you have the rows or when you don't.
You are calling:
var rows = loginM.findUser(req.body, res);
and you expect to get the rows there, but you won't. What you'll get is undefined and you'll get it quicker than the database query is even started. It works like this:
me.findUser = function(params, res) {
// (1) you save the username in a variable
var username = params.username;
// (2) you pass a function to getConnection method
pool.getConnection(function (err, connection) {
console.log("Connection ");
if (err) {
console.log("ERROR 1 ");
res.send({"code": 100, "status": "Error in connection database"});
return;
}
connection.query('select Id, Name, Password from Users ' +
'where Users.Name = ?', [username], function (err, rows) {
connection.release();
if (!err) {
return rows;
} else {
return false;
}
});
//connection.on('error', function (err) {
// res.send({"code": 100, "status": "Error in connection database"});
// return;
//});
});
// (3) you end a function and implicitly return undefined
}
The pool.getConnection method returns immediately after you pass a function, before the connection to the database is even made. Then, after some time, that function that you passed to that method may get called, but it will be long after you already returned undefined to the code that wanted a value in:
var rows = loginM.findUser(req.body, res);
Instead of returning values from callbacks you need to call some other functions or methods from them (like some callbacks that you need to call, or a method to resolve a promise).
Returning a value is a synchronous concept and will not work for asynchronous code.
How promises should be used
Now, if your function returned a promise:
me.findUser = function(params, res) {
var username = params.username;
return new Promise(function (res, rej) {
pool.getConnection(function (err, connection) {
console.log("Connection ");
if (err) {
rej('db error');
} else {
connection.query('...', [username], function (err, rows) {
connection.release();
if (!err) {
res(rows);
} else {
rej('other error');
}
});
});
});
}
then you'll be able to use it in some other part of your code in a way like this:
app.post('/login/', function(req, res, next) {
var promise = new Promise(function (resolve, reject) {
// rows is a promise now:
var rows = loginM.findUser(req.body, res);
rows.then(function (rowsValue) {
console.log("Success");
resolve(rowsValue);
}).catch(function (err) {
console.log("Failed");
reject(err);
});
});
// ...
Explanation
In summary, if you are running an asynchronous operation - like a database query - then you can't have the value immediately like this:
var value = query();
because the server would need to block waiting for the database before it could execute the assignment - and this is what happens in every language with synchronous, blocking I/O (that's why you need to have threads in those languages so that other things can be done while that thread is blocked).
In Node you can either use a callback function that you pass to the asynchronous function to get called when it has data:
query(function (error, data) {
if (error) {
// we have error
} else {
// we have data
}
});
otherCode();
Or you can get a promise:
var promise = query();
promise.then(function (data) {
// we have data
}).catch(function (error) {
// we have error
});
otherCode();
But in both cases otherCode() will be run immediately after registering your callback or promise handlers, before the query has any data - that is no blocking has to be done.
Summary
The whole idea is that in an asynchronous, non-blocking, single-threaded environment like Node.JS you never do more than one thing at a time - but you can wait for a lot of things. But you don't just wait for something and do nothing while you're waiting, you schedule other things, wait for more things, and eventually you get called back when it's ready.
Actually I wrote a short story on Medium to illustrate that concept: Nonblacking I/O on the planet Asynchronia256/16 - A short story loosely based on uncertain facts.

Nodejs get mysql insertId and handle errors

i want to simply handle error and get insertId after created new row on database, in my code which i get help from mysqljs/mysql official page, i can't do them, how can i fix my code to do them?
var post = {deviceImei: deviceInformation.deviceImei, created_at: currentDateTime, updated_at: currentDateTime};
var query = connection.query('INSERT INTO users SET ?', post, function (err, result) {
if (err)
throw err;
return result;
});
query.on('error', function (err) {
console.log(err)
}).on('fields', function (fields) {
console.log(fields)
}).on('result', function (row) {
console.log(row.insertId);
}).on('end', function () {
// all rows have been received
});
after insert or get some error i cant any message on shell by console

The correct way to do that is documented here:
connection.query('INSERT INTO users SET ?', post, function(err, result) {
if (err) throw err;
console.log(result.insertId);
});

You can use native ES6 promises supported in newest versions of node
var insertQuery = function(post) {
return new Promise(function(resolve, reject) {
connection.query('INSERT INTO users SET ?', post, function (err, result) {
if (err)
reject(error);
resolve(result);
}
});
}
The you can handle after
insertQuery(post)
.then(function(result) {
console.log(result.inserId);
})
.catch(function(error) {
console.log(err);
})
There are libs like 'q' or 'bluebird' which are easy to use too

Sequelize JS node updating multiple entities with new data from previous promise

I am new to sequelize and NodeJS promises in general. My app basically saves tweets from the Twitter API, but also needs to update some of the saved tweets' data in realtime, like the Retweet count or the Liked count.
But it seems like after fetching the new data, when trying to run all the update on my tweet instances, nothing happens. The promise doesn't go through.
To sum up : I find 100 saved tweets, chain on a callback that fetches their new data from Twitter, and then chain on updating every single 100 tweets with the new data. The later update doesn't go through.
var Sequelize = require('sequelize');
...
//Getting 100 tweets previously saved in DB
Sequelize.query("SELECT * FROM tweets WHERE ORDER BY id DESC LIMIT 100", { model: Model }).then(function(result) {
if(result.length == 0) {
callback(false);
} else {
var ids = [];
var realData = {};
for (var i in result) {
realData[result[i].dataValues.id_str] = result[i];
ids.push(result[i].dataValues.id_str);
}
//getting twitter data for 100 tweets previously saved in DB
twitVendor.get('statuses/lookup', {
id : ids.join(',')
}, function (err, tweets, response) {
if (typeof err == 'undefined') {
//to get a synchronous saving of all tweets
//this could be cleaned up with a Sequelize.Promise.push(...)
var i = 0;
var saving = false;
while (i < tweets.length) {
if (!saving) {
saving = true;
console.log('Updating tweet ', tweets[i].id_str);
//updating tweet with new data from twitter
Sequelize.query("UPDATE tweets SET retweet_count = "+tweets[i].retweet_count+", favorite_count = "+tweets[i].favorite_count+" WHERE id_str = '"+tweets[i].id_str+"'", {
model: Model
}).then(function(result) {
console.log('Updated tweet');
saving = false;
i++;
}).catch(function (err) {
console.log('Failed to update post ', err);
saving = false;
i++;
});
}
}
callback(true);
console.log("Updated tweets");
} else {
console.log("Failed :", err);
callback(false, err);
}
});
}
}).catch(function (err) {
console.log("Failed :", err);
callback(false, err);
})
EDIT : If you want to execute the above code, I'd recommend using this Twit to hit the Twitter API : https://github.com/ttezel/twit
To get credentials to hit the API, you will need to set up an app on Twitter though : https://apps.twitter.com/
EDIT 2 : I already tried to use transactions and pure Sequelize functions to make my queries, but the issue stood still.

Don't nest promises inside of promises. Instead, chain them by returning promises. If you are returning something that is not a promise, use Promise.resolve(value) to turn it into a promise. And certainly don't put promises inside of callbacks, or even mix them at all; instead create a promise that calls the action, and then in the callback resolve the promise.
Here's my attempt to rewrite what you're trying to do. You may need to wrap the first in a Promise.resolve to take advantage of returning the new promise:
Sequelize.query("SELECT * FROM tweets WHERE ORDER BY id DESC LIMIT 100"
, { model: Model }).then(function (results) {
if (results.length == 0) {
return Promise.reject(false); //reject to hit the catch of the promise. Change false to error.
}
var ids = [];
var realData = {};
for (var i in result) {
realData[result[i].dataValues.id_str] = result[i];
ids.push(result[i].dataValues.id_str);
}
return new Promise((resolve, reject) => {
twitVendor.get('status/lookup', {
id: ids.join(',')
}, function (err, tweets, response) {
if (err) {
reject(false); //reject to hit the catch of the promise. Change false to error message
}
resolve(tweets);
})
})
}).then(function (tweets) {
function updateTweet(tweet) {
return sequelize.query(...);
}
var updatesInParallel = tweets.map(updateTweet);
return Promise.all([updatesInParallel]);
}).then(function () {
callback(true);
}).catch(function (error) {
console.log("failed: ", error);
callback(false)
});

How do you do a "join" on an array in mongoose (mongodb) with node.js?

How do you do a "join" (i know it is not the correct term) with an array of messages in mongoose?
I tried looping over all the messages and querying to get the user info but it is not working:
messages.forEach(function (message, index) {
User.findById(message.userId, function (err, user) {
messages[index].user = user
})
})
console.log(messages) // the user info is not attatched
So how is this accomplished with mongoose and node.js?

the biggest problem with your code is, that you assume the code to run synchronously - but it doesn't. it runs asynchronously. so messages is not yet set when you execute
console.log(messages);
do something like this instead:
var userIds = [id1, id2, id3];
User.find({"_id": {$in: userIds}}, function (err, users) {
console.log(users);
});
edit
ok, i see. you want to add the userInfo to the different messages.
easiest way to acieve this, is to use the async module: https://github.com/caolan/async
async.map(messages, getUserInfo, function (err, result) {
if (err) {
console.log(err);
return;
}
// log all msg with userinfo
console.log(result);
});
function getUserInfo (msg, callback) {
User.findById(msg.userId, function (err, user) {
if (err) {
callback(err);
return;
}
msg.user = user;
callback(null, msg);
});
}