promises

Promises are a fundamental concept in JavaScript for managing asynchronous operations. They represent a value that may be available now, or in the future, or never. Understanding promises is crucial for handling asynchronous code more effectively and avoiding callback hell.

Key Concepts of Promises
State: A promise can be in one of three states:

Pending: The initial state, neither fulfilled nor rejected.
Fulfilled: The operation completed successfully.
Rejected: The operation failed.
Value: Once a promise is fulfilled, it has a value. If it’s rejected, it has a reason (an error).

Settled: A promise is settled if it is either fulfilled or rejected.

Creating Promises
You can create a promise using the Promise constructor, which takes a function (executor) with two arguments: resolve and reject.

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const myPromise = new Promise((resolve, reject) => {
  const success = true; // Simulate an async operation

  if (success) {
    resolve('Operation was successful!');
  } else {
    reject('Operation failed.');
  }
});
Handling Promises
then Method
Used to handle a fulfilled promise and to chain multiple promises.

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myPromise.then(value => {
  console.log(value); // "Operation was successful!"
}).catch(error => {
  console.error(error); // "Operation failed."
});
catch Method
Used to handle a rejected promise.

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myPromise.catch(error => {
  console.error(error); // "Operation failed."
});
finally Method
Runs regardless of the promise’s outcome.

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myPromise.finally(() => {
  console.log('Promise has been settled.');
});
Chaining Promises
You can chain multiple then calls to handle a sequence of asynchronous operations.

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myPromise
  .then(value => {
    console.log(value);
    return 'Next operation';
  })
  .then(nextValue => {
    console.log(nextValue);
  })
  .catch(error => {
    console.error(error);
  })
  .finally(() => {
    console.log('All done!');
  });
Promise Methods
Promise.all
Waits for all promises to settle and returns an array of their results. If any promise rejects, it immediately rejects with that reason.

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const promise1 = Promise.resolve('First');
const promise2 = Promise.resolve('Second');
const promise3 = Promise.resolve('Third');

Promise.all([promise1, promise2, promise3])
  .then(values => {
    console.log(values); // ["First", "Second", "Third"]
  })
  .catch(error => {
    console.error(error);
  });
Promise.race
Returns the first promise that settles (either fulfills or rejects).

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const promise1 = new Promise((resolve) => setTimeout(resolve, 500, 'First'));
const promise2 = new Promise((resolve) => setTimeout(resolve, 100, 'Second'));

Promise.race([promise1, promise2])
  .then(value => {
    console.log(value); // "Second"
  })
  .catch(error => {
    console.error(error);
  });
Promise.allSettled
Waits for all promises to settle and returns an array of their results with their status.

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const promise1 = Promise.resolve('First');
const promise2 = Promise.reject('Error');
const promise3 = Promise.resolve('Third');

Promise.allSettled([promise1, promise2, promise3])
  .then(results => {
    results.forEach((result) => console.log(result.status));
    // "fulfilled"
    // "rejected"
    // "fulfilled"
  });
Promise.any
Returns the first promise that fulfills, ignoring rejections.

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const promise1 = Promise.reject('Error 1');
const promise2 = Promise.reject('Error 2');
const promise3 = Promise.resolve('First successful');

Promise.any([promise1, promise2, promise3])
  .then(value => {
    console.log(value); // "First successful"
  })
  .catch(error => {
    console.error('All promises were rejected', error);
  });
Using Async/Await
async and await provide a more readable way to work with promises, allowing you to write asynchronous code that looks synchronous.

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async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Error fetching data:', error);
  } finally {
    console.log('Fetch operation complete.');
  }
}

fetchData();
Summary
Promises simplify asynchronous programming by providing a clear and manageable way to handle asynchronous operations.
then, catch, and finally methods help in handling fulfilled, rejected, and settled promises.
Promise methods like all, race, allSettled, and any provide utilities for managing multiple promises.
Async/await syntax allows for writing asynchronous code in a synchronous style, improving readability.