🧠 Understand Synchronous and Asynchronous JavaScript Once and For All

🔹 What Is Synchronous JavaScript?

Synchronous JavaScript runs line by line — each operation waits for the previous one to finish before starting the next. Example:

console.log("A");
console.log("B");
console.log("C");
  

Output:

A
B
C

Each statement runs in order — nothing happens at the same time.

🔹 What Is Asynchronous JavaScript?

Asynchronous JavaScript means tasks don’t wait for each other — JavaScript can continue running other code while waiting for something time-consuming to finish. But since JavaScript is single-threaded, how does it do that? 🤔 Through the Web APIs.

⚙️ How JavaScript Handles Asynchronous Code

The Web API is a set of built-in browser features (written in C++) that allow JavaScript to run tasks outside its main thread, like:

• Network requests (fetch, XMLHttpRequest)
• Timers (setTimeout, setInterval)
• DOM manipulation
• Geolocation
• WebSockets

🧩 Example: Asynchronous Code in Action

console.log("A");

setTimeout(() => {
  console.log("B");
}, 1000);

console.log("C");
  

Output:

A
C
B

🎯 Quick Interview Question

console.log("A");

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

console.log("C");
  

Answer:

A
C
B

Even with a delay of 0ms, the callback still goes through the Web API → Task Queue → Event Loop before it runs.

🌍 How Other Languages Handle Async

Language	Async Mechanism	Description
JavaScript	Web APIs + Event Loop	Single-threaded but offloads work via the browser or Node.js
Python	asyncio / threading	Uses coroutines or OS threads
Java	Thread pools / CompletableFuture	Uses real multithreading
C#	async/await + Task Scheduler	Managed by the .NET runtime
Go	Goroutines + Scheduler	Lightweight threads managed by Go runtime

✅ Final Summary

• Synchronous: runs in order, blocking other code.
• Asynchronous: runs independently without blocking.
• JavaScript uses Web APIs, Task Queues, and the Event Loop.
• Other languages use true multithreading.