Building a JavaScript Event Loop Visualizer: Making Async Code Easier to Understand

Understanding how JavaScript handles asynchronous operations is crucial for every developer, yet it remains one of the most challenging concepts to grasp. That's why I built the JavaScript Event Loop Visualizer – an interactive tool that makes these complex concepts visual and intuitive.

The Problem

When I was learning JavaScript, I struggled to understand:

• Why does Promise.then() execute before setTimeout(..., 0)?
• What exactly is the Call Stack and how does it work?
• How do Web APIs fit into the picture?
• What's the difference between Microtask and Task queues?

Reading articles and watching videos helped, but I needed something interactive – a tool where I could see the execution step-by-step and truly understand what's happening under the hood.

The Solution

I created an open-source web application that visualizes:

• 📚 Call Stack – currently executing functions
• 🌐 Web APIs – async operations running in the background
• ⚡ Microtask Queue – high-priority tasks (Promises, async/await)
• 📋 Task Queue – standard async tasks (setTimeout, setInterval)
• 🗄️ Memory – Stack and Heap visualization for primitives, objects, and functions

Key Features

Step-by-Step Execution

The visualizer breaks down code execution into individual steps, showing exactly what happens at each moment. You can:

• Run examples in automatic mode (great for demonstrations)
• Use manual mode to control each step (perfect for learning)
• Track every action in the history panel

5 Built-in Examples

I included examples that cover the most important concepts:

1. Memory & Variables – How primitives and objects are stored
2. setTimeout – Basic async code handling
3. Promise vs setTimeout – Understanding task priorities
4. Complex Example – Nested Promises and timers
5. Fetch API – Real network requests

Beautiful, Modern UI

Built with React 19, TypeScript, and shadcn/ui components:

• 🎨 Dark and light themes
• 📱 Fully responsive design
• ⌨️ Keyboard shortcuts for power users
• 🌍 Bilingual (English & Ukrainian)

Technical Implementation

Stack

• React 19 – Latest features including the new compiler
• TypeScript 5.9 – Type safety throughout
• Tailwind CSS – Utility-first styling
• shadcn/ui – Beautiful, accessible components
• Vite 6 – Lightning-fast build tool

Architecture

The visualizer is built with a custom state machine that:

1. Parses example code into discrete steps
2. Manages execution state (Call Stack, queues, Web APIs)
3. Updates UI reactively based on current state
4. Handles timing for async operations

interface SimulatorState {
  callStack: StackItem[]
  webAPIs: WebAPIItem[]
  microtaskQueue: Task[]
  taskQueue: Task[]
  history: HistoryEntry[]
  currentLine: number
  stepCount: number
}

Key Challenges

1. Timing Coordination

Simulating setTimeout and Promise timing required careful coordination between real JavaScript timers and the visualization state.

2. State Management

Managing complex state with multiple queues and maintaining consistency across components was solved using React Context and custom hooks.

3. Internationalization

Supporting multiple languages meant extracting all strings into translation files and making examples dynamic based on the selected language.

What I Learned

Building this project taught me:

• Deep understanding of the Event Loop mechanism
• Advanced React patterns and performance optimization
• Creating intuitive educational interfaces
• Importance of responsive design and accessibility
• How to structure a maintainable open-source project

Impact

Since launching on GitHub, the project has:

• Helped developers worldwide understand JavaScript async
• Become a teaching tool in coding bootcamps
• Received contributions from the community
• Been featured in JavaScript learning resources

Try It Yourself

🔗 Live Demo: vitalii4reva.com/visual-js-event-loop
💻 Source Code: github.com/vitalii4reva/visual-js-event-loop

The project is fully open-source under MIT license. Contributions, feedback, and stars are always welcome! ⭐

What's Next?

I'm planning to add:

• More examples (async/await, event listeners, AJAX)
• Animation playback controls (pause, rewind, speed)
• Code editor to create custom examples
• Sharing feature for teachers and students
• Performance profiling visualization

Final Thoughts

Building educational tools is incredibly rewarding. If my visualizer helps even one person understand the Event Loop, all the effort was worth it.

Do you have ideas for improvement? Open an issue on GitHub or reach out on Twitter @vitalii4reva. I'd love to hear from you!

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Have you struggled with understanding JavaScript's Event Loop? What concepts do you find most challenging? Share in the comments below!