Change to the following

      if (typeof data.count === 'function') {
        count = data.count();
      } else if (typeof data.size !== 'undefined') {
        count = data.size;
      } else {
        count = data.length;
      }

To handle the length of a Map()

Yeah, I agree. I had the size there before, but then I've noticed that MoriJS has only count method.

I am thinking if it wouldn't be better to use iterator even here to count entries. That would be unified way. I would like to avoid adding another property/method in a future if some other data structure implementing iterator would use different name for this. Some library could even have a like size being method or count being property.

Any conclusion here?

Alright, I've change it. It uses the size property as it's the most common (only mori doesn't have it), otherwise it counts entries through the iterator. I am not using length because it's deprecated in Immutable anyway and others doesn't have it at all.

Another small change to use Number.isSafeInteger for the size property. It should be safer way in case some library has size as a function.