Question

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ERR_UNHANDLED_REJECTION UnhandledPromiseRejection when awaited inside an array of promise Objects

rated 0 times [ 16] [ 6] / answers: 1 / hits: 11136 / 3 Years ago, mon, june 21, 2021, 12:00:00

I am trying to understand async await with promises and for..of loop. I have a function which generates promise by resolving if an even number is passed to it and by rejecting if odd number is passed to it. Inside an async function I am tying to await but getting UnhandledPromiseRejection even though I am catching the error.
My question is what points i am missing. Why I am getting UnhandledPromiseRejection and how to handle it. I need conceptual information

function genPromise(value) {

    return new Promise((resolve, reject) => {

        if (value % 2) {

            resolve(`The value ${value} is odd`);

        } else {

            reject(`The value ${value} is even`);

        }

    })

}



(async function () {



    x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    let y = x.map(d => genPromise(d))

    try {

        for await (let k of y) {

            console.log(k);

        }

    } catch (err) {

        console.log(err)

    }

})();

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amari

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answered 3 Years ago lesli · Accepted Answer

for await( is only intended for use with async iterables. You do not have an async iterable - only a normal iterable, in which case it's equivalent to for..of:

for (const k of y) {

  console.log(k);

}

The k promise is not being caught, hence the rejection.

Await k inside the loop, without for await:

function genPromise(value) {

    return new Promise((resolve, reject) => {

        if (value % 2) {

            resolve(`The value ${value} is odd`);

        } else {

            reject(`The value ${value} is even`);

        }

    })

}



(async function () {



    x = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    let y = x.map(d => genPromise(d))

    try {

        for (const k of y) {

            console.log(await k);

        }

    } catch (err) {

        console.log('error', err)

    }

})();

If you wanted to make an async iterable from the array of numbers, then you'd do something like:

function* genPromise(arr) {

    for (const value of arr) {

        yield new Promise((resolve, reject) => {

            if (value % 2) {

                resolve(`The value ${value} is odd`);

            } else {

                reject(`The value ${value} is even`);

            }

        });

    }

}



(async function () {

    const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    try {

        for await (const k of genPromise(arr)) {

            console.log(k);

        }

    } catch (err) {

        console.log('error', err)

    }

})();