Promises & its Properties

🔹 What is a Promise?

• A Promise object represents the eventual completion (or failure) of an asynchronous operation & its resulting value.
• A Promise is an object that represents a value that may be:
  • Pending -> operation not finished yet.
  • Fulfilled -> operation completed successfully.
  • Rejected -> operation failed.
• Eg:

    const promise = new Promise((resolve, reject)=>{
        setTimeout(()=> resolve("Success!"), 1000)
    })

    promise.then(res=> console.log(res)).catch((err)=> {console.log(err)})

🔹 Key Promise Static Methods (Properties)

• 1. Promise.all(iterable)
  • Runs promise in parallel.
  • Resolves only if all succeed, rejects if all fails.
  • Eg:

        Promise.all([Promise.resolve(1), Promise.resolve(2)])
        .then(res=> console.log(res))                       // [1,2]

• 2. Promise.allSettled(iterable)
  • Waits for all promise to settle(fulfilled or rejected).
  • Always return array with {status, value/reason}.
  • Eg:

        Promise.allSettled([Promise.resolve(1), Promise.reject('Err')])
        .then(res=> console.log(res))
        // [ {status:"fulfilled", value:1}, {status:"rejected", reason:"Err"} ]

• 3. Promise.any(iterable)
  • Resolves as soon as any one succeeds.
  • Rejects only if all fails.
  • Eg:

        Promise.any([Promise.reject("fail"), Promise.resolve("win")])
        .then(res => console.log(res));                 // "win"

• 4. Promise.race(iterable)
  • Resolves/rejects with the first promise that settles (success or failure).
  • Eg:

        Promise.race([
            new Promise(res=> setTimeout(()=> res('fast'), 100)),
            new Promise(res=> setTimeout(()=> res('slow'), 1000)),
        ]).then(res=> console.log(res))             // "fast"

🔹 Instance Methods

• .then(onFulFilled) -> runs on success.
• .catch(onRejected) -> runs on failure.
• .finally(onFinally) -> always runs after completion (success or failure).
• Eg:

    promise
    .then(res => console.log(res))
    .catch(err => console.error(err))
    .finally(() => console.log("Done!"));

🔹 Summary Table

Method	Behavior
Promise.all	All must succeed, else reject
Promise.allSettled	Returns result of all (fulfilled/rejected)
Promise.any	Resolves on first success
Promise.race	Resolves/rejects on first finished
.then()	Handle success
.catch()	Handle failure
.finally()	Always run after completion

👉 Quick memory hook:

• all = “everyone must win”
• allSettled = “give me the report card”
• any = “just one winner is enough”
• race = “whoever finishes first wins (success or fail)”