JavaScript Async Internals: Visualizing the Event Loop's Hidden Choreography
See the Invisible
Interactive simulators visualize what's hidden from view.
Hands-On Labs
Step through executions tick by tick. Manipulate state.
Why, Not Just What
Deep architectural understanding for mid-senior developers.
AI Sandbox Lab
Real-world scenarios. Find the flaws AI misses.
Become the Engineer Who Supervises AI
As AI generates more code, understanding what that code does becomes more valuable, not less. Someone must verify AI output, debug failures, and make architectural decisions.
Build Your Architectural EdgeYou've Written Async Code for Years. Can You Predict What Runs First?
Why does
Promise.resolve().then()
fire before
setTimeout(..., 0)? Even though setTimeout was scheduled first, and the timeout is
zero milliseconds.
If you can't explain exactly why, you're debugging async code by trial and error. You're writing race conditions without knowing it. And you're guessing at interview questions that senior engineers answer with precision.
The Invisible Queue Problem
JavaScript maintains multiple queues with different priorities. Macrotasks, microtasks, rendering callbacks. They're completely invisible in your code. Your debugger won't show you what's queued or why one callback runs before another.
The Gap
Without understanding the event loop algorithm, you can't trace async execution in your head. Every nested Promise chain, every interleaved setTimeout, every async/await desugaring becomes a mystery you solve by running the code.
See What No Debugger Shows You
This course uses interactive simulators to visualize the event loop's internal choreography. Watch call stack frames push and pop in real-time. See callbacks move from Web APIs to the macrotask queue. Observe the microtask queue drain completely before the next macrotask executes. These aren't animations of concepts. They're accurate simulations of the actual algorithm that schedules your async code.
Mental Models You'll Build
Execution contexts, stack frames, LIFO ordering. The foundation that async mechanisms build upon.
Why setTimeout isn't JavaScript. How Web APIs run concurrently while JS stays single-threaded.
setTimeout, setInterval, I/O callbacks, DOM events. One macrotask per loop iteration.
Promise.then, queueMicrotask, MutationObserver. Why microtasks drain completely first.
Where requestAnimationFrame fits. Why long JS blocks rendering. How to maintain 60fps.
How await creates microtask continuations. Trace any async function execution order.
Course Curriculum
Visual cheatsheet
Practice verifying AI-generated code
Stop Guessing. Start Tracing.
After this course, you'll annotate async code with queue states in your head. You'll predict execution order before running code. You'll spot microtask starvation bugs in code review. You'll answer event loop questions in interviews with the confidence of someone who has watched it execute step by step.
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