Making async callback functions in nodejs

Question

I have confusing code that I'd like to modularize. I have a general ledger implemented in Mongodb. Transferring credits from john to adam appends the following document in the db.dummyTx:

{
  "debitAccount": "john",
  "creditAccount": "adam",
  "amount": 10
}

I'd like to create a single function transfer(from, to, amount, callback()) where callback receives the transaction document/object.

I've created the following using the 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 the account of the sender
        transaction.debitAccount = to; // with reference to the reciever
        transaction.amount = amount; // of the given amount
        transaction.save(function(err) {
          callback(null, transaction);
        });
      },
      function(transaction, callback) {
        console.log("Credited User " + transaction.creditAccount +
          " and debited User " + transaction.debitAccount + " by amount " +
          transaction.amount + "credits");
        callback(null, transaction);
      },
      function(transaction, callback) {
        userBalance(transaction.creditAccount);
        userBalance(transaction.debitAccount);
        callback(null, transaction);
      }
    ],
    acallback(err, transaction)
  );
}

My rationale was that if I pass function(err,transaction){if err console.log(err);} as acallback, it would run as the final callback in the end. However, it says err is undefined in acallback(err, transaction)

Bear with me, I just discovered async yesterday, so I'm a figurative five year old.

My second thought was to save the chain of functions into an Array named transfer and to call it as async(transfer,function(err,transaction){if err console.log(err)}; if I can't get this to work.

Edit: I'd also like the acallback parameter to be optional.

Why do you ignore errors from `transaction.save`? Also, why do you use `waterfall` at all here when you have only a single async function? — Bergi, May 11 '15 at 20:41
@Bergi It's simple really, because I'm stupid. Thank you, I've now changed it to `callback(err, transaction)`. And I just realized that I could do the same thing with `async.series`. I've got another problem now though, the `acallback` parameter now executes after the third function, with a `null` error value, and then outputs the final function normally when I run `transfer(2, 1, 2, function(err, transaction) { if (err) { console.log(transaction); }});`. [Pastebin of output](http://pastebin.com/CuN6j2yB) — Amin Shah Gilani, May 11 '15 at 21:00

score 1 · Accepted Answer · answered May 11 '15 at 20:09

1

If you have defined the function acallback, then you should just pass that, and not the parameters. In other words, instead of this:

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

...use this:

...
      }
    ],
    acallback
  );
}

To make acallback() optional, you can do a number of things. A couple things that leap to mind:

Before calling, async.waterfall(), check to see if acallback() is defined. If it is not, set it to a no-op function.
Before calling async.waterfall(), check to see if acallback() is defined. If it is not, invoke async.waterfall() without it. If it is, invoke it with it.

answered May 11 '15 at 20:09

Trott

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Thank you for that, the code runs now. Only, the `acallback` is executed after the third function, instead of the last. I ran it with `transfer(2,1,2,function(err, transaction){console.log(transaction);});`. [Pastebin to output](http://pastebin.com/CuN6j2yB). Also, the `to` in the third function is now `undefined`. – Amin Shah Gilani May 11 '15 at 20:22
If a function in the waterfall sends an error, the waterfall is aborted and the callback invoked immediately. So check the value of `err` in your callback and it will hopefully indicate what's going wrong. – Trott May 11 '15 at 20:23
It's `null`, however, I notice that the value of `to` in the third function is now `undefined`. – Amin Shah Gilani May 11 '15 at 20:26
It's `undefined` because of a typo in the code, sorry. I fixed it. However, `acallback` is still being executed after the third function, with the `err` outputing to `null` – Amin Shah Gilani May 11 '15 at 20:34

Making async callback functions in nodejs

1 Answers1