My function could return a promise or value, how to avoid it?

Question

I think it's bad practice to have different return types. So, this is my function, and I want it to always return a promise.

I tried to simplify the scenario. Let's say I have two lines of products (line X and line Y) and the way I'm retrieving a product by name, from each line is totally different from the other.

Please also note that ideally, I wanted to have a function to "or" two promises and return whichever that resolves successfully. But I couldn't think of a better way to achieve this!

ProductService.findProductByName = function findProductByName(name) {
  return LineXService.findOneByName(name) // promise
    .then(function _returnProduct(product) {
      return product
        ? product // value
        : LineYService.findOneByName(name)); // promise
    })
};

wait... that results in always returning a promise. where's the question? — Kevin B, Jun 23 '16 at 19:11
@KevinB `product` isn't a promise, but something returned by a promise. — tadman, Jun 23 '16 at 19:12
Right, but it's being returned in a .then callback. the overall function is still returning a promise. — Kevin B, Jun 23 '16 at 19:12
In general terms you can wrap it up in `resolve` but in this *particular* case it might not be relevant. That's a good note. — tadman, Jun 23 '16 at 19:13

score 1 · Accepted Answer · answered Jun 23 '16 at 19:11

You're right that returning a consistent thing is better than an ambiguous result. Normally you can cast an arbitrary value into a promise easily enough. For example using resolve:

if (product) {
  return Promise.resolve(product);
}

return LineYService.findOneByName(name);

It's worth noting, as Kevin B observes, that this is irrelevant in this particular case since your outer function always returns a promise. The need to promisify a value is only a concern if this is not part of a surrounding promise.

My function could return a promise or value, how to avoid it?

1 Answers1