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.

In theory `x instanceof Promise` would work, but the reason for this package is that there are many non-standard Promise implementations in the JS world.

Promises were not always part of the standard and for many years were implemented in user space, by many different implementations. Using duck typing like this was the only way to allow packages to interact with each other, as requiring an entire stack to say only use Bluebird promises is not realistic at all.

I'm totally with you on this. It's dumb that this is a problem but it is actually a problem.