V8 Engine, Single-Threaded Event Loop (Microtasks vs Macrotasks)

1Concept

JavaScript runs on a single thread powered by an Event Loop (V8 Engine + Libuv in Node.js). The Call Stack executes synchronous code. Microtasks (Promise.then, queueMicrotask) take highest priority and drain completely before Macrotasks (setTimeout, setInterval, I/O) execute.

2Architecture Diagram

[ Call Stack (Sync) ] ──► [ Microtask Queue (Promises) ] ──► [ Macrotask Queue (setTimeout/IO) ]
        ▲                                │
        └──────────────(Event Loop)──────┘

3Code Example

console.log("1. Synchronous Start");

setTimeout(() => {
    console.log("4. Macrotask (setTimeout 0ms)");
}, 0);

Promise.resolve().then(() => {
    console.log("3. Microtask (Promise.then)");
});

console.log("2. Synchronous End");

4Expected Output

1. Synchronous Start
2. Synchronous End
3. Microtask (Promise.then)
4. Macrotask (setTimeout 0ms)

5Key Takeaways

• ✓Microtasks always drain before the next macrotask executes.
• ✓Never block the single thread with long synchronous loops.
• ✓JavaScript uses non-blocking asynchronous I/O.