Schedule multiple promise-based tasks on idle browser time.

idleMap.ts

port async function idleMap<Item, Output>(
    iterable: Iterable<Item>,
    op: (item: Item) => Promise<Output>
): Promise<Output[]> {
    return new Promise((resolve) => {
        let iter = iterable[Symbol.iterator]();
        let buf: Output[] = [];

        async function process(deadline: RequestIdleCallbackDeadline) {
            do {
                let result = iter.next();
                if (result.done) {
                    return resolve(buf);
                }

                buf.push(await op(result.value));
            } while (!deadline.didTimeout);

            requestIdleCallback(process);
        }

        requestIdleCallback(process);
    });
}