Promise with state and result properties for synchronous inspection

stateful-promise.js

// credit: https://stackoverflow.com/a/46647783/8457586
const newStatefulPromise = (executor, ...args) => {
  let currentState = "pending";
  let currentResult;
  let promise;

  const setResult = (state, result) => {
    if (currentState !== "pending") {
      return;
    }

    if (promise) {
      // executor had run asynchronously
      promise.state = state;
      promise.result = result;
    } else {
      // executor had run synchronously, promise not yet assigned
      currentState = state;
      currentResult = result;
    }
  };

  const wrapper = (resolve, reject) => {
    const resolveWrapper = (result) => {
      if (result instanceof Promise) {
        result.then(resolveWrapper).catch(rejectWrapper);
      } else {
        setResult("resolved", result);
        resolve(result);
      }
    };

    const rejectWrapper = (result) => {
      if (result instanceof Promise) {
        // always reject
        result.then(rejectWrapper).catch(rejectWrapper);
      } else {
        setResult("rejected", result);
        reject(result);
      }
    };

    executor(...args, resolveWrapper, rejectWrapper);
  };

  promise = new Promise(wrapper);
  promise.state = currentState;
  promise.result = currentResult;

  if (currentState === "rejected") {
    // executor ran synchronously and rejected; catch it here so that the
    // callee does not need to catch it, but can just inspect the state and
    // result
    promise.catch(() => {});
  }
  return promise;
};

// Example:
// let p1 = newStatefulPromise((res, rej) => res('foo'));
// console.log(p1.state, p1.result); // fulfilled
//
// let p2 = newStatefulPromise((res, rej) => res(
//    new Promise((res) => setTimeout(() => res('foo'), 10))
// ));
// console.log(p2.state, p2.result); // pending
// p2.then(() => console.log(p2.state, p2.result))
//
// let p3 = newStatefulPromise((res, rej) => setTimeout(() => res('foo'), 10));
// console.log(p3.state, p3.result); // pending
// p3.then(() => console.log(p3.state, p3.result))