Check whether a promise is finished / resolved / rejected

promise-is-resolved.js

function delay(msec, value) {
  return new Promise(done => window.setTimeout((() => done(value)), msec));
}

function isResolved(promise) {
  return Promise.race([delay(0, false), promise.then(() => true, () => false)]);
}

function isRejected(promise) {
  return Promise.race([delay(0, false), promise.then(() => false, () => true)]);
}

function isFinished(promise) {
  return Promise.race([delay(0, false), promise.then(() => true, () => true)]);
}


isFinished(Promise.resolve(42)).then(result => console.log(result ? "Resolved" : "Not resolved"));
// => "Resolved"

isFinished(delay(100)).then(result => console.log(result ? "Resolved" : "Not resolved"));
// => "Not resolved"

isFinished(delay(1)).then(result => console.log(result ? "Resolved" : "Not resolved"));
// => "Resolved"
// oh...

isResolved => promise.then( resolved=true, reject=false)
isRejcted => promise.then( resolved=false, reject=true)
isFinished => promise.then( resolved=true, reject=true)

race the given promise(A) with a promise(B) which is definitely resolved in the next macro task.
if B is settled it will win the race, otherwise A will.