How do I get data from a query OUTSIDE the query in Node.JS?
I can get results printed to the console INSIDE the function but I can't get the data OUTSIDE to use in other places in my application.
this.getMyQuestion = function(id) {
var query = connection.query('select * from questions where id = ' + connection.escape(id), function(err, result) {
if(err) {
console.error(err);
return;
}
//console.log(result[0].question); //displays in console
return(result[0].question);
});
}
var test = this.getMyQuestion(1);
console.log(test) //returns undefined.
You're using an asynchronous function, so by the time you run the console.log(test) after calling your function, it hasn't finished running and hasn't returned anything yet.
That's what callbacks are for. You pass a callback to your function, and when it finishes execution, it calls that callback function instead of returning a value like it is now.
For example :
this.getMyQuestion = function(id, callback) {
var query = connection.query('select * from questions where id = ' + connection.escape(id), function(err, result) {
callback(null, result[0].question);
});
}
this.getMyQuestion(1, function(err, question){
// Do what you want
});
You can also promises to get data from async functions.
Related
I send two query sequentially
Query the data from A tables, and then accoring to the result, query the data from B table.
So, I query the data like that,
var async = require('async');
var mysql = require('mysql');
var config = require('./config.json');
var connection = mysql.createConnection({
host : config.dbhost,
user : config.dbuser,
password : config.dbpassword,
database : config.dbname
});
exports.handler = (event, context, callback) => {
// TODO implement
var tasks = [
function (callback) {
connection.query("SELECT email FROM Visitor WHERE id =?;", [1], function (err, row) {
if (err) return callback(err);
if (row.length == 0) return callback('No Result Error');
callback(null, row[0]);
})
},
function (data, callback) {
connection.query("SELECT id,signtime FROM Board WHERE email =?;", data.email, function (err, row) {
if (err) return callback(err);
if (row.length == 0) {
return callback('No Result Error');
}else {
callback(null, row[0])
}
})
}
];
async.waterfall(tasks, function (err, result) {
if (err)
console.log('err');
else
***return result;***
console.log('done');
connection.end();
});
};
I log the data with console.log(), it take the data in command line.
But in lambda, put the function into exports.handler, it response null.
If I change the 'return result' to callback(result), it occurs error.
I think it maybe too simple to solve this problem
If you know about that, please help me
In the first case, response is null because you didn't use neither Promise, nor callback to let the Lambda sandbox know that the job is done. In the second case, you used the callback, but you passed the result as the first argument to it. Lambda programming model for Node.js follows a principle called "error first callback". Long story short, if any error occurred during execution, you should go with callback(error), and if everything is ok and you need to return some result from lambda, you should go with callback(null, result). So basically on your line before console.log('done'); use callback(null, result) and it will work for you.
In the code
var stuff_i_want = '';
stuff_i_want = get_info(parm);
And the function get_info:
get_info(data){
var sql = "SELECT a from b where info = data"
connection.query(sql, function(err, results){
if (err){
throw err;
}
console.log(results[0].objid); // good
stuff_i_want = results[0].objid; // Scope is larger than function
console.log(stuff_i_want); // Yep. Value assigned..
}
in the larger scope
stuff_i_want = null
What am i missing regarding returning mysql data and assigning it to a variable?
============ New code per Alex suggestion
var parent_id = '';
get_info(data, cb){
var sql = "SELECT a from b where info = data"
connection.query(sql, function(err, results){
if (err){
throw err;
}
return cb(results[0].objid); // Scope is larger than function
}
==== New Code in Use
get_data(parent_recording, function(result){
parent_id = result;
console.log("Parent ID: " + parent_id); // Data is delivered
});
However
console.log("Parent ID: " + parent_id);
In the scope outside the function parent_id is null
You're going to need to get your head around asynchronous calls and callbacks with javascript, this isn't C#, PHP, etc...
Here's an example using your code:
function get_info(data, callback){
var sql = "SELECT a from b where info = data";
connection.query(sql, function(err, results){
if (err){
throw err;
}
console.log(results[0].objid); // good
stuff_i_want = results[0].objid; // Scope is larger than function
return callback(results[0].objid);
})
}
//usage
var stuff_i_want = '';
get_info(parm, function(result){
stuff_i_want = result;
//rest of your code goes in here
});
When you call get_info this, in turn, calls connection.query, which takes a callback (that's what function(err, results) is
The scope is then passed to this callback, and so on.
Welcome to javascript callback hell...
It's easy when you get the hang of it, just takes a bit of getting used to, coming from something like C#
I guess what you really want to do here is returning a Promise object with the results. This way you can deal with the async operation of retrieving data from the DBMS: when you have the results, you make use of the Promise resolve function to somehow "return the value" / "resolve the promise".
Here's an example:
getEmployeeNames = function(){
return new Promise(function(resolve, reject){
connection.query(
"SELECT Name, Surname FROM Employee",
function(err, rows){
if(rows === undefined){
reject(new Error("Error rows is undefined"));
}else{
resolve(rows);
}
}
)}
)}
On the caller side, you use the then function to manage fulfillment, and the catch function to manage rejection.
Here's an example that makes use of the code above:
getEmployeeNames()
.then(function(results){
render(results)
})
.catch(function(err){
console.log("Promise rejection error: "+err);
})
At this point you can set up the view for your results (which are indeed returned as an array of objects):
render = function(results){ for (var i in results) console.log(results[i].Name) }
Edit
I'm adding a basic example on how to return HTML content with the results, which is a more typical scenario for Node. Just use the then function of the promise to set the HTTP response, and open your browser at http://localhost:3001
require('http').createServer( function(req, res){
if(req.method == 'GET'){
if(req.url == '/'){
res.setHeader('Content-type', 'text/html');
getEmployeeNames()
.then(function(results){
html = "<h2>"+results.length+" employees found</h2>"
html += "<ul>"
for (var i in results) html += "<li>" + results[i].Name + " " +results[i].Surname + "</li>";
html += "</ul>"
res.end(html);
})
.catch(function(err){
console.log("Promise rejection error: "+err);
res.end("<h1>ERROR</h1>")
})
}
}
}).listen(3001)
Five years later, I understand asynchronous operations much better.
Also with the new syntax of async/await in ES6 I refactored this particular piece of code:
const mysql = require('mysql2') // built-in promise functionality
const DB = process.env.DATABASE
const conn = mysql.createConnection(DB)
async function getInfo(data){
var sql = "SELECT a from b where info = data"
const results = await conn.promise().query(sql)
return results[0]
}
module.exports = {
getInfo
}
Then, where ever I need this data, I would wrap it in an async function, invoke getInfo(data) and use the results as needed.
This was a situation where I was inserting new records to a child table and needed the prent record key, based only on a name.
This was a good example of understanding the asynchronous nature of node.
I needed to wrap the all the code affecting the child records inside the call to find the parent record id.
I was approaching this from a sequential (PHP, JAVA) perspective, which was all wrong.
Easier if you send in a promise to be resolved
e.g
function get_info(data, promise){
var sql = "SELECT a from b where info = data";
connection.query(sql, function(err, results){
if (err){
throw err;
}
console.log(results[0].objid); // good
stuff_i_want = results[0].objid; // Scope is larger than function
promise.resolve(results[0].objid);
}
}
This way Node.js will stay fast because it's busy doing other things while your promise is waiting to be resolved
I've been working on this goal since few weeks, without any result, and I finally found a way to assign in a variable the result of any mysql query using await/async and promises.
You don't need to understand promises in order to use it, eh, I don't know how to use promises neither anyway
I'm doing it using a Model class for my database like this :
class DB {
constructor(db) {
this.db = db;
}
async getUsers() {
let query = "SELECT * FROM asimov_users";
return this.doQuery(query)
}
async getUserById(array) {
let query = "SELECT * FROM asimov_users WHERE id = ?";
return this.doQueryParams(query, array);
}
// CORE FUNCTIONS DON'T TOUCH
async doQuery(queryToDo) {
let pro = new Promise((resolve,reject) => {
let query = queryToDo;
this.db.query(query, function (err, result) {
if (err) throw err; // GESTION D'ERREURS
resolve(result);
});
})
return pro.then((val) => {
return val;
})
}
async doQueryParams(queryToDo, array) {
let pro = new Promise((resolve,reject) => {
let query = queryToDo;
this.db.query(query, array, function (err, result) {
if (err) throw err; // GESTION D'ERREURS
resolve(result);
});
})
return pro.then((val) => {
return val;
})
}
}
Then, you need to instantiate your class by passing in parameter to constructor the connection variable given by mysql. After this, all you need to do is calling one of your class methods with an await before. With this, you can chain queries without worrying of scopes.
Example :
connection.connect(function(err) {
if (err) throw err;
let DBModel = new DB(connection);
(async function() {
let oneUser = await DBModel.getUserById([1]);
let allUsers = await DBModel.getUsers();
res.render("index.ejs", {oneUser : oneUser, allUsers : allUsers});
})();
});
Notes :
if you need to do another query, you just have to write a new method in your class and calling it in your code with an await inside an async function, just copy/paste a method and modify it
there are two "core functions" in the class, doQuery and doQueryParams, the first one only takes a string as a parameter which basically is your mysql query. The second one is used for parameters in your query, it takes an array of values.
it's relevant to notice that the return value of your methods will always be an array of objects, it means that you'll have to do var[0] if you do a query which returns only one row. In case of multiple rows, just loop on it.
How do I set a variable to a query? I am trying to use functions and callbacks in node.js to work through async, but I am not sure how to get a query to equal to a variable. What I am trying to do in this code is take a friend collection that belongs to a user and return the friends result(which I don't think I am doing correctly in the query insertAll) and then find the user's info for each of the query. And then return the results as a render. I am not sure how to call render either with this...
Here is my code:
exports.contactList = function(req, res) {
var insertFriend = function(data, callback) {
var friend = User.findById({_id: user.friendStatus.fuId}, function() {
callback(null, data);
}, friend);
};;
var insertAll = function(coll, callback) {
var queue = coll.slice(0),
friendX;
(function iterate(){
if(queue.length === 0) {
callback();
return;
}
friendX = queue.splice(0,1)[0];
insertFriend(friendX, function(err, friendX) {
if(err) {throw err;}
console.log(friendX + ' inserted');
process.nextTick(iterate);
});
})();
};
insertAll([Friend.findOne({userId: req.signedCookies.userid})], function(){
});
};
A Query object is returned if you do not pass a callback.
From http://mongoosejs.com/docs/queries.html:
When a callback function:
is passed, the operation will be executed immediately with the results passed to the
callback.
is not passed, an instance of Query is returned, which provides a special QueryBuilder
interface for you.
I'm developing an application and need to add many items at a time.
How can I do that with node.js?
This is the npm module for parse.com but there is no method like
insertAll("Foo", [objs...], ...)
I don't want to insert single object every time.
Write a convenience function that interfaces between your application and parse.com. You will have to write the iteration code once (or debug mine)
var async = require('async');
var parseApp = require('node-parse-api').Parse;
var APP_ID = "";
var MASTER_KEY = "";
var parseApp = new Parse(APP_ID, MASTER_KEY);
function insertAll(class, objs, callback){
// create an iterator function(obj,done) that will insert the object
// with an appropriate group and call done() upon completion.
var insertOne =
( function(class){
return function(obj, done){
parseApp.insert(class, obj, function (err, response) {
if(err){ return done(err); }
// maybe do other stuff here before calling done?
var res = JSON.parse(response);
if(!res.objectId){ return done('No object id') };
done(null, res.objectId);
});
};
} )(class);
// async.map calls insertOne with each obj in objs. the callback is executed
// once every iterator function has called back `done(null,data)` or any one
// has called back `done(err)`. use async.mapLimit if throttling is needed
async.map(objs, insertOne, function(err, mapOutput){
// complete
if(err){ return callback(err) };
// no errors
var objectIds = mapOutput;
callback(null, objectIds);
});
};
// Once you've written this and made the function accessible to your other code,
// you only need this outer interface.
insertAll('Foo', [{a:'b'}, {a:'d'}], function(err, ids){
if(err){
console.log('Error inserting all the Foos');
console.log(err);
} else {
console.log('Success!);
};
});
I am performing database connection in nodejs module. But its callback is not called.
Here is my module-
*getChapterList.js
var mysql=require('mysql');
var client=mysql.createClient({
user:'mysql',
password:''
});
client.useDatabase('test');
module.exports.get_chapter_list = function(subject_id){
client.query("select distinct chapter_id from course_associations where subject_id="+subject_id+" and status_id=1",
function(err,results,fields){
return results;
});
return "hello";
};
Now i am calling this module as-
rs=require('./getChapterList');
rs.get_chapter_list(1);
// Output: hello
but expected o/p is results array.
Googled alot.but no result..
The callback will be called after the query completes and the return value of results will be passed back to the method that created the callback, which then discards it.
The reason why the output is "hello", is because that's what the get_chapter_list function returns.
What happens is:
You call the get_chapter_list function
The client.query fires off a request to the database
The client.query function returns.
The get_chapter_list function returns "hello".
The SQL query completes and calls the callback
Your callback method is called and does nothing (it just returns the results, but that return value is handed back to the caller of the callback (somewhere within client.query) which discards it).
To get what you want you'll probably need to think asynchronously.
Define the method as
module.exports.get_chapter_list = function(subject_id, callback){
client.query("select distinct chapter_id from course_associations where subject_id="+subject_id+" and status_id=1",
function(err,results,fields){
// doesn't contain any error handling
callback(results);
});
};
and then call the method:
rs.get_chapter_list(1, function(results) {
console.log(results); // or whatever you need to do with the results
});
You need to return the results asynchronously:
exports.get_chapter_list = function(subject_id, callback) {
client.query("select ...", callback);
};
...
var rs = require('./getChapterList');
rs.get_chapter_list(1, function(err, results) {
if (err) {
// handle error
} else {
// do something with results
console.log(results);
}
});
scttnlsn answer is correct, as I have faced this issue and had solved by just passing the callback function as a argument.
Try for that:
var mysql=require('mysql');
var client=mysql.createClient({
user:'mysql',
password:''
});
client.useDatabase('test');
module.exports.get_chapter_list = function(subject_id, callback){
client.query("select distinct chapter_id from course_associations where subject_id="+subject_id+" and status_id=1",
function(err,results,fields){
callback( results );
});
return "hello";
};
And then
var rs = require('./getChapterList');
rs.get_chapter_list(1, function(results) {
console.log(results);
}
});
This will print the desire output.