Interview question

What is a Promise in JavaScript and what states can it have? JavaScript में Promise क्या है और इसकी क्या states होती हैं?

Answer

A Promise is an object representing the eventual completion or failure of an asynchronous operation. It acts as a placeholder for a value that isn't available yet.

State	Meaning
Pending	Initial state, operation not yet completed
Fulfilled	Operation completed successfully, has a resolved value
Rejected	Operation failed, has a reason/error

// Creating a promise
const myPromise = new Promise((resolve, reject) => {
    const success = true;
    setTimeout(() => {
        if (success) {
            resolve('Operation succeeded');
        } else {
            reject('Operation failed');
        }
    }, 1000);
});

console.log(myPromise); // Promise { <pending> }

myPromise
    .then(result => console.log(result)) // 'Operation succeeded' after 1s
    .catch(error => console.log(error));

// A promise can only settle once - either fulfilled or rejected, never both
const p = new Promise((resolve, reject) => {
    resolve('first');
    reject('second'); // ignored, promise already settled
});
p.then(val => console.log(val)); // 'first'

// Once settled, state and value are locked in permanently
const settled = Promise.resolve(42);
settled.then(val => console.log(val)); // 42, every time it's used

Promise एक object है जो asynchronous operation के eventual completion या failure को represent करता है। यह ऐसे value के लिए placeholder है जो अभी available नहीं है।

State	अर्थ
Pending	Initial state, operation पूरा नहीं हुआ
Fulfilled	Operation successfully पूरा हुआ
Rejected	Operation fail हुआ

const myPromise = new Promise((resolve, reject) => {
    const success = true;
    setTimeout(() => {
        if (success) resolve('Operation succeeded');
        else reject('Operation failed');
    }, 1000);
});

console.log(myPromise); // Promise { <pending> }

myPromise
    .then(result => console.log(result))
    .catch(error => console.log(error));

// Promise एक बार ही settle होता है
const p = new Promise((resolve, reject) => {
    resolve('first');
    reject('second'); // ignore हो जाता है
});
p.then(val => console.log(val)); // 'first'

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