JavaScript: Async & Advanced Features

JavaScript: Async & Advanced Features is a free practice course on Abekus built around 950+ MCQs covering everything between the event loop and ES2023. If you already know basic JavaScript and you have hit the wall where Promises, async/await, generators, Proxy, and the event loop stop making intuitive sense, this is the course that drills those gaps until they close.

The format is one question at a time, with an explanation that fires the moment you click a wrong answer. No videos, no passive reading — retrieval practice on the topics interviewers actually ask about and that production bugs actually come from.

• Format — 950+ free MCQs with instant explanations after every wrong answer.
• Duration — about 10.6 hours of focused practice; most learners finish over 2–3 weeks at ~30 questions a day.
• Level — intermediate to advanced. Assumes you know JavaScript basics (variables, functions, objects, arrays, basic control flow).
• Cost — free, with a free completion certificate.
• Audience — working frontend / Node devs filling depth gaps, candidates prepping for product-company JS interviews, MERN bootcamp graduates, college students doing placement prep.
• Companion course — Node.js for learners targeting server-side JavaScript roles.

Three audiences specifically. First, working JavaScript developers who can ship features but have never sat down with the event loop spec — the people who write await in a loop, ship it, and wonder later why their integration takes 30 seconds instead of 3. Second, candidates preparing for JavaScript interviews at product companies, where the questions are output-prediction (setTimeout vs Promise.then ordering), pattern selection (when do you need a generator), and trap recognition (Proxy traps that silently break ===). Third, MERN-stack bootcamp graduates and engineering students who have built React apps but never had to debug a memory leak from a stale closure inside a WeakMap-less cache.

This is not a course for people who have never written JavaScript before. If you do not know what const versus let means, or how an object literal works, start somewhere else and come back.

Foundations — the async model

• Event Loop & Concurrency Model — event loop mechanics, task queues vs microtask queues, timers and scheduling. Predict the output of any combination of setTimeout, Promise.then, and queueMicrotask.
• Promises — Promise basics, chaining and error propagation, and the static methods (Promise.all, allSettled, race, any) with the right scenarios for each.
• Async & Await — what an async function actually returns, how await suspends execution, await patterns for sequential vs parallel work, and async error handling with try/catch.

Depth — modern language features

• Iterators & Iterables — the iterator protocol, the iterable protocol, and why lazy evaluation matters when you are streaming a million records.
• Generators — generator basics, two-way communication via yield, generator delegation, and patterns like infinite sequences and pull-based async streams.
• Map, Set, WeakMap & WeakSet — when each beats a plain object or array, and the specific cases where WeakMap and WeakSet prevent memory leaks.
• Proxy & Reflect — Proxy basics, the trap set (get, set, has, deleteProperty, ownKeys, and friends), the Reflect API, and where meta-programming silently breaks invariants downstream.
• Modern Syntax & Optional Features — optional chaining, nullish coalescing, logical assignment operators, top-level await, and the rest of the ES2020–2023 additions you actually need.

Interview readiness — patterns and trap recognition

• Async Patterns & Error Handling — control-flow patterns (sequential, parallel, batched), structured error handling, retry/backoff, and cancellation with AbortController.
• Review & Mastery — concept comparisons (callbacks vs Promises vs async/await; Map vs Object; Proxy vs class accessor), pattern selection (which tool for which problem), and in-practice questions on combining everything.

One of the most-Googled JavaScript questions, and one this course makes you stop guessing at. Callbacks were the original async primitive — fast, flexible, and prone to nesting into pyramids that no one could maintain. Promises gave a structured way to chain async work and propagate errors, but introduced their own confusion (returned Promise vs awaited value, then-callback execution order, error-swallowing chains). Async/await is sugar on top of Promises — every async function returns one, every await unwraps one — but it makes sequential async code read like synchronous code and lets you use try/catch around it.

The course works through this with output-prediction MCQs (does this log fire before that one?), pattern questions (parallelise these three requests — three correct ways, one anti-pattern), and trap questions (this await in a loop turns 3 parallel requests into 9 sequential seconds — fix it). By the time you finish the Async & Await and Async Patterns topics, you will not have to think twice about which pattern to use.

Active recall, not passive reading. Watching a 12-minute YouTube video on the event loop feels productive, but a week later you will still mis-predict whether a Promise.then fires before a setTimeout(fn, 0). Reading the MDN page is better, but the test is not whether you understand it on the page — it is whether you can produce the answer cold, two days later, on a whiteboard or in a debugger.

MCQ-based retrieval practice forces the recall every single question. You see the code, you predict the output, you commit to an answer, and the explanation either confirms your reasoning or shows you exactly where it broke. That is the feedback loop that moves concepts from "I think I understand" to "I can use this in production". The 950+ questions in this course are spaced across the 10 topics so you cover each concept multiple times in different framings.

One question at a time. You read a snippet (often a 5–15 line async function), pick the option that matches what the snippet logs, returns, or throws, and submit. Wrong answers trigger an explanation that walks through the actual evaluation order — usually with the microtask / macrotask trace, or the Proxy trap firing, or the generator's resume value. Right answers move on, fast.

The Abekus AI guide watches which topics you keep getting wrong and surfaces them more often. If you nail Promises but keep mis-predicting generator outputs, the next session will be generator-heavy. The certificate at the end is gated on completing each top-level topic, not on a single end-of-course exam, so the depth signal is honest.

Plan on roughly 10.6 hours of focused practice. The math: 958 active questions × ~40 seconds per question (reading the snippet, picking an answer, skimming the explanation when you are wrong) = ~640 minutes = 10.6 hours.

Most learners spread that across 2–3 weeks at 30 questions a day. Some compress it into a long weekend before an interview; that works for refresher mode but retention is weaker than spacing it out. Trust the spacing — async semantics, in particular, only stick after you have seen the same trap rephrased four or five different ways.

If you are targeting server-side JavaScript roles, take Node.js alongside this course — Node's async story (streams, event emitters, the libuv thread pool) builds directly on the event loop and Promise foundations here. If you are targeting frontend roles, the React and DOM-event practice tracks pair well; an async-aware frontend dev is the one who does not ship double-fire button handlers or stale-closure data fetches.

A JavaScript Fundamentals course on Abekus is in the pipeline and not yet publicly listed; once it ships it will be the recommended prerequisite for anyone uncertain about JS basics. Until then, if the Quick facts section above made you nervous, brush up on basics first via any beginner JavaScript resource and come back. The Node.js course is also a reasonable adjacent practice ground for learners who want to apply async patterns to real I/O.