javascript - Making async callback functions in nodejs -

i have confusing code i'd modularize. have general ledger implemented in mongodb. transferring credits john adam appends following document in db.dummytx:

{   "debitaccount": "john",   "creditaccount": "adam",   "amount": 10 } 

i'd create single function transfer(from, to, amount, callback()) callback receives transaction document/object.

i've created following using async module:

function transfer(from, to, amount, acallback) {   async.waterfall([       function(callback) {         userbalance(from);         userbalance(to);         callback(null);       },       function(callback) {         var transaction = new dummytx();         transaction.creditaccount = from; // in account of sender         transaction.debitaccount = to; // reference reciever         transaction.amount = amount; // of given amount         transaction.save(function(err) {           callback(null, transaction);         });       },       function(transaction, callback) {         console.log("credited user " + transaction.creditaccount +           " , debited user " + transaction.debitaccount + " amount " +           transaction.amount + "credits");         callback(null, transaction);       },       function(transaction, callback) {         userbalance(transaction.creditaccount);         userbalance(transaction.debitaccount);         callback(null, transaction);       }     ],     acallback(err, transaction)   ); } 

my rationale if pass function(err,transaction){if err console.log(err);} acallback, run final callback in end. however, says err undefined in acallback(err, transaction)

bear me, discovered async yesterday, i'm figurative 5 year old.

my second thought save chain of functions array named transfer , call async(transfer,function(err,transaction){if err console.log(err)}; if can't work.

edit: i'd acallback parameter optional.

if have defined function acallback, should pass that, , not parameters. in other words, instead of this:

...       }     ],     acallback(err, transaction)   ); } 

...use this:

...       }     ],     acallback   ); } 

to make acallback() optional, can number of things. couple things leap mind:

• before calling, async.waterfall(), check see if acallback() defined. if not, set no-op function.

• before calling async.waterfall(), check see if acallback() defined. if not, invoke async.waterfall() without it. if is, invoke it.