48Implement an asynchronous task runner with configurable concurrency

Implement a TaskRunner class. Unlike a pool over a fixed list, tasks can be added at any time, and the concurrency can change while it runs.

• new TaskRunner({ concurrency = 2 })
• add(task): task is a function returning a promise (or a value). Returns a promise that settles with that task's result.
• setConcurrency(n): change the limit. Raising it starts waiting tasks immediately; lowering it lets running tasks finish but starts no more until there is room.
• onIdle(): a promise that resolves when nothing is running and nothing is waiting.
• running and waiting: the current counts.

const runner = new TaskRunner({ concurrency: 2 });
const thumbnail = runner.add(() => resize(image, 200));
const preview = runner.add(() => resize(image, 800));
runner.setConcurrency(navigator.hardwareConcurrency ?? 4);
await runner.onIdle();

Requirements

• Tasks start in the order they were added.
• A failing task rejects only its own promise; the runner keeps going.
• A synchronous throw in a task counts as a failure of that task.