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?

• konijn: Just to be absolutely sure, are you saying that _hasDataTypeA can throw an exception, or did you mean that it can return false?
• aebabis: @konijn Yes, I'm saying _hasDataTypeA could, hypothetically, throw an exception.
• konijn: So what should happen in that case ?
• aebabis: @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: @konijn Did you want to try to put an answer? You can get some bounty even if the answer isn't accepted.

Answer 1

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]();

• aebabis: getTypeAData and getTypeBData return promises, so you don't need to wrap them at the end.
• aebabis: I call then on the return value of SomeBackendAccessObject.getTypeAData().