Async and await

Problem

What becomes confusing, fragile, or impossible without understanding async and await? 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

Async and await belongs to “Promises are state machines”. async and await is inspected by introducing one realistic failure and proving root cause before repair.

For async and await, 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: Capture healthy async and await baseline from: Trace resolve, chained transform, thrown error, catch, finally, and recovery.

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: Record async and await behavior at: Resolve with thenable and compare Promise.all empty/nonempty.
4. Failure case: Diagnose this exact async and await failure before editing: Create unhandled rejection then attach handler late.

RESULT
Evidence identifies first divergence, repair changes only proven cause, and rerun restores: State/reaction trace explains flattening, propagation, recovery, and host rejection reporting. 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:async and await"));
setTimeout(() => record("task:async and await"), 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

Prove root cause: async and await

1. Create second case from blank file: Record async and await behavior at: Resolve with thenable and compare Promise.all empty/nonempty.
2. Then create controlled failure: Diagnose this exact async and await failure before editing: Create unhandled rejection then attach handler late.
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

• Evidence identifies first divergence, repair changes only proven cause, and rerun restores: State/reaction trace explains flattening, propagation, recovery, and host rejection reporting.
• Order ends as sync:start > sync:end > microtask > task. Each line is a prefix of that final sequence.
• Controlled async and await failure produces captured evidence; repair restores stated invariant without hiding error.

Hints

Solution

Attempt both exercises before opening reference approach.