40Implement Promise.resolve() and Promise.reject()

Implement promiseResolve(value) and promiseReject(reason) without using Promise.resolve or Promise.reject.

promiseResolve(value):

• returns value itself when it is already a native promise;
• adopts a thenable (any object or function with a then method): the returned promise settles however the thenable does;
• otherwise returns a promise fulfilled with value.

promiseReject(reason) returns a promise rejected with reason, never unwrapping it, even if reason is itself a promise.

const p = fetch("/api");
promiseResolve(p) === p; // true

const legacyThenable = { then(onOk) { onOk("data"); } };
await promiseResolve(legacyThenable); // "data"

Requirements

• Call a thenable's then asynchronously (in a microtask), as the built-in does.
• If reading value.then throws, reject with that error.
• If the thenable calls its callbacks more than once, only the first call counts.