promisePool.ts

const promisePool = <T>(list: Iterable<() => PromiseLike<T>>, limit: number): Promise<T[]> => {
    const result: Promise<T>[] = [];

    const iterator = list[Symbol.iterator]();

    return new Promise((resolve, reject) => {
        const settle = (next: ReturnType<typeof iterator["next"]>) => {
            if (next.done) {
                Promise.all(result).then(resolve, reject);
                return;
            }

            const value = Promise.resolve(next.value());
            result.push(value);
            value.then(() => {
                settle(iterator.next());
            }, reject);
        };
        for (let i = 0; i < limit; i += 1) {
            settle(iterator.next());
        }
    });
};

const a = (n: number): Promise<number> =>
    new Promise((resolve) => {
        setTimeout(() => {
            console.log(n);
            resolve(n);
        }, n);
    });

const b = [() => a(300), () => a(500), () => a(100), () => a(300), () => a(500), () => a(100)];

const c = await promisePool(b, 3);
console.log(c);