Execute Asynchronous Functions in Parallel

Problem Statement

In this problem, you are provided with an array named functions, wherein each element of this array is an asynchronous function that returns a promise. The task is to process these functions in parallel and track their completions. The output should be a new promise, promise, that handles the results from each of the asynchronous functions in the array.

• Resolution Scenario: promise should resolve if all the asynchronous functions resolve their promises successfully. The result should be an array containing the resolved values of these promises, maintaining the same order as they appear in the input array functions.

• Rejection Scenario: promise should reject if any of the asynchronous function's promises get rejected. Additionally, the rejection reason should be the reason provided by the first promise that gets rejected.

Importantly, the challenge specifies not to use the built-in Promise.all function, which typically handles such scenarios, hence requiring a manual implementation of promise handling.

Examples

Example 1

Input:

functions = [
  () => new Promise(resolve => setTimeout(() => resolve(5), 200))
]

Output:

{"t": 200, "resolved": [5]}

Explanation:

promiseAll(functions).then(console.log); // [5]
The single function was resolved at 200ms with a value of 5.

Example 2

Input:

functions = [
() => new Promise(resolve => setTimeout(() => resolve(1), 200)),
() => new Promise((resolve, reject) => setTimeout(() => reject("Error"), 100))
]

Output:

{"t": 100, "rejected": "Error"}

Explanation:

Since one of the promises rejected, the returned promise also rejected with the same error at the same time.

Example 3

Input:

functions = [
() => new Promise(resolve => setTimeout(() => resolve(4), 50)),
() => new Promise(resolve => setTimeout(() => resolve(10), 150)),
() => new Promise(resolve => setTimeout(() => resolve(16), 100))
]

Output:

{"t": 150, "resolved": [4, 10, 16]}

Explanation:

All the promises resolved with a value. The returned promise resolved when the last promise resolved.

Constraints

• functions is an array of functions that returns promises
• 1 <= functions.length <= 10

Approach and Intuition

Given the constraints and explanations in the examples, let's construct the approach to solve this problem:

1. Initialization of Variables:

   • Create an array to store the results of the promises.
   • Initiate a counter to keep track of completed promises.

2. Handling Each Function:

   • Iterate over the array of functions.
   • Immediately invoke each function (since they are asynchronous and return promises).
   • Attach .then and .catch handlers to each promise returned by the functions.

3. Resolve Handling:

   • In the .then handler, update the results array with the resolved value at the respective index.
   • Increment the counter of completed promises.
   • If the counter indicates all promises are resolved, the main promise should resolve with the array of results.

4. Reject Handling:

   • In the .catch handler, immediately reject the main promise with the encountered error, if it is the first to fail.
   • Ensure to handle synchronization issues to avoid multiple rejections.

5. Concurrency Assurance:

   • All functions are triggered in the same synchronous loop, ensuring they run in parallel.
   • JavaScript’s event loop and the inherent nature of promises handle the concurrency.

By following these steps, one can mimic what Promise.all offers but with explicit control over its internal workings, appropriately handling all given conditions from the problem statement.

Solutions

var executeAllPromises = function(promisesArray) {
    return new Promise((resolve, reject) => {
        if(promisesArray.length === 0) {
            resolve([]);
            return;
        }
        
        const results = new Array(promisesArray.length).fill(null);
        let completionCount = 0;

        promisesArray.forEach((promise, index) => {
            promise().then((result) => {
                results[index] = result;
                completionCount++;
                if(completionCount === promisesArray.length) {
                    resolve(results);
                }
            }).catch((error) => {
                reject(error);
            })
        });
    });
};