async iterable operators (map, filter) using async generators

example.ts

const getName = async function*() {
    yield 'bob';
    yield 'baz';
    yield 'bar';
};

const getAge = (name: string) => {
    const responseJson = fetch('https://httpbin.org/get')
        .then(response => response.json())
        // Let's pretend the API is returning an age for us
        .then(() => Math.round(Math.random() * 100));
    return responseJson;
};

// https://github.com/ReactiveX/IxJS/blob/3218613193258c4d817a4dcbf2423df0e070d516/src/asynciterable/operators/filter.ts
const filter = async function*<T, T2>(
    source: AsyncIterable<T>,
    predicate: (item: T, index: number) => boolean,
) {
    let i = 0;
    for await (const item of source) {
        if (predicate(item, i++)) {
            yield item;
        }
    }
};

// https://github.com/ReactiveX/IxJS/blob/3218613193258c4d817a4dcbf2423df0e070d516/src/asynciterable/operators/map.ts
const map = async function*<T, T2>(
    source: AsyncIterable<T>,
    selector: (item: T, index: number) => T2 | Promise<T2>,
) {
    let i = 0;
    for await (const item of source) {
        const result = await selector(item, i++);
        yield result;
    }
};

const stringAsyncIterable = map(
    filter(getName(), name => name === 'bob'),
    name => getAge(name).then(age => `${name}'s current age: ${age}`),
);

(async () => {
    for await (let value of stringAsyncIterable) {
        console.log(value);
    }
})();