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How do .then(), .catch(), and .finally() work in Promise chains? Promise chains में .then(), .catch(), और .finally() कैसे काम करते हैं?

Answer

These methods let you handle a promise's eventual result. Each returns a new Promise, which is what enables chaining.

Method	Runs when	Purpose
.then()	Promise fulfills	Handle success value, can chain more logic
.catch()	Promise rejects (anywhere upstream)	Handle errors
.finally()	Always, regardless of outcome	Cleanup logic (hide spinner, close connection)

function fetchUser(id) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (id > 0) resolve({ id, name: 'John' });
            else reject(new Error('Invalid ID'));
        }, 500);
    });
}

fetchUser(1)
    .then(user => {
        console.log('User:', user);
        return user.id; // returned value passed to next .then()
    })
    .then(id => {
        console.log('ID was:', id);
    })
    .catch(error => {
        console.log('Error:', error.message); // catches errors from ANY step above
    })
    .finally(() => {
        console.log('Request finished'); // always runs
    });

// A .catch() placed early stops error propagation for subsequent .then()
fetchUser(-1)
    .then(user => console.log(user)) // skipped due to rejection
    .catch(error => {
        console.log('Caught:', error.message);
        return 'default user'; // recovers the chain
    })
    .then(value => console.log('Continues with:', value)); // runs normally

ये methods promise के eventual result को handle करने देते हैं। हर method एक नया Promise return करता है, इसीलिए chaining possible है।

Method	कब चलता है	उद्देश्य
.then()	Promise fulfill होने पर	Success value handle करना
.catch()	Promise reject होने पर	Errors handle करना
.finally()	हमेशा	Cleanup logic

function fetchUser(id) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (id > 0) resolve({ id, name: 'John' });
            else reject(new Error('Invalid ID'));
        }, 500);
    });
}

fetchUser(1)
    .then(user => {
        console.log('User:', user);
        return user.id;
    })
    .then(id => console.log('ID was:', id))
    .catch(error => console.log('Error:', error.message))
    .finally(() => console.log('Request finished'));

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