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2721. Execute Asynchronous Functions in Parallel

Description

Given an array ofΒ asynchronous functionsΒ functions, return a new promise promise. Each function in the array accepts no argumentsΒ and returns a promise. All the promises should be executed in parallel.

promise resolves:

  • When all the promises returned fromΒ functionsΒ were resolved successfully in parallel.Β The resolvedΒ value ofΒ promise should be an array of all the resolved values of promises in the same order as they were in theΒ functions. The promise should resolve when all the asynchronous functions in the array have completed execution in parallel.

promise rejects:

  • When anyΒ of the promisesΒ returned fromΒ functionsΒ were rejected.Β promise should alsoΒ rejectΒ with the reason of the first rejection.

Please solve it without using the built-inΒ Promise.allΒ function.

Β 

Example 1:

Input: functions = [
Β  () => 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.

Β 

Constraints:

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

Solutions

Solution 1

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async function promiseAll<T>(functions: (() => Promise<T>)[]): Promise<T[]> {
    return new Promise<T[]>((resolve, reject) => {
        let cnt = 0;
        const ans = new Array(functions.length);
        for (let i = 0; i < functions.length; ++i) {
            const f = functions[i];
            f()
                .then(res => {
                    ans[i] = res;
                    cnt++;
                    if (cnt === functions.length) {
                        resolve(ans);
                    }
                })
                .catch(err => {
                    reject(err);
                });
        }
    });
}

/**
 * const promise = promiseAll([() => new Promise(res => res(42))])
 * promise.then(console.log); // [42]
 */

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