49Implement a priority-based task scheduler

Implement a PriorityScheduler class. Tasks wait in a queue, and whenever a slot is free the highest-priority waiting task runs next.

• new PriorityScheduler({ concurrency = 1 })
• schedule(task, priority = 0): task is a function returning a promise or value. Returns a promise for its result. A higher number means more urgent.
• size: the number of waiting tasks.

const scheduler = new PriorityScheduler({ concurrency: 2 });
scheduler.schedule(() => prefetchImages(), 0);
scheduler.schedule(() => loadVisibleRows(), 10); // runs before the prefetch if both are waiting

Requirements

• Tasks with equal priority run in the order they were scheduled (FIFO).
• Running tasks are never interrupted; priority only decides which waiting task starts next.
• Use a binary heap, so scheduling and picking the next task are O(log n), even with thousands of waiting tasks.
• A failing task rejects only its own promise.