asyncjsのよくありそうな処理に型を律義につけるとこうなるらしい

asyncjs_typed.ts

import async, {AsyncFunction, AsyncResultArrayCallback} from "async";

const main = (): void => {
    const funcs0: AsyncFunction<number, Error>[] = [1, 2, 3].map((num: number): AsyncFunction<number, Error> => {
        return (callback: (err: Error | null, result?: number) => void): void => {
            callback(null, num * 2);
        }
    });

    const after: AsyncResultArrayCallback<number, Error> = (errors: Error | undefined | null, results: (number | undefined)[] | undefined): void => {
        if (errors) {
            console.error(errors);
        } else {
            console.log(results);
        }
    };

    async.parallel<number, Error>(funcs0, after);
};

main();