🎯 Promise.any() Implementation

Promise.any() takes an iterable of promises and returns a single promise that resolves as soon as any of the input promises fulfills. If all promises reject, it rejects with an AggregateError containing all rejection reasons.

🧠 Understanding Promise.any

Atom 1: Purpose

Promise.any returns a promise that:

• ✅ Resolves as soon as any input promise resolves
• ❌ Rejects only if all input promises reject

Atom 2: Normalization

• Convert all inputs to promises with Promise.resolve(...)

Atom 3: Resolve on First Fulfilled

• On first .then, resolve the outer promise.

Atom 4: Track Rejections

• Maintain count of rejections.
• If all promises reject, reject with AggregateError , which contains all rejection reasons.

Atom 5: Edge Case

• If input is empty → reject immediately with AggregateError

✅ Implementation: promiseAny

function promiseAny(iterable) {
  return new Promise((resolve, reject) => {
    const promises = Array.from(iterable);
    const errors = [];
    let rejectedCount = 0;

    if (promises.length === 0) {
      return reject(new AggregateError([], "All promises were rejected"));
    }

    promises.forEach((p, i) => {
      Promise.resolve(p)
        .then(resolve)
        .catch((err) => {
          errors[i] = err;
          rejectedCount++;
          if (rejectedCount === promises.length) {
            reject(new AggregateError(errors, "All promises were rejected"));
          }
        });
    });
  });
}

🧪 Example:

promiseAny([
  Promise.reject("fail1"),
  new Promise(res => setTimeout(() => res("success"), 100)),
  Promise.reject("fail2")
]).then(console.log)
  .catch(console.error); // → "success"

❌ If all reject:

promiseAny([
  Promise.reject("fail1"),
  Promise.reject("fail2")
]).then(console.log)
  .catch(err => {
    console.error(err instanceof AggregateError); // true
    console.error(err.errors);                   // ["fail1", "fail2"]
  });