Node.js: Asynchronous callback confusion

Question

I am trying to figure out how to create asynchronous functions for a web app. I am doing a database query, manipulating the data to a format that is more convenient, and then trying to set my router to pass back that file.

//Module 1
//Module 1 has 2 functions, both are necessary to properly format
function fnA(param1){
    db.cypherQuery(query, function(err, result){
        if(err){
            return err;
        }
        var reformattedData = {};
        //code that begins storing re-formatted data in reformattedData

        //the function that handles the rest of the formatting
        fnB(param1, param2);
    });
});

function fnB(param1, reformattedData){
    db.cypherQuery(query, function(err, result){
        if(err){
            return err;
        }
        //the rest of the reformatting that uses bits from the second query 
        return reformattedData;
    });
});

exports.fnA = fnA;

Then in my router file:

var db = require('module1');

router.get('/url', function(req,res,next){
    db.fnA(param1, function(err, result){
        if (err){
            return next(err);
        }
        res.send(result);
    });
});

When I tried to test this out by hitting the URL indicated by the router, it just loads indefinitely.

I know what I have above is wrong, since I never wrote my function to require a callback. When I tried figuring out how to rewrite it though, I got really confused - How do I write my function to have a callback when the asynchronous stuff happens inside it?

Can someone help me rewrite my functions to use callbacks correctly, so that when I actually use the function, I can still work with the asynchronous response?

Ben Diamant · Accepted Answer · 2015-02-11 11:31:46Z

1

You use db.fa from your router file, and pass the second parameter as a callback function. but the function signature don't have the cb param and doesnt use it.

The main idea - you try to initiate an async operation and cannot know when it ends, so you send it a callback function to get triggered when all operations are done.

Fixed code should be like that:

//Module 1
//Module 1 has 2 functions, both are necessary to properly format
function fnA(param1, cb1){
    db.cypherQuery(query, function(err, result){
        if(err){
            cb1(err); <-- return error to original call
        }
        var reformattedData = {};
        //code that begins storing re-formatted data in reformattedData

        //the function that handles the rest of the formatting
        fnB(param1, param2, cb1);
    });
});

function fnB(param1, reformattedData, cb1){
    db.cypherQuery(query, function(err, result){
        if(err){
            cb1(err); <-- return error to original call
        }
        //the rest of the reformatting that uses bits from the second query 
        cb1(false, dataObjectToSendBack); <--- This will call the anonymouse function in your router call
    });
});

exports.fnA = fnA;

Router file:

var db = require('module1');

router.get('/url', function(req,res,next){
    db.fnA(param1, function(err, result){ <-- This anonymous function get triggered last 
        if (err){
            return next(err);
        }
        res.send(result);
    });
});

edited Feb 11, 2015 at 11:31

answered Feb 11, 2015 at 8:47

Ben Diamant

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5 Comments

jfriend00 Over a year ago

the return err is not correct because returning it where you are will do nothing. You need a way to tell the callback that there's an error because right now, you're not communicating anything to the outside world when there's an error.

Ben Diamant Over a year ago

@jfriend00 you are right, that was a copy paste of Dude's code. fixed

Dude Over a year ago

Ok, this is making much more sense now. Thanks a lot Ben! Just to make sure that I understand this now, does this mean that cb1 is not defined until it is called in the router file? By the code that says: ...function(err, result){ if(err)...

Ben Diamant Over a year ago

@Dude yep, its an anonymous function

Dude Over a year ago

@Ben, you are awesome! This core concept finally snapped into place for me, and makes perfect sense now. Man, I love this place.

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