Add Two Promises

Problem Statement

In this task, you are provided with two promises: promise1 and promise2. Each promise, when resolved, produces a numerical value. The core challenge is to craft a new promise which, upon resolution, provides the sum of the numbers obtained from the resolutions of promise1 and promise2. This problem tests your ability to handle asynchronous JavaScript operations and combine their results effectively.

Examples

Example 1

Input:

promise1 = new Promise(resolve => setTimeout(() => resolve(2), 20)),
promise2 = new Promise(resolve => setTimeout(() => resolve(5), 60))

Output:

7

Explanation:

The two input promises resolve with the values of 2 and 5 respectively. The returned promise should resolve with a value of 2 + 5 = 7. The time the returned promise resolves is not judged for this problem.

Example 2

Input:

promise1 = new Promise(resolve => setTimeout(() => resolve(10), 50)),
promise2 = new Promise(resolve => setTimeout(() => resolve(-12), 30))

Output:

-2

Explanation:

The two input promises resolve with the values of 10 and -12 respectively. The returned promise should resolve with a value of 10 + -12 = -2.

Constraints

• promise1 and promise2 are promises that resolve with a number

Approach and Intuition

When approaching the problem of combining the results of two resolved promises, it's essential to handle the asynchronous nature of JavaScript promises. Here's how you can think about and solve this problem:

1. Both promise1 and promise2 resolve to a numerical value as confirmed by the constraints.
2. Your task is to ensure that you only attempt to sum these numbers once both promises have fully resolved. Attempting to access their values prematurely will not yield the correct results since promises are asynchronous.

Using Promise Handling Functions:

• Promise.all():
  • Utilize Promise.all() to await the resolution of both promise1 and promise2.
  • Promise.all() takes an array of promises and returns a new promise that resolves when all the promises in the array have resolved or if any promise is rejected.
  • The output of Promise.all() is an array containing the resolved values of each promise in the same order as they were passed in.
  • You can then sum the numerical values from the resolved array.

Implementation Steps:

1. Pass promise1 and promise2 within an array to Promise.all(). This will handle waiting for both promises to resolve.
2. Use .then() on the result of Promise.all() to access the array of resolved values.
3. Inside the .then() callback, extract the values, sum them, and resolve this sum with a new promise.
4. You also need to consider error handling. If any promise rejects, handle this gracefully.

This approach ensures that you're combining the data from both promises accurately and efficiently, respecting the asynchronous nature of JavaScript operations.

Solutions

/**
 * @param {Promise} firstPromise
 * @param {Promise} secondPromise
 * @return {Promise}
 */
var concatenatePromises = async function (firstPromise, secondPromise) {
  return new Promise((resolve, reject) => {
    let pendingPromises = 2;
    let totalResult = 0;
    
    [firstPromise, secondPromise].forEach(async singlePromise => {
      try {
        const partialResult = await singlePromise;
        totalResult += partialResult;
        pendingPromises--;

        if (pendingPromises === 0) {
          resolve(totalResult);
        }
      } catch (error) {
        reject(error);
      }
    });
  });
};