Coding Challenge Practice - Question 94

Source: Dev.to

Problem Description

Implement Promise.any(). The function receives an array of promises and returns a single promise that resolves with the value of the first fulfilled promise, or rejects with an AggregateError containing all rejection reasons if every promise rejects.

Boilerplate

function any(promises) {
  // your code here
}

Implementation Details

• Store each rejection reason, track how many promises have fulfilled, and keep the total number of promises.
• If the input array is empty, reject immediately with an AggregateError.

const errors = [];
let rejectedCount = 0;
const total = promises.length;

if (total === 0) {
  reject(new AggregateError([], 'All promises were rejected'));
  return;
}

• Iterate over the promises, normalising each with Promise.resolve.

promises.forEach((promise, index) => {
  Promise.resolve(promise).then(
    // As soon as one promise fulfills, resolve
    value => {
      resolve(value); // first fulfilled value wins
    },
    // If a promise rejects, store the error
    error => {
      errors[index] = error;
      rejectedCount++;

      // If all promises have rejected, reject with AggregateError
      if (rejectedCount === total) {
        reject(new AggregateError(errors, 'All promises were rejected'));
      }
    }
  );
});

Final Code

function any(promises) {
  return new Promise((resolve, reject) => {
    const errors = [];
    let rejectedCount = 0;
    const total = promises.length;

    if (total === 0) {
      reject(new AggregateError([], "All promises were rejected"));
      return;
    }

    promises.forEach((promise, index) => {
      Promise.resolve(promise).then(
        value => {
          resolve(value);
        },
        error => {
          errors[index] = error;
          rejectedCount++;

          if (rejectedCount === total) {
            reject(new AggregateError("All promises were rejected", errors));
          }
        }
      );
    });
  });
}