✅ Promise.allSettled() Implementation

Promise.allSettled() returns a promise that resolves after all input promises have settled (either fulfilled or rejected). Unlike Promise.all(), it never rejects—it always returns an array of result objects describing each promise's outcome.

🧠 Understanding Promise.allSettled

Atom 1: Purpose

Promise.allSettled returns a promise that:

• ✅ Resolves when all input promises settle (either resolve or reject)
• ❌ Never rejects — result always contains an array of outcomes

Atom 2: Result Format

Each result is an object with shape:

• If resolved: { status: "fulfilled", value: result }
• If rejected: { status: "rejected", reason: error }

Atom 3: Normalize Inputs

Use Promise.resolve(...) to handle non-promise inputs.

Atom 4: Track Completion

• Maintain results[]
• Track how many have settled
• Resolve once all have settled

Atom 5: Edge Case

• Empty input → resolve to [] immediately

✅ Implementation: promiseAllSettled

function promiseAllSettled(iterable) {
  return new Promise((resolve) => {
    const promises = Array.from(iterable);
    const results = [];
    let settledCount = 0;

    if (promises.length === 0) return resolve([]);

    promises.forEach((p, i) => {
      Promise.resolve(p)
        .then(value => {
          results[i] = { status: "fulfilled", value };
        })
        .catch(reason => {
          results[i] = { status: "rejected", reason };
        })
        .finally(() => {
          settledCount++;
          if (settledCount === promises.length) {
            resolve(results);
          }
        });
    });
  });
}

🧪 Example:

promiseAllSettled([
  Promise.resolve(1),
  Promise.reject("fail"),
  3
]).then(console.log);

/*
[
  { status: "fulfilled", value: 1 },
  { status: "rejected", reason: "fail" },
  { status: "fulfilled", value: 3 }
]
*/