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Build an Async Priority Task Scheduler

Q: Build an Async Priority Task Scheduler

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Medium > **How to read this doc:** the real, defining behavior a priority scheduler must get right — that tasks NOT YET STARTED run in priority order, while an ALREADY-RUNNING task is never interrupted — was verified directly with a real, logged execution sequence: a low-priority task allowed to keep running, followed correctly by high-before-medium for the two tasks queued after it. ## 1. The problem, restated A scheduler accepting async tasks with a numeric priority, running them respecting a configured concurrency limit — but critically, among tasks that have NOT yet started, a higher-priority one must run BEFORE a lower-priority one, regardless of which was scheduled first. JavaScript's single-threaded, run-to-completion nature means a genuinely ALREADY-RUNNING task cannot be interrupted mid-execution — priority only affects which task starts NEXT, once a slot is free. ## 2. Cla

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The problem

Problem, as an interviewer would state it: "Build a task scheduler that runs queued ASYNC tasks by PRIORITY, not insertion order — a higher-priority task queued later should still run before a lower-priority one that was queued earlier, as long as neither has started yet."

Examples:

Code

scheduler.schedule(lowPriorityTask, 1);
scheduler.schedule(highPriorityTask, 10); // queued SECOND, but should run FIRST

Clarifying questions expected:

Can an already-RUNNING task be preempted by a higher-priority one arriving later, or does priority only affect the order of tasks that have not started yet?
What real concurrency limit applies — one task at a time, or several running simultaneously?
Should two tasks with the SAME priority run in their original insertion order (a stable sort)?

Code / implementation expected: Yes — real, direct, logged proof that tasks queued after an already-started one correctly run in PRIORITY order, not insertion order.

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utility
algorithm

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Solution

Target Audience: Engineers preparing for practical JavaScript coding interviews. Difficulty: Medium

How to read this doc: the real, defining behavior a priority scheduler must get right — that tasks NOT YET STARTED run in priority order, while an ALREADY-RUNNING task is never interrupted — was verified directly with a real, logged execution sequence: a low-priority task allowed to keep running, followed correctly by high-before-medium for the two tasks queued after it.

1. The problem, restated

A scheduler accepting async tasks with a numeric priority, running them respecting a configured concurrency limit — but critically, among tasks that have NOT yet started, a higher-priority one must run BEFORE a lower-priority one, regardless of which was scheduled first. JavaScript's single-threaded, run-to-completion nature means a genuinely ALREADY-RUNNING task cannot be inter

The solution is hidden so you can try first

7 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal, direct proof: two tasks queued after an already-running low-priority task correctly run in priority order (high before medium), not insertion order

Practice

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