Rebuild an async task runner

Problem

What becomes confusing, fragile, or impossible without understanding rebuild an async task runner? This lesson answers that through explanation, a worked example, two runnable exercises, and a reference solution. No teacher-supplied worksheet is required.

Event loops let one thread coordinate many delayed operations. Promises standardized continuation and error composition over callback-heavy code.

Concept explanation

Rebuild an async task runner belongs to “Async runtime rebuild”. rebuild an async task runner ships as one thin, inspectable slice of an observable task runner with cancellation.

For rebuild an async task runner, trace concrete input, state transition, output, and failure through a state-and-time rule spanning synchronous frames, host tasks, microtasks, rendering, cancellation, or backpressure.

Synchronous frames run to completion. Hosts schedule tasks; promise reactions enter microtask queues; rendering occurs only at host-selected opportunities. Apply that model to supplied normal, boundary, and failure cases; each case below names its input and expected evidence.

Example bank

Compare normal, boundary, failure, and cross-layer cases. Predict each observation before revealing the explanation.

Worked example

Start from supplied lab.mjs. Focus: Add rebuild an async task runner to an observable task runner with cancellation; demo it with: Build task runner with concurrency limit, AbortSignal, progress stream, and result aggregation.

1. Run: node lab.mjs
2. Save baseline evidence. Record ordering, async stacks, queue delay, cancellation signals, stream state, and cleanup paths.
3. Boundary case: Document and test shipped slice under: Run zero, one, and more-than-limit tasks.
4. Failure case: Add recovery behavior and a regression check for: Reject and abort during active work, then verify queued/active cleanup.

RESULT
Fresh clone or blank directory can run documented slice and reproduce: Trace never exceeds limit; result order policy is documented; all terminal paths settle and release resources. Starter-level baseline: Order ends as sync:start > sync:end > microtask > task. Each line is a prefix of that final sequence.

Starter material

Create lab.mjs, paste this exact content, then run the command below.

const events = [];
const record = (value) => { events.push(value); console.log(events.join(" > ")); };

record("sync:start");
queueMicrotask(() => record("microtask:rebuild an async task runner"));
setTimeout(() => record("task:rebuild an async task runner"), 0);
record("sync:end");

STOP / CLEANUPScripts exit after queued work completes. Press Ctrl+C only if your experiment creates an intentional infinite loop.

Independent exercise

Make it recoverable: rebuild an async task runner

1. Create second case from blank file: Document and test shipped slice under: Run zero, one, and more-than-limit tasks.
2. Then create controlled failure: Add recovery behavior and a regression check for: Reject and abort during active work, then verify queued/active cleanup.
3. Use Chromium Performance · Network · async debugger to prove behavior, then repair controlled failure.
4. Compare result against supplied acceptance checks and reference approach before marking complete.

Expected result

• Fresh clone or blank directory can run documented slice and reproduce: Trace never exceeds limit; result order policy is documented; all terminal paths settle and release resources.
• Order ends as sync:start > sync:end > microtask > task. Each line is a prefix of that final sequence.
• Controlled rebuild an async task runner failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.