Q242- Explain the difference between Promise.all, Promise.any, and Promise.race?

JavaScript provides these methods to manage multiple asynchronous operations.

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✅ 1. Promise.all()

✔ Purpose:

Wait for ALL promises to succeed.
If ANY promise fails → the entire result fails.

✔ Best for:

• Fetching multiple API calls that all are required
• Loading dashboard data
• Running tasks in parallel

✔ Behavior:

• Returns array of results
• Rejects immediately if any promise rejects

Example:

Promise.all([
  fetch("/user"),
  fetch("/orders"),
  fetch("/cart")
])
  .then(res => console.log("All success"))
  .catch(err => console.log("At least one failed"));

---

✅ 2. Promise.any()

✔ Purpose:

Returns the result of the first successful promise.
It ignores failures unless all fail.

✔ Best for:

• Fastest server response
• Fallback requests
• Multiple CDN URLs → take the first one that works

✔ Behavior:

• Resolves on first success
• Rejects only if ALL promises fail

Example:

Promise.any([
  fetch("https://cdn1.com/file"),
  fetch("https://cdn2.com/file"),
  fetch("https://cdn3.com/file")
])
.then(res => console.log("First successful response"))
.catch(err => console.log("All failed"));

---

✅ 3. Promise.race()

✔ Purpose:

Returns the result of the first promise that settles
(doesn’t matter if it's resolved or rejected).

✔ Best for:

• Timeout or cancellation logic
• Taking the quickest operation (success or failure)

✔ Behavior:

• Whichever promise finishes first → resolve or reject immediately

Example:

const fetchData = fetch("/api/data");
const timeout = new Promise((_, reject) =>
  setTimeout(() => reject("Timeout"), 2000)
);

Promise.race([fetchData, timeout])
  .then(res => console.log("Completed first"))
  .catch(err => console.log("First returned error:", err));

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🔥 Summary Table

Method	Resolves When	Rejects When	Use Case
Promise.all	All succeed	Any fails	Load multiple required APIs
Promise.any	First success	All fail	Fastest success, CDN fallback
Promise.race	First settles (success/fail)	First to settle is rejection	Timeout logic, speed competition

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🎯 Short Interview Answer

Promise.all waits for all promises and fails if one fails.
Promise.any returns the first successful promise and ignores failures unless all fail.
Promise.race returns the first promise that settles, whether success or failure.