How to Use
Pick a lesson from the learning path. They go from the call stack to a final mixed-queue challenge.
Read the code, then predict the console output by choosing one of the options.
Press Play, or step forward one frame at a time, and watch items move between queues.
Read the narration under the stage. It explains every move the event loop makes.
Finish all seven lessons, then test yourself with the Node.js Fundamentals interview drill.
What Is This Tool?
The Node.js event loop is the scheduler behind every callback, promise and timer in your server. This visualizer turns it into a guided learning path: seven short lessons, each with real code, an animated call stack, nextTick and microtask queues, and the libuv phases (timers, poll, check), so you can see why the console prints in the order it does.
Why It Matters
Event loop ordering is one of the most common Node.js interview topics and the root cause of many production bugs: starved timers, surprising log order and blocked requests. Understanding it makes async code predictable.
The four things to remember
1) Synchronous code always finishes first. 2) process.nextTick runs before promises. 3) Microtasks drain completely between every callback. 4) The loop visits timers, then poll, then check, in that order.
Who is this for?
Developers learning Node.js, frontend engineers moving to the backend, and anyone preparing for a technical interview. No setup is needed: everything runs in your browser.