Push object into local array from asynchronous callback function

Question

I use node.js on my server and I use redis key-store for storing data about my characters. Every connection has it own character. I want to get all data about characters(Person, has its name, age, profession, ...) into Characters array, so I can then selectively send it to connected clients.

var Characters = [];
for (var ID in Connections) {
    redis_client.HGETALL(ID, function(err, result) {
        if (result) {
            Characters.push(result);
        }
    });
}
console.log(Characters);

I have read, that this is due to asynchronous vs synchronous problem, so I made global variable character.

//global variables
var character;
//function code
var Characters = [];
for (var ID in Connections) {
    redis_client.HGETALL(ID, function(err, result) {
        character = result;
    });
    if(character) {
         console.log(character); // returns correct result
         // make copy of character
        Characters.push(JSON.parse(JSON.stringify(character)));
        character = undefined;
    }
}
console.log(Characters); // array of 1 character * number of connection
                                                    //BUT I expect different character for each connection

Kamrul · Accepted Answer · 2014-06-21 12:27:50Z

1

there are different ways,

the easiest way would be creating calling the async function one after another, as follows

    var Characters = [];
    var objectKeys = Object.keys(Connections);
    var ID = 0; 
    if (ID < objectKeys.length) 
        doCall(objectKeys[ID]);
    else 
        console.log(Characters);
   function doCall(key) {

        redis_client.HGETALL(key, function(err, result) {
            if (result) {
                Characters.push(result);
            }
            ID++;
            if ( ID < objectKeys.length)
               doCall(objectKeys[ID]);
            else 
               console.log(Characters);
        });
   }

answered Jun 21, 2014 at 12:27

Kamrul

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1 Comment

James07 Over a year ago

Hi, could you explain me why if I replace console.log(Characters); at the 3rd row from th end with for (var ID in Connections) { Connections[ID].emit("snapshot", { MyIndex: Indices[ID], Characters: Characters });} (its socketio sending function) then whole code doesn't work(even on line where is ID++; - ID is undefined and turns into NaN. BUT: If i put this for cycle into another function and only call this function instead of console.log(Characters); code works as expected?

Ben · Accepted Answer · 2014-06-21 13:00:49Z

1

Regular for loop doesn't work correctly when you have async call and global variables. Here is a loop version that would work (make sure async is installed. if not do "npm install async"):

var async = require('async');   
var Characters = [];

async.each(Connections, function(ID, callback) {
  redis_client.HGETALL(ID, function(err, result) {
    if (result) {
        Characters.push(result);
    }
    callback(null);
  });
}, function(err) { // this function gets called when the loop is done
  console.log("finish:");
   // print out your array
  for (var i = 0, len = Characters.length; i < len; i++) {
    console.log(Characters[i]);
  }
});

edited Jun 21, 2014 at 13:00

answered Jun 21, 2014 at 12:55

Ben

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3 Comments

James07 Over a year ago

async.each wont interate through Connection array. I have: for (ID in Connections) {console.log(ID);} and this prints keys to objects in Connections. And under this command I have async.each(Connections, ... But it wont do any operation, even when previous "for" prints some keys, it will just print "finish".

Ben Over a year ago

async.each() should. Did you call callback(null) as in the answer? without that, async.each() won't continue to process the next item in the array

Ben Over a year ago

Here is the example I ran and it works as expected, just not using HGETALL(): var async = require('async'); var Characters = []; var Connections = [1, 2, 3]; async.each(Connections, function(ID, callback) { Characters.push(ID); callback(null); }, function(err) { console.log("finish: %d", Characters.length); for (var i = 0, len = Characters.length; i < len; i++) { console.log(Characters[i]); } });

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