if-else flow in promise (bluebird)

ifelse.js

(conditionA 
  ? fs.writeFileAsync(file, jsonData)
  : Promise.resolve())
.then(functionA);

var waitFor;
if (conditionA)
    waitFor = fs.writeFileAsync(file, jsonData);
else
    waitFor = Promise.resolve(undefined); // wait for nothing,
                                          // create fulfilled promise
waitFor.then(function() {
    return functionA();
});