Using a Wrapper Promise

Question

I wrote some code for an application that uses the following pattern:

function _getData() {
    return new Promise(function(resolve, reject) {
        if(_hasDataTypeA()) {
            SomeBackendAccessObject.getTypeAData().then(resolve, reject);
        } else {
            SomeBackendAccessObject.getTypeBData().then(_convertToTypeA).then(resolve, reject);
        }
    });
}

My rationale for writing the code this way is that if _hasDataTypeA throws an exception, we're still able to return a Promise. This means the caller doesn't have to do a try block and a Promise .catch; but this code feels smelly to me. Is there a better way to write it?

Just to be absolutely sure, are you saying that _hasDataTypeA can throw an exception, or did you mean that it can return false? — konijn
– konijn, Commented Oct 30, 2014 at 14:41
@konijn Yes, I'm saying _hasDataTypeA could, hypothetically, throw an exception. — aebabis
– aebabis, Commented Oct 30, 2014 at 15:53
@konjin (I think) I would like a Promise to catch the error so that the caller of _getData can see it by calling .catch, but I'm not entirely confident in this premise either. — aebabis
– aebabis, Commented Oct 30, 2014 at 16:16
@konijn Did you want to try to put an answer? You can get some bounty even if the answer isn't accepted. — aebabis
– aebabis, Commented Nov 1, 2014 at 18:02

Sukima · Accepted Answer · 2014-11-05 22:14:34Z

2

First thing I noticed is that your fulfilling the wrapper promise by way of other promises inside it. Seems like and extra step that isn't needed. Especially the final .then(resolve, reject) feels like a big anti-pattern especially since it is a repeated pattern. So the rational is to catch the exception. My take on this is to wrap only _hasDataTypeA in it's own promise and then use that result to manage the following promises:

function _hasDataTypeAPromised() {
  try {
    return Promise.resolve(_hasDataTypeA());
  } catch (e) {
    return Promise.reject(e);
  }
}

function _getData() {
  return _hasDataTypeAPromised()
    .then(function(hasDataTypeA) {
      if(hasDataTypeA) {
        return SomeBackendAccessObject.getTypeAData();
      } else {
        return SomeBackendAccessObject.getTypeBData();
      }
    });
}

You could even ternary-ize the method with:

var method = hasDataTypeA ? 'getTypeAData' : 'getTypeBData';
return SomeBackendAccessObject[method]();

edited Nov 5, 2014 at 22:14

answered Nov 5, 2014 at 13:17

Sukima

1365 bronze badges

\$\begingroup\$ getTypeAData and getTypeBData return promises, so you don't need to wrap them at the end. \$\endgroup\$

aebabis
– aebabis

2014-11-05 16:06:24 +00:00
Commented Nov 5, 2014 at 16:06
\$\begingroup\$ I call then on the return value of SomeBackendAccessObject.getTypeAData(). \$\endgroup\$

aebabis
– aebabis

2014-11-05 21:46:25 +00:00
Commented Nov 5, 2014 at 21:46

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