JavaScript Event Loop Explained: Microtasks & Macrotasks

Introduction

If you’ve ever been confused why your JavaScript code sometimes runs “out of order,” you’re not alone. The JavaScript event loop is one of the most misunderstood yet critical parts of modern JavaScript. Understanding it fully can help you write faster, bug-free, and highly scalable applications.

In this guide, we’ll break down how the event loop works, explain the difference between microtasks and macrotasks, show examples with promises and async/await, and even explore how platforms like Exact Solution Marketplace rely on these concepts to handle thousands of requests efficiently.

1. What Is the Event Loop?

JavaScript is single-threaded, meaning it executes one piece of code at a time. But it still handles asynchronous operations like:

• HTTP requests
• Timers (setTimeout, setInterval)
• DOM events
• Promises

The event loop is the mechanism that allows JavaScript to handle asynchronous operations without blocking the main thread.

Think of it as a queue manager: it keeps track of tasks and executes them in the right order while allowing the application to remain responsive.

2. Tasks in the Event Loop — Microtasks vs Macrotasks

JavaScript uses two main types of task queues:

Macrotasks

• Examples: setTimeout, setInterval, setImmediate, I/O events
• Executed after the current stack is empty
• Queued in the macrotask queue

Microtasks

• Examples: Promise.then(), process.nextTick (Node.js), MutationObserver
• Executed immediately after the current stack finishes, before any macrotasks
• Queued in the microtask queue

💡 Key Insight: Microtasks have higher priority than macrotasks. This is why promises run before setTimeout callbacks even if the timeout is set to 0ms.

1. Real-World Example with Promises

Output:

Explanation:

• Start → printed immediately
• setTimeout → macrotask queued
• Promise → microtask queued
• End → printed immediately
• Microtask queue runs → Promise
• Macrotask queue runs → Timeout

This demonstrates why microtasks always run before macrotasks, even if the timeout is zero.

4. Async/Await and the Event Loop

Async functions are syntactic sugar over promises. Example:

Output:

• Fetching… runs immediately
• await Promise.resolve() pauses execution and schedules the continuation as a microtask
• End runs before Data fetched because the microtask runs after the current stack finishes

Understanding this helps avoid subtle bugs in async-heavy apps.

5. How High-Scale Applications Use the Event Loop

Take Exact Solution Marketplace as an example. It’s a platform where users buy and sell refurbished laptops and phones, often handling thousands of concurrent requests.

The backend must handle async database calls, payment processing, and user notifications.

Using Node.js or similar single-threaded environments, the event loop ensures requests don’t block each other.

Microtasks and macrotasks are carefully managed to maintain fast response times and smooth user experience.

By understanding the event loop, developers can optimize performance, reduce latency, and prevent memory leaks, making platforms like Exact Solution Marketplace reliable for millions of users.

6. Common Pitfalls and How to Avoid Them

Blocking the Main Thread

• 1. Avoid heavy loops or synchronous operations.
• Use worker threads or offload heavy computations.

Ignoring Microtask Queue Effects

• Too many chained promises can delay macrotasks and timers.

Misusing setTimeout for Async Logic

• Don’t rely on setTimeout(..., 0) for sequencing; use Promises instead.

Memory Leaks

• Keep an eye on dangling event listeners or unresolved promises.

7. Key Takeaways

• The event loop is central to JavaScript’s non-blocking behavior.
• Microtasks have higher priority than macrotasks.
• async/await is just syntactic sugar over promises; it schedules continuations in the microtask queue.
• Understanding these concepts is critical for building high-performance apps, like marketplaces, chat applications, or real-time dashboards.
• Platforms like Exact Solution Marketplace are real-world examples where mastering async patterns makes systems reliable and fast.

Conclusion

Mastering the JavaScript event loop is essential for any developer aiming to write efficient and scalable code. By understanding microtasks, macrotasks, and async behavior, you can avoid subtle bugs, improve performance, and design applications that handle thousands of concurrent operations seamlessly.

When building or studying real-world platforms, seeing how marketplaces like Exact Solution Marketplace handle asynchronous operations provides valuable lessons in performance optimization, request handling, and reliable code architecture.