sample.js

function getSpecialData(arg1) { 
  return someDbCall(arg1) 
  .then((result) => {
      if (result.isComplete) {
        return result;
      }
      else {
        return someOtherDbCall(result.foo);
      }
  });
}

return foo
.then(getSpecialData) //this is what I mean by composing. 
.then(bar);