Function thenableFactory

After I created the thenable function, my code became easier, because I could write the same code regardless whether the input was synchronous or asynchronous. But by wrapping something with thenable, the check whether it was a Promise or not was done on every invocation.

In a library that is about iterators, we expect this to be called many times. So it feels like it could make sense to create a version that 'remembers' the conclusions from the first run, and that will use that knowledge in the second run (assuming that every next element in an iterator will be a promise if the first was a promise and vice versa)

A few tests seemed to indicate that calling isPromise often if about 10x slower than checking if a variable is true or false (or is a specific symbol), so there should be gain to be made with this.

Example

// instead of
for (x of [1, 2, 3]) {
  thenable(x).then((v) => console.log(v));
}
// do something like
const cachedThenable = thenableFactory(1);
for (x of [1, 2, 3]) {
  cachedThenable(x).then((v) => console.log(v))
}

Returns

a thenable(...)-like function that has assumptions built-in based on the first x