Scope issue calling a callback function inside a loop in node.js

Question

With the help of glortho in this thread i built this code:

for(var i=0;i<datos.length;i++){
   bittrex.getticker(datos[i].Currency, function(err, data){
        if (err){
            console.log('ERROR:', err);
            return 'ERROR:'+ err;
        } else {
            if (data.message!='INVALID_MARKET') {
                 this.LasValueBTC=data.result.Last;
            } else {
                 this.LasValueBTC='';   
            }  
        }
    }.bind(datos[i]));
}

The problem is that outside the callback function the datos array is not updated...As it is written at the moment if i console.log(this) inside the function works great and this.LastValueBTC exists in my json, but outside the function if i console.log(datos) after the loop, the LastValueBTC does not exist..and i need to do a res.send(datos) after the loop..

Barış Uşaklı · Accepted Answer · 2014-07-10 06:00:37Z

What you need to do is wait for all the callbacks to complete and then call res.send.

var count = datos.length;
for(var i=0;i<datos.length;i++){
   bittrex.getticker(datos[i].Currency, function(err, data){
        if (err){
            console.log('ERROR:', err);
            return 'ERROR:'+ err;
        } else {
            if (data.message!='INVALID_MARKET') {
                 this.LasValueBTC=data.result.Last;
            } else {
                 this.LasValueBTC='';   
            }  
            count--;
            if (count === 0) {
                res.send(datos);
            }
        }
    }.bind(datos[i]));
}

Or using async

async.each(datos, function(dato, next) {
     bittrex.getticker(dato.Currency, function(err, data) {
        if (err){
            console.log('ERROR:', err);
            next(err);
        } else {
            if (data.message!='INVALID_MARKET') {
                 dato.LasValueBTC = data.result.Last;
            } else {
                 dato.LasValueBTC='';   
            }  
            next();
        }
    });
}, function(err) {
   res.send(datos);
});

randomness · Accepted Answer · 2015-04-21 06:42:46Z

1

Go through this article http://www.richardrodger.com/2011/04/21/node-js-how-to-write-a-for-loop-with-callbacks/#.VTXnFa2eDGc

It gives a good conceptual overview on what happens if you put functions inside for loop

answered Apr 21, 2015 at 6:42

randomness

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