Async and Errors in Typescript

asyncErrors.ts

const promiseAndError = async (n: number): Promise<any> => {

    if (n == 666) {
        throw new Error("truly bad news");
    }

    if ( n < 0 )
        throw new Error("Negative n illegal");

    if ( n % 2 == 1 )
        return Promise.resolve({result: "odd"});
    else
        return Promise.resolve({result: "even"});
}

// https://2ality.com/2016/03/promise-rejections-vs-exceptions.html
// https://thecodebarbarian.com/async-functions-in-javascript.html#error-handling
//
// https://twitter.com/bterlson/status/713402741733785600?s=20

// "TC39 says no. Originally, param initializer exprs would throw out of async
// functions but now rejects promise. For the best, IMO."
//
// So, do a reject

const isEvenOrNegative = async (n: number): Promise<boolean> => {

    console.log(`>>> Testing ${n}`);
    try {

        if (n == 999) {
            // Sneaky n thing throws from here
            throw new Error("999 almost got through.");
        }

        return promiseAndError(n).then(
            ({result}) => {
                return new Promise((resolve, reject) => {
                    resolve(result == "even");
                })});
    } catch(err) {
        // This is the failure of promiseAndError
        return Promise.reject(err);
    }
};

const main = async () => {

    // test could be parameterized...

    try {
        console.log(await isEvenOrNegative(666));
    } catch (err) {
        console.log(`try/catch: ${err}`);
    }

    try {
        console.log(await isEvenOrNegative(999));
    } catch (err) {
        console.log(`try/catch: ${err}`);
    }


    // This needs await to execute the reject.
    await isEvenOrNegative(666).then(
        (x) => console.log(`Shouldnt' be ${x}`)
    ).catch(
        (err) => console.log(`promise.catch: ${err}`));

    await isEvenOrNegative(999).then(
        (x) => console.log(`Shouldnt' be ${x}`)
    ).catch(
        (err) => console.log(`promise.catch: ${err}`));

    // 0
    try {
        console.log(await isEvenOrNegative(0));
    } catch (err) {
        console.log('no way');
    }

    // < 0
    try {
        console.log(await isEvenOrNegative(-1));
    } catch (err) {
        console.log(`should be neg: ${err}`);
    }

    await isEvenOrNegative(-999).then(
        (x) => console.log(`Shouldnt' be ${x}`)
    ).catch(
        (err) => console.log(`promise.catch neg: ${err}`));

}


main();