Difference between `Promise.all()` and `Promise.allSettled()`

The core difference between Promise.all() and Promise.allSettled() is that Promise.all() rejects immediately if any of the promises reject whereas Promise.allSettled() waits for all of the promises to settle (either resolve or reject) and then returns the result.

Initialize §

const promise1 = Promise.resolve('Promise 1 resolved');
const promise2 = Promise.reject('Promise 2 rejected');

Using Promise.all() §

Promise.all([promise1, promise2])
  .then((values) => {
    console.log(values);
  })
  .catch((error) => {
    console.log('An error occurred in Promise.all():', error);
  });
 
// Output:
// An error occurred in Promise.all(): Promise 2 rejected

In the above code, the Promise.all() rejects immediately when any of the promise2 rejects.

Using Promise.allSettled() §

Promise.allSettled([promise1, promise2]).then((results) => {
  results.forEach((result, index) => {
    if (result.status === 'fulfilled') {
      console.log(
        `Promise ${index + 1} was fulfilled with value:`,
        result.value
      );
    } else {
      console.log(
        `Promise ${index + 1} was rejected with reason:`,
        result.reason
      );
    }
  });
});
 
// Output:
// Promise 1 was fulfilled with value: Promise 1 resolved
// Promise 2 was rejected with reason: Promise 2 rejected

In the above code, the Promise.allSettled() waits for all of the promises to settle (either resolve or reject) and then returns the result.