JavaScript is Single-Threaded

🚦 What does "single-threaded" mean?

• A thread is like a worker that can do tasks.
• JavaScript has only one worker (one thread).
• That worker can do only one thing at a time.

So if JS is running some code, it cannot start another piece of code at the same exact time on another thread.

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🛑 Why does this matter?

• If one task is slow (e.g. a heavy loop or waiting for a file), the single worker gets stuck.
• This means your whole app (like a webpage) may freeze until the task finishes.

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🍕 Analogy

Imagine a pizza chef in a small restaurant:

• There’s only one chef (single thread).
• If the chef is busy making a pizza, he cannot simultaneously make pasta.
• To handle waiting times (like pizza baking in the oven), the chef can set a timer and move to the next order.
• That timer is like callbacks, promises, or async/await.

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⚡ How does JS still feel fast?

JavaScript uses the event loop + asynchronous operations:

• Slow tasks (network calls, timers, etc.) are handed off to the browser / Node.js background helpers.
• When they’re done, they say “Hey, I’m ready” and the single thread picks it up later.

This makes JS feel like it’s multitasking, even though only one thing runs at a time.

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💻 Code Example

console.log("Start");

setTimeout(() => {
  console.log("Async task done");
}, 0);

console.log("End");

Output

Start
End
Async task done

👉 Even though the timeout is 0ms, the callback is delayed until the main thread finishes current tasks (Start and End).

This proves that JavaScript has only one main thread.

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✅ Summary

JavaScript is single-threaded = one main worker runs tasks one after another.

With the event loop + async, it can still handle multiple tasks smoothly without blocking.

Originally published on: Bitlyst