Microtask scheduler task runner

Q: Microtask scheduler task runner

**Target Audience:** Engineers preparing for practical JavaScript coding interviews. **Difficulty:** Hard > **How to read this doc:** the real ordering guarantee this question specifically tests — that queued microtasks run before ANY real, pending macrotask, even one scheduled EARLIER — was verified directly against a genuine `setTimeout(0)` call made before the runner's own tasks were pushed. ## 1. The problem, restated JavaScript's event loop drains the ENTIRE microtask queue completely, after every single synchronous stretch of code, BEFORE processing even one macrotask (a `setTimeout` callback, a UI event, etc.) — regardless of which was scheduled first in real wall-clock terms. A microtask-based runner batches pushed callbacks to run together via `queueMicrotask`, guaranteeing this real ordering. ## 2. Clarifying questions a strong candidate asks | Question | Why it matters | | :--- | :--- | | Multiple pushes batched into one microtask? | Yes, genuinely the real, common conv

The problem

Problem, as an interviewer would state it: "Build a task runner that batches multiple pushed tasks and runs them all together as MICROTASKS — the whole batch must run before any real, already-scheduled macrotask (like a setTimeout), even one scheduled earlier."

Examples:

Code

setTimeout(() => console.log("macrotask"), 0);
runner.push(() => console.log("microtask 1"));
runner.push(() => console.log("microtask 2"));
// real order: microtask 1, microtask 2, THEN macrotask -- despite setTimeout being called first

Clarifying questions expected:

Should multiple pushes made synchronously in the same tick be batched into ONE real microtask, or does each get its own separate microtask?
Does the runner need to guarantee tasks run in the order they were pushed?
What is the real, defining difference between a microtask and a macrotask that makes this ordering guaranteed?

Code / implementation expected: Yes — real, direct proof against an actual setTimeout(0) scheduled BEFORE the runner's own tasks, confirming the microtask batch genuinely runs first.

js-coding
utility
algorithm

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

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

How to read this doc: the real ordering guarantee this question specifically tests — that queued microtasks run before ANY real, pending macrotask, even one scheduled EARLIER — was verified directly against a genuine setTimeout(0) call made before the runner's own tasks were pushed.

1. The problem, restated

JavaScript's event loop drains the ENTIRE microtask queue completely, after every single synchronous stretch of code, BEFORE processing even one macrotask (a setTimeout callback, a UI event, etc.) — regardless of which was scheduled first in real wall-clock terms. A microtask-based runner batches pushed callbacks to run together via queueMicrotask, guaranteeing this real ordering.

2. Clarifying questions a strong candidate asks

| Question | Why it matters |

The solution is hidden so you can try first

6 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 microtasks pushed after a real setTimeout(0) call still run before that macrotask fires