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runWithLimit: promise pool

Write a function runWithLimit(tasks, limit). tasks is an array of functions that each return a promise. Run them all, but with no more than limit running at once: as soon as one finishes, the next one starts.

Return a promise with the results in the order of tasks. If a task fails, the overall promise rejects with that error and no new tasks are started.

function runWithLimit(tasks, limit) {
  // ...
}

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Tests (6)

let active = 0;
let peak = 0;
const task = (v) => async () => { active++; peak = Math.max(peak, active); await wait(20); active--; return v; };
const result = await runWithLimit([1, 2, 3, 4, 5].map(task), 2);
expect(result).toEqual([1, 2, 3, 4, 5]);
expect(peak).toBe(2);

const order = [];
const task = (v, ms) => async () => { await wait(ms); order.push(v); return v; };
const result = await runWithLimit([task('a', 60), task('b', 10), task('c', 10)], 2);
expect(result).toEqual(['a', 'b', 'c']);
expect(order).toEqual(['b', 'c', 'a']);

expect(await runWithLimit([], 3)).toEqual([]);

const error = new Error('fail');
await expect(runWithLimit([async () => 1, async () => { throw error; }], 2)).rejects.toBe(error);

const started = fn();
const tasks = [1, 2, 3].map((v) => () => { started(v); return wait(30).then(() => v); });
const promise = runWithLimit(tasks, 1);
await wait(10);
expect(started).toHaveBeenCalledTimes(1);
expect(await promise).toEqual([1, 2, 3]);
expect(started).toHaveBeenCalledTimes(3);

const started = Date.now();
await runWithLimit([1, 2, 3, 4].map(() => () => wait(40)), 4);
expect(Date.now() - started < 120).toBe(true);