How to create asynchronous callbacks in node js?

Question

var express = require('express');
var GoogleUrl = require('google-url');
var favicon = require('serve-favicon');
var mongo = require('mongodb').MongoClient;
var app = express();
var db;
var googleUrl = new GoogleUrl({key: 'AIzaSyB5i1sdnt6HwzrvsPTBu0FbPiUZrM_BCsk'});
var PORT = 8080;

mongo.connect('mongodb://localhost:27017/url-shortener', function(err, newDb){
   if(err){
       throw new Error('Database failed to connect');
   } else{
       console.log('Successfully connected to MongoDB on port 27017');
   }
   db=newDb;
   db.createCollection('sites', {
      autoIndexID: true 
   });
});

app.use(favicon(__dirname+'/public/favicon.ico'));

app.get('/new/*', function(req, res){
    var doc;
   console.log('This is the url: '+req.params[0]);
   googleUrl.shorten(req.params[0], function(err, shortUrl){
       if(err){
           console.log(err);
       }else{
           console.log(shortUrl);
       }
       doc = check_db(req.params[0], shortUrl, db);
   });

The below res.json statement runs and returns an undefined variable before the other functions have a chance of returning a value.

   res.json(doc);
});


app.listen(process.env.PORT, function(){
    console.log('Express listening on: '+PORT);
});

function check_db(longUrl, shortUrl, db){
    db.collection('sites').findOne({
        'longUrl': longUrl, 
        'shortUrl': shortUrl
    }, function(err, doc){
        if(err){
            console.log(err);
        }if(doc){

The res.json statement is executed before the below statements are executed and a value can be returned.

            console.log('This site already exists on the database');
            return doc;
        }else{
            save_db(longUrl, shortUrl, db);
        }
    });

}

function save_db(longUrl, shortUrl, db){
    db.collection('sites').insert({
        'longUrl': longUrl, 
        'shortUrl': shortUrl
    }, function(err, doc){
        if(err){
            throw err
        }else{
            console.log(doc);
        }
        db.close();
    });
}

In the above code, the res.json statement executes before the functions defined below the GET request have a chance to complete execution, with the result that res.json returns an undefined variable. I know that I have to implement asynchronous functionality into my app (potentially promises?), but I am at a complete loss as to how to do so!

Angrysheep · Accepted Answer · 2016-09-01 16:58:52Z

A callback is just an argument, in a function call, so

googleUrl.shorten(req.params[0], function(err, shortUrl){
   if(err){
       console.log(err);
   }else{
       console.log(shortUrl);
   }
   doc = check_db(req.params[0], shortUrl, db);
});
res.json(doc);

behaves just like

foo(a, b);
bar();

Your call to #googleUrl.shorten() is immediately followed by your call to #res.json(), just like the call to #foo() is immediately followed by a call to #bar().

Your callback function:

function(err, shortUrl){
   if(err){
       console.log(err);
   }else{
       console.log(shortUrl);
   }
   doc = check_db(req.params[0], shortUrl, db);
}

Is executed asynchronously, meaning it does not interrupt regular control flow. When you need to postpone execution of a statement like #res.json(doc), you must make sure that it's execution is also taken out of regular control flow. To do that, you'll need to accept a callback argument in both #check_db() and #save_db(). The new function signatures will look like this:

function save_db(longUrl, shortUrl, db, callback)

and

function check_db(longUrl, shortUrl, db, callback)

Then pass a callback function, which accepts an argument 'doc', and executes res.json(doc), to your db functions. eg:

doc = check_db(req.params[0], shortUrl, db, function(doc){
  res.json(doc);
});

Note: You'll simply pass 'callback' to #save_db() from #check_db().

Finally, call the callback, rather than returning a value, once your asynchronous functions have completed execution. eg:

db.collection('sites').findOne({
    'longUrl': longUrl, 
    'shortUrl': shortUrl
}, function(err, doc){
    if(err){
        console.log(err);
    }if(doc){
        console.log('This site already exists on the database');
        callback(doc);
    }else{
        save_db(longUrl, shortUrl, db, callback);
    }
});

I've left the changes to #save_db() up to you. Good luck!

Ebrahim Pasbani · Accepted Answer · 2016-09-01 16:19:58Z

Change check_db and save_db to be async

function check_db(longUrl, shortUrl, db, cb){
    db.collection('sites').findOne({
        'longUrl': longUrl, 
        'shortUrl': shortUrl
    }, function(err, doc){
        if(err){
            console.log(err);
            process.nextTick(function(){cb(err)});
        }else if(doc){
            console.log('This site already exists on the database');
            process.nextTick(function(){cb(null, doc)});
        }else{
            save_db(longUrl, shortUrl, db, cb);
        }
    });

}

function save_db(longUrl, shortUrl, db, cb){
    db.collection('sites').insert({
        'longUrl': longUrl, 
        'shortUrl': shortUrl
    }, function(err, doc){
        db.close();
        process.nextTick(function(){cb(err, doc)});
    });
}

And use like this :

app.get('/new/*', function(req, res){
    var doc;
   console.log('This is the url: '+req.params[0]);
   googleUrl.shorten(req.params[0], function(err, shortUrl){
       if(err){
           console.log(err);
       }else{
           console.log(shortUrl);
       }
       check_db(req.params[0], shortUrl, db, function(err, doc){
          if(err) return res.json(err);
             res.json(doc);
       });
   });
});

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