Setting variable via asynchronous function inside for loop

Question

I've come across a situation where I have an asynchronous function inside of a for loop. I've done my prerequisite searching and now know that forEach() or map() might be the solution I need. But all of the examples I see just console.log() the result of each async function. How would it work to have to set the results of each async function into a variable, and then only return the variable?

Here's some slimmed down code of what I'm doing: (this is all in node, by the way)

var clients={
    "102323":{stuff about this client},
    "242341":{stuff about that client}
};
var messages={};
for (var id_client in clients) {
    mysql.query("SELECT * FROM messages WHERE id_client='"+id_client+"' ORDER BY date", function(err, rows) {
        if (typeof rows !== 'undefined') messages[id_client]=rows;
    });
}
//do other stuff with messages variable

With this, messages predictably is null. Which I understand.

But even when I transition this to using map() instead of for(), like this...

var messages={};
Object.keys(clients).map(function(id_client){
    mysql.query("SELECT * FROM messages WHERE id_client='"+id_client+"' ORDER BY date", function(err, rows) {
        if (typeof rows !== 'undefined') messages[id_client]=rows;
    });
});

...messages ends up being null.

Finally, just want to note that I do know how to wrap mysql.query() into another function with a callback and all that to get around the whole asynchronous thing. I just don't know how that all works if being iterated on inside of a loop.

There is no difference in your map call vs your for loop - FYI. — tymeJV
– tymeJV, Commented Aug 24, 2016 at 12:42
@tymeJV - there is a difference ... id_client will be "correct" for each iteration of .map ... in the for loop, it would be the last client every time — Jaromanda X
– Jaromanda X, Commented Aug 24, 2016 at 12:54
the similarity is WHEN messages will be populated - i.e. well after the next line of code (not shown in either snippet) — Jaromanda X
– Jaromanda X, Commented Aug 24, 2016 at 12:55
Also, this way of inserting variables inside your queries is highly discouraged, see github.com/mysqljs/mysql#escaping-query-values for more information, i'd personally go with mysql.query("SELECT * FROM messages WHERE id_client = ? ORDER BY date", ['id_client'], function(err, rows) { — roberrrt-s
– roberrrt-s, Commented Aug 24, 2016 at 12:59

Sergii Shvager · Accepted Answer · 2016-08-24 12:52:14Z

3

You can use Promise.

var messages = {};

Promise.all(Object.keys(clients).map((id_client) => {
  return new Promise((resolve, reject) => {
    mysql.query("SELECT * FROM messages WHERE id_client='" + id_client + "' ORDER BY date", function(err, rows) {
      if (typeof rows !== 'undefined') {
        messages[id_client] = rows;
        resolve(rows);
      }
    });
  });
})).then(results => {
  console.log(messages);
});

answered Aug 24, 2016 at 12:52

Sergii Shvager

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Vad · Accepted Answer · 2016-08-24 12:51:13Z

1

Here is a pure JavaScript version:

var clients={
    "102323":{stuff about this client},
    "242341":{stuff about that client}
};
var messages={};
var cnt = 2;

function done() {
    console.log(messages);
}

mysql.query("SELECT * FROM messages WHERE id_client='"+id_client+"' ORDER BY date", function(err, rows) {
    cnt--;
    if (typeof rows !== 'undefined') messages[id_client]=rows;
    if(!cnt) done();
});

answered Aug 24, 2016 at 12:51

Vad

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DrakaSAN · Accepted Answer · 2016-08-24 12:45:23Z

0

I presume you are console.log()ing just after map. But map didn't finished yet, since it's callback is asynchonous.

You can use async to get a asynchnous version of map, which will work:

'use strict';
let async = require('async'),
    messages = {};

async.map(Object.keys(clients), (id_client) => {
    mysql.query("SELECT * FROM messages WHERE id_client='"+id_client+"' ORDER BY date", (error, rows) => {
        if (typeof rows !== 'undefined') {
            messages[id_client]=rows;
        }
        callback(error);
    });
}, (error) => {
    if(error) {
        console.log(error);
    }
    console.log(messages);
});

answered Aug 24, 2016 at 12:45

DrakaSAN

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