promise_resolve.js

// Demo: converting a thenable to a Promise

const fulfilledThenable = {
    then(reaction) {
        reaction('hello');
    }
};
const promise = Promise.resolve(fulfilledThenable);
console.log(promise instanceof Promise); // true
promise.then(x => console.log(x)); // hello

Regarding thenables, @bahmutov teached me to start with a promise so you don't have to wrap all library calls with Promise.resolve().

Promise.resolve()
  .then(thenableReturningFn)

instead of

Promise.resolve(thenableReturningFunction())
  .then(...)