Task Scheduler with Priority
Problem statement
Implement a small TaskScheduler class with concurrency and priority support.
API
const scheduler = new TaskScheduler(2);
scheduler.add(() => fetch('/a'), { priority: 10 });
scheduler.add(() => fetch('/b'), { priority: 1 });Requirements
- The constructor receives
concurrency. add(task, options)returns a promise for that task's result.- Run at most
concurrencytasks at once. - Higher priority tasks should start before lower priority queued tasks.
- For equal priority, preserve FIFO order.
- Support synchronous return values, async results, and thrown errors.
This is a senior-style async question because it combines queue modeling, promise resolution, fairness, and concurrency control.
Requirements & constraints
- Do not run more than `concurrency` tasks at once.
- Higher priority means earlier start, not preemption of already-running tasks.
- Preserve FIFO ordering for same priority.
How to approach Task Scheduler with Priority
The strategy an interviewer expects you to reach for.
A priority task scheduler is a promise pool with a queue: VIP work jumps ahead without starving concurrency rules. Naive move Always await tasks in arrival order, or sort the whole queue on every insert without a stable tie-break. Where it breaks Equal priorities should keep FIFO order. Finishing a task must drain the highest waiting work up to concurrency. The fix Enqueue {priority, seq, task, resolve, reject}. On add/finish, _drain: while running < concurrency, pop best priority (then lowest seq), run it, settle its promise. Remember: Priority first, insertion sequence second — stable under ties. Say this in the interview: I'd queue tasks with priority and a monotonic sequence, then drain into a fixed concurrency limit so higher priorities run first while equal priorities stay FIFO.
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