I was rejected from my BirdEye Frontend Software Engineer interview due to this question

I was asked to implement a polyfill for Promise.all, but with a twist:

Instead of resolving when the longest-running promise finishes (like Promise.all does), the custom implementation had to resolve after the cumulative time of all promises.

I attempted a solution — but it didn’t work as expected.

const promise1 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve('promise1 resolved')
    }, 1000)
})

// const promise2 = new Promise((resolve, reject) => {
//     setTimeout(() => {
//         reject("promise2 rejected")
//     }, 2000)
// })

const promise3 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve("promise3 resolved")
    }, 4000)
})

const finalVal = Promise.all([promise1,
    // promise2,
    promise3
])

function customAll(promises) {
    return new Promise((resolve, reject) => {
        let ans = [];
        promises.forEach((p, i) => {
            p.then((res) => {
                ans[i] = res;
                if (ans.length === promises.length) {
                    resolve(ans)
                }
            }).catch((err) => {
                reject(err);
            })
        })
    })
}

Promise.customAll = customAll

const finalVal2 = Promise.customAll([promise1,
    // promise2,
    promise3
])

console.log(finalVal.then((res1) => console.log("res1", res1)), finalVal2.then((res2) => console.log("res2", res2)))

Here’s a near-correct version I came up with (still not perfect):

function customAll(promises) {
    return new Promise((resolve, reject) => {
        const results = [];
        async function runSequentially() {
            try {
                for (let i = 0; i < promises.length; i++) {
                    results[i] = await promises[i]();
                }
                resolve(results);
            } catch (err) {
                reject(err);
            }
        }
        runSequentially();
    });
}

Promise.customAll = customAll;

function createTasks() {
    let ans = []
    for (let i = 0; i < 4; i++) {
        ans.push(() => new Promise((resolve, reject) => setTimeout(() => resolve(`Resolved ${i}`), 1000)))
    }
    return ans;
}
const tasks = createTasks()
console.log(tasks)
Promise.customAll(tasks).then((res) => console.log(res))

If you have a better solution or thoughts on this, I’d love to hear them in the comments. Your input is truly appreciated 🙏

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