Rename isPromise to isPromiseLike or isThenable

[aleclarson]

Currently, isPromise is checking for a then method, but it should really be doing Object.prototype.toString.call(value) === "[object Promise]" instead.

The question is, do we rename the current implementation to isPromiseLike or isThenable?

[Minhir]

What type should it return?

[aleclarson]

Probably value is { then: Function }

[Minhir]

In such case isThenable looks like the perfect name

[aleclarson]

TypeScript has a built-in PromiseLike<T> type we could use, but there's no real way to safely ensure a then method has the signature of then<TResult1 = T, TResult2 = never>(onfulfilled?: ((value: T) => TResult1 | PromiseLike<TResult1>) | undefined | null, onrejected?: ((reason: any) => TResult2 | PromiseLike<TResult2>) | undefined | null): PromiseLike<TResult1 | TResult2>.

Also, I checked and value is { then: Function } properly narrows the type string | PromiseLike<string> | Promise<string> to be PromiseLike<string> | Promise<string>.

[anion155]

Also, I checked and value is { then: Function } properly narrows the type string | PromiseLike<string> | Promise<string> to be PromiseLike<string> | Promise<string>.

It will work for union types with Promise<unknown>, but it would not work for something like unknown. I'd say it's pretty safe to assume that anything that has then function field is PromiseLike<unknown>

[aleclarson]

@anion155 There are workarounds for that, but I'm thinking using isPromiseLike as a name and value is PromiseLike<unknown> as the return type is the most pragmatic solution. Like you said, reserving then methods for promise-like functionality is a best practice at this point.