I have a for loop that pulls data from a MySQL server. I would like the four values to be put into variables so I can use them later. Here's the code I have; for some reason, it says thev is undefined?
create();
function create(){
for(var i=0;i<4;i++){
var thev=[];
client.query('SELECT curattend FROM table1 WHERE ind=?',[i], function(err,result){
thev[i] = result[0].curattend;
});
}
return thev;
}
console.log(thev[2]);
I would appreciate any advice on this problem.
There are a lot of problems here.
thev is local to create. You don’t assign the return value of create to anything, so it’s still not going to be defined.
var thev = []; should not be inside the for loop. It’ll only end up containing one element. Or it would, but…
The callback to query is not just there for fun; it’s an asynchronous call, and is 100% sure to not have happened by the time you actually return from the function.
I would just do it using the async library:
function range(start, end) {
var result = [];
while(start < end) {
result.push(start);
start++;
}
return result;
}
async.map(range(0, 4), function(i, callback) {
client.query('SELECT curattend FROM table1 WHERE ind = ?', [i], function(err, result) {
if(err) return callback(err);
callback(null, result[0].curattend);
});
}, function(err, thev) {
// Continue
});
Related
I am new to NodeJs and I'm finding the Non Blocking and Asynchronous nature of JS extremely difficult to understand and handle,
I have a piece of code which is supposed to Iterate an array
and for every iteration, I'm supposed to make a DB update.
Can someone provide the correct implementation of Async library functions and help fix my code?
Code example -
function updateFunction(conn, requestBody, callback) {
let arr = [];
async.each(requestBody.arr, function(item, callback) {
let sqlData = []
let columns = "";
if(item.columnData != null){
sqlData.push(item.columnData);
columns += "`columnName` = ?,";
}
if(columns != ''){
columns = columns.substring(0,columns.length-1);
let sqlQuery = 'UPDATE someTable SET '+columns
+' WHERE id = "' + item.id + '"';
conn.query(sqlQuery, sqlData, function (err, result) {
if (err) {
return callback(err, false);
}
})
}
else{
return callback(null, false);
}
columns = "";
sqlData = [];
},
function(err, results) {
//Code never reaches here, don't know why
if (err) {
return callback(err, false);
}
else{
return callback(null, true);
}
});
} // END
During your database query call, on a successful query your callback is not called, therefore causing your code to never reach the final callback.
You will want to add another return statement after your if (err) { return callback(err); } to let async know your database query is finished.
And another thing, according to the docs, the async each method's final callback does not invoke with results in its callback.
A callback which is called when all iteratee functions have finished, or an error occurs. Invoked with (err).
Therefore, it is not required for you to pass a value into the callback statement within your iteratee function.
Modify your code to do this and it will work.
conn.query(sqlQuery, sqlData, function (err, result) {
if (err) {
return callback(err);
}
return callback(null);
})
Hope this helps.
conn.query(sqlQuery, sqlData, async function (err, result) {
if (err) {
return await callback(err, false);
}
})
Something like this.. so the function callback is async here and we gave await which actually waits until the return call is finished..
I'm having trouble with some asynchronous database queries. I'm using the Async library for Node.js. I have some code forming an object before it hits the add() function. In my code, I have a loop the executes to create the object multiple times.
for(var i = 0; i < 10; i++){
var obj.id = i + '_idstring';
add(obj);
}
As you can see, each object is slight different. I need to perform a lookup in my database to see if the object already exists (1st waterfall function) and if it does not, add it to my database (2nd waterfall function). My issue is, loaObj.id always equals 10_idstring. Is this a problem with how I'm passing the object through the callbacks? How should I be handling this?
function add(loaObj) {
async.waterfall([
function(loaObj, callback) {
var sql = "";
connection.query(sql, function(error, results, fields) {
callback(error, results, loaObj);
});
},
function(results, loaObj, callback) {
if (results.length > 0) {
sendMessage();
} else {
var sql = "";
connection.query(sql, function(error, results, fields) {
callback(error, loaObj);
});
}
}
], function(err, loaObj) {
console.log(err);
if (err) {
sendMessage();
} else {
sendMessage();
}
});
}
Because you are using Object, which will be passed by "copy of a reference", then obj.id will be overridden with every loop and it will settle down to 10_idstring -where the loop stops-.
One simple solution is to pass new Object every time like below:
for(var i = 0; i < 10; i++){
add({id: `${i}_idstring`});
}
I'm trying to write a function which i can use in an if statement for logic. I've used callback functions to avoid having undefined response due to the asynchronous nature of Javascript. Am i going about this all wrong? is there a better way?
if(emailExists(Email, someCallback)){
//email does exist, do stuff
}
Basically i just want a function to tell me if an email exists in my database to return true, But even with all the precaution i took i'm still getting undefined when i run this code.
function someCallback(e){
console.log(e);
return e;
}
function emailExists(input, callback) {
pg.connect(conString, function(err, client, done){
//Connect to database
client.query('select email from users', function(err, result){
//select all emails from database
var tempArray = [];
for(var x = 0; x < result.rows.length; x++){
tempArray.push(result.rows[x].email)
} //create array of emails
callback(tempArray.includes(input));
});
});
}
I would recommend using a Promise instead, that makes working with async much more convenient. I didn't use pg-js before and don't know if it does support promises by itself - if so, you can simply use the promise returned by it; if not, below code will work for you:
function emailExists(input) {
return new Promise((resolve, reject) => {
pg.connect(conString, function (err, client, done) {
client.query('select email from users', function (err, result) {
var tempArray = [];
for (var x = 0; x < result.rows.length; x ++) {
tempArray.push(result.rows[x].email)
}
resolve(tempArray.includes(email));
});
});
});
}
You can then use it like this:
emailExists("foo#bar.baz")
.then(exists => {
if (exists) {
// email exists
} else {
// not
}
});
You just need to put your if statement in the callback.
emailExists(Email, function(hasMail){
if(hasMail){
//email does exist, do stuff
}
});
Promise would be good but not required.
I am trying to implement a for loop that iterates through a list and subsequently calls two functions, only if the first function results are found.
The issue is that the second function (search.similar) might be taking longer to fetch results.
With the code below, when I run, all of the appropriate output from (search.locate) is correct, but only the last element's results from myList are stored from the (search.similar) function.
ie. all_results = [[cat_res1,mouse_res2],[dog_res1,mouse_res2],[mouse_res1,mouse_res2]]
How do I fix this to append the right results in the right order?
ie. all_results = [[cat_res1,cat_res2],[dog_res1,dog_res2],[mouse_res1,mouse_res2]]
var search = require('./search');
var myList = ['cat','dog','mouse'];
var all_results = [];
for (i=0; i<myList.length; i++){
/* locate function*/
search.locate(myList[i], function (err, searchResult){
if (err){
console.log("Error");
return;
}
if (!searchResult){
console.log("Cannot find it");
return;
}
/*similarity function*/
/* seems to take longer*/
search.similar(myList[i], function (err, similarResult){
if (err){
return;
}
if (!similarResult){
return;
}
var res1 = searchResult.data;
var res2 = similarResult.data;
/* append results to array*/
all_results.push([res1,res2]);
}
});
}
Javascript can be thought of as asynchronous, in that the execution of particular functions do not necessarily happen synchronously, however "describing JavaScript as asynchronous is perhaps misleading. It's more accurate to say that JavaScript is synchronous and single-threaded with various callback mechanisms"
In order to accomplish your goal, though you may still get some ordering issues with the top array, you will need to wrap your .similar() call in another function that takes both arguments. Your reference to the "item" on the top search is changing:
function searchNestedSimilar(item, topRes) {
search.similar(item, function (err, similarResult) {
if (err){
return;
}
if (!topRes){
return;
}
var res1 = topRes.data
var res2 = similarResult.data
// append results to array
all_results.push([res1,res2])
}
}
function searchLocate(item) {
search.locate(item, function (err, searchResult) {
if (err){
console.log("Error");
return;
}
if (!searchResult){
console.log("Cannot find it");
return;
}
searchNestedSimilar(item, searchResults);
}
I encapsulated both calls to keep it modular, but since "item" is in the closure, you really only need the searchLocate() function to wrap your capture your item reference during iteration.
This is a good case for Promises (see Bluebird JS for example http://bluebirdjs.com/docs/getting-started.html) or you could do it with
async.map().
This page talks about it well, too. http://promise-nuggets.github.io/articles/14-map-in-parallel.html
There are many Stack Overflows discussing Promises as well. Understanding JS Promises for example.
A rough example of how to write this with a Promise:
var search = require('./search');
var myList = ['cat','dog','mouse']
var all_results = []
var Promise = require('bluebird');
var locate = Promise.promisify(search.locate);
var similar = Promise.promisify(search.similar);
for (i = 0; i < myList.length; i++){
// locate function
locate(myList[i], function (err, searchResult) {
if (err) {
console.log("Error");
return;
}
if (!searchResult){
console.log("Cannot find it");
return;
}
}).then(function(result) {
//similarity function
similar(myList[i], function (err, similarResult) {
if (err){
return;
}
if (!similarResult){
return;
}
var res1 = searchResult.data
var res2 = similarResult.data
// append results to array
all_results.push([res1,res2])
}).finally(function() {
// NOP
});
});
}
I am doing a for loop to find the result from mongodb, and concat the array. But I am not getting the final results array when the loop is finished. I am new to node.js, and I think it's not working like objective-c callback.
app.get('/users/self/feed', function(req, res){
var query = Bill.find({user: req.param('userId')});
query.sort('-createdAt');
query.exec(function(err, bill){
if (bill) {
var arr = bill;
Following.findOne({user: req.param('userId')}, function(err, follow){
if (follow) {
var follows = follow.following; //this is a array of user ids
for (var i = 0; i < follows.length; i++) {
var followId = follows[i];
Bill.find({user: followId}, function(err, result){
arr = arr.concat(result);
// res.send(200, arr);// this is working.
});
}
} else {
res.send(400, err);
}
});
res.send(200, arr); //if put here, i am not getting the final results
} else {
res.send(400, err);
}
})
});
While I'm not entirely familiar with MongoDB, a quick reading of their documentation shows that they provide an asynchronous Node.js interface.
That said, both the findOne and find operations start, but don't necessarily complete by the time you reach res.send(200, arr) meaning arr will still be empty.
Instead, you should send your response back once all asynchronous calls complete meaning you could do something like:
var billsToFind = follows.length;
for (var i = 0; i < follows.length; i++) {
var followId = follows[i];
Bill.find({user: followId}, function(err, result){
arr = arr.concat(result);
billsToFind -= 1;
if(billsToFind === 0){
res.send(200, arr);
}
});
}
The approach uses a counter for all of the inner async calls (I'm ignoring the findOne because we're currently inside its callback anyway). As each Bill.find call completes it decrements the counter and once it reaches 0 it means that all callbacks have fired (this works since Bill.find is called for every item in the array follows) and it sends back the response with the full array.
That's true. Your codes inside for will be executed in parallel at the same time (and with the same value of i I think). If you added console.log inside and after your for loop you will found the outside one will be printed before inside one.
You can wrap the code that inside your for into array of functions and execute them by using async module (https://www.npmjs.org/package/async) in parallel or series, and retrieve the final result from async.parallel or async.series's last parameter.