And the source code of the library is:
function isPromise(obj) {
return !!obj && (typeof obj === 'object' || typeof obj === 'function') && typeof obj.then === 'function';
}Here's my off-the-cuff take that will not be popular.
A function like this should be a package. Or, really, part of standard js, maybe.
A) The problem it solves is real. It's dumb, but JS has tons of dumb stuff, so that changes nothing. Sometimes you want to know "is this thing a promise", and that's not trivial (for reasons).
B) The problem it solves is not straightforward. If you Google around you'll get people saying "Anything with a .then is a promise' or other different ways of testing it. The code being convoluted shows that.
Should this problem be solved elsewhere? Sure, again, JavaScript is bad and no one's on the other side of that argument, but it's what we have. Is "just copy paste a wrong answer from SO and end up with 50 different functions in your codebase to check something" like other languages that make package management hard so much better ? I don't think so.
I'd say that it should rather be a part of the type system. Some kind of `obj isa Promise` should be the way to do this, not random property checks. But that's JS...
The thing is that there is the Promise "class", which is provided by the environment, but there is a interface called PromiseLike, which is defined as having a method called then that takes one or two functions. Now, JS doesn't have nominal typing for interfaces, so you have to do "random property checks".
Typescript partially solves that by declaring types, but if you have a any variable, you still need to do some probing to be able to safely convert it to a PromiseLike, because TypeScript goes to great lengths to not actually produce physical code on its output, attempting to be just a type checker.
Perhaps if TS or an extension allowed "materializing" TS, that is `value instanceof SomeInterface` generated code to check for the existence of appropriate interface members, this could be avoided, but alas, this is not the case.
It's not perfect and a bit of a bolt-on, but io.ts works reasonably well in this area: