all_settled.md

date	2025-03-27 08:09:10 +0530
description	Promise.allSettled waits for all promises to settle and returns their outcomes. Never rejects, always returns success and error results.
premium	true

✅ 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 }
]
*/

---

Q1: What is the key difference between Promise.all and Promise.allSettled?

• Promise.allSettled runs promises sequentially
• Promise.allSettled is faster than Promise.all
• Promise.allSettled never rejects and returns all outcomes regardless of failures
• Promise.allSettled only works with async/await syntax

Q2: What is the result format for a rejected promise in Promise.allSettled?

• { success: false, error: reason }
• { fulfilled: false, value: reason }
• { status: "rejected", reason: error }
• { state: "error", message: reason }

Q3: What does Promise.allSettled return when passed an empty array?

• It throws an error
• It returns a promise that never settles
• It resolves immediately with an empty array []
• It rejects with an AggregateError