JavaScript Event Loop, Microtasks, and Macrotasks Explained Visually with Clear Examples and Execution Order

The Event Loop, Microtasks, and Macrotasks Explained Visually

1️⃣ What Is the Event Loop?

JavaScript is single-threaded, meaning it can do one thing at a time. But with asynchronous code, it handles waiting tasks (e.g., timeouts, fetch) using the event loop.

📌 Key Components:

• Call Stack (executes code)

• Web APIs (browser-side: timers, fetch, etc.)

• Task Queues (for microtasks and macrotasks)

• Event Loop (glue that checks if the call stack is empty)

The event loop constantly checks the call stack and pushes tasks from the queues when it's empty.

 

2️⃣ Microtasks vs Macrotasks: What’s the Difference?

🔹 Microtasks:

• Promise.then(), catch(), finally()

• queueMicrotask()

🔹 Macrotasks:

• setTimeout(), setInterval()

• setImmediate() (Node.js)

• UI rendering, I/O tasks

🧠 Microtasks are always executed before macrotasks once the call stack is clear.

3️⃣ Visual Execution Flow Example

console.log("Start");

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

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

console.log("End");

Execution Order:

1. "Start"

2. "End"

3. "Microtask"

4. "Macrotask"

Why?

• console.log("Start") and console.log("End") run first (synchronous).

• Then microtask queue is checked → "Microtask".

• Then macrotask queue → "Macrotask".

4️⃣ Practical Scenarios Where Order Matters

✅ Handling async data

fetch("/data")
  .then(() => console.log("Promise resolved"));

console.log("After fetch");

"After fetch" logs before "Promise resolved"

✅ Nested Promises & Microtasks

Promise.resolve().then(() => {
  console.log("Microtask 1");
  Promise.resolve().then(() => {
    console.log("Nested Microtask");
  });
});

Output:

• Microtask 1

• Nested Microtask (queued after 1st finishes)

5️⃣ Common Mistakes and Best Practices

❌ Mistake: Assuming setTimeout(..., 0) executes immediately
✅ Fact: It goes to macrotask queue and waits for the stack & microtasks to clear

❌ Mistake: Overusing long synchronous code that blocks the event loop
✅ Solution: Break heavy tasks into smaller async chunks using setTimeout(fn, 0) or requestIdleCallback()

✅ Use Promise.resolve() to prioritize code with microtask scheduling

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