Q328- Difference between Promise.all vs Promise.allSettled?

Both are used to handle multiple Promises, but they behave very differently when some Promises fail.

---

🔹 Promise.all()

✅ What it does

• Waits for all Promises to resolve
• Fails fast if any Promise rejects

Promise.all([
  Promise.resolve("A"),
  Promise.reject("Error"),
  Promise.resolve("B")
])
.then(console.log)
.catch(console.error);

❌ Output

Error

🧠 Explanation

• As soon as one Promise rejects, Promise.all rejects
• Remaining Promises are ignored

---

📌 When to use Promise.all()

✔ All operations are required
✔ Failure of one means failure of whole task
✔ Example: loading critical data

---

🔹 Promise.allSettled()

✅ What it does

• Waits for all Promises to complete
• Never rejects
• Returns result of each Promise

Promise.allSettled([
  Promise.resolve("A"),
  Promise.reject("Error"),
  Promise.resolve("B")
])
.then(console.log);

✅ Output

[
  { status: "fulfilled", value: "A" },
  { status: "rejected", reason: "Error" },
  { status: "fulfilled", value: "B" }
]

🧠 Explanation

• Every Promise is tracked
• Useful for partial success handling

---

🔥 Key Differences (Quick Table)

Feature	Promise.all	Promise.allSettled
Waits for all	❌ No	✅ Yes
Rejects on failure	✅ Yes	❌ No
Returns results	Only if all succeed	Always
Use case	All-or-nothing	Partial success

---

🎯 Short Interview Answer

Promise.all() resolves only if all Promises resolve and rejects immediately on the first failure, while Promise.allSettled() waits for all Promises to finish and returns their individual outcomes regardless of success or failure.

---

⭐ One-line summary

Use Promise.all for all-or-nothing, Promise.allSettled when every result matters.