Node.js calling a callback function inside loop scope issue

Question

I am building the following 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') {
                 datos[i].LasValueBTC=data.result.Last;
            } else {
                 datos[i].LasValueBTC='';   
            }  
        }
    });
}

ok i get an error message: "Cannot set property 'LasValueBTC' of undefined". So i guessed that i variable is not being understood inside the callback function.. I tried without success:

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') {
                 datos[i].LasValueBTC=data.result.Last;
            } else {
                 datos[i].LasValueBTC='';   
            }  
        }
    })(i)); //and also i tried }).bind(this,i)); 
}

How is this solved?

Regards,

glortho · Accepted Answer · 2014-07-09 19:02:00Z

0

The problem is likely that getticker is asynchronous while the for loop is synchronous, so the value of i is probably not what you're expecting it to be by the time you access datos[i]. A simple solution for this is as follows:

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]));
}

answered Jul 9, 2014 at 19:02

glortho

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4 Comments

Egidi Over a year ago

thanks @glortho. After doing what you suggest 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..

glortho Over a year ago

Okay @Aitor that's a different (but related) question, which I can also help you with. Post that separately and let me know when it's up.

Egidi Over a year ago

mmm i can only post every 90 minutes... :(

glortho Over a year ago

Okay, an easy solution to your issue can be found by using this library to call a callback once all asynchronous functions have finished: github.com/caolan/async , but you may need to spend some time becoming familiar with the library first.

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