JavaScript Promise API

JavaScript Promise API

The Promise API provides powerful methods to work with multiple promises efficiently.

🔹 1. Promise.all() – Wait for All Promises

✅ Resolves when all promises succeed
❌ Rejects if any fail

let p1 = Promise.resolve(10);
let p2 = new Promise((resolve) => setTimeout(() => resolve(20), 2000));
let p3 = Promise.reject("Error occurred!");

Promise.all([p1, p2, p3])
    .then((results) => console.log("✅ Success:", results))
    .catch((error) => console.log("❌ Error:", error));

✅ Output:

❌ Error: Error occurred!

✔ If one fails, everything fails!

🔹 2. Promise.allSettled() – Handle All Outcomes

✅ Waits for all promises
✔ Returns both resolved & rejected results

let p1 = Promise.resolve(10);
let p2 = new Promise((resolve) => setTimeout(() => resolve(20), 2000));
let p3 = Promise.reject("Error occurred!");

Promise.allSettled([p1, p2, p3]).then((results) => console.log(results));

✅ Output:

[
    { "status": "fulfilled", "value": 10 },
    { "status": "fulfilled", "value": 20 },
    { "status": "rejected", "reason": "Error occurred!" }
]

✔ Handles errors gracefully!

🔹 3. Promise.race() – First One Wins

✅ Resolves/rejects as soon as the first promise settles

let fast = new Promise((resolve) => setTimeout(() => resolve("🏎 Fast!"), 500));
let slow = new Promise((resolve) => setTimeout(() => resolve("🐌 Slow!"), 2000));
let error = new Promise((_, reject) => setTimeout(() => reject("❌ Failed!"), 1000));

Promise.race([fast, slow, error])
    .then((result) => console.log("✅ Winner:", result))
    .catch((error) => console.log("❌ Error:", error));

✅ Output (after 500ms):

✅ Winner: 🏎 Fast!

✔ First resolved/rejected promise decides the result.

🔹 4. Promise.any() – First Success Wins

✅ Resolves when any promise resolves
❌ Fails only if all fail

let fail1 = new Promise((_, reject) => setTimeout(() => reject("❌ Error 1"), 500));
let success = new Promise((resolve) => setTimeout(() => resolve("🎉 Success!"), 1000));
let fail2 = new Promise((_, reject) => setTimeout(() => reject("❌ Error 2"), 1500));

Promise.any([fail1, success, fail2])
    .then((result) => console.log("✅ First success:", result))
    .catch((error) => console.log("❌ All failed:", error.errors));

✅ Output (after 1 sec):

✅ First success: 🎉 Success!

✔ Only rejects if all fail!

🔹 5. Promise.resolve() & Promise.reject()

✅ Promise.resolve(value)

Returns a promise that resolves instantly.

Promise.resolve(42).then(console.log); // ✅ 42

❌ Promise.reject(error)

Returns a promise that rejects instantly.

Promise.reject("Something went wrong!").catch(console.log); // ❌ Something went wrong!

🔹 Summary

Method	Resolves when	Rejects when	Use case
Promise.all()	All promises resolve	Any promise rejects	Run multiple promises and fail fast
Promise.allSettled()	All promises settle	Never rejects	Get both success & failure results
Promise.race()	First promise resolves/rejects	First promise rejects	Get first completed result
Promise.any()	First promise resolves	Only if all reject	Get first success & ignore failures
Promise.resolve(value)	Instantly resolves	Never	Create resolved promise
Promise.reject(error)	Never	Instantly rejects	Create rejected promise

🚀 Need examples? Just ask! 😊