Multithreading in JS?

JavaScriptMiddle~3 min read

Short answer:

JavaScript was deliberately designed as a single-threaded language, to be simple, safe, and predictable - especially in the browser, where it originally ran.

Now let's break this down in detail.

1. Historical reason

When JavaScript was created (in 1995 at Netscape), it was needed for managing pages - handling clicks, forms, animations, the DOM, and so on.

In the browser, everything revolves around one tree of elements - the DOM. If two threads modified the DOM at the same time, that would cause data races, conflicts, unpredictable states.

So: JS was made single-threaded, to guarantee that only one thread can change the DOM at any given moment.

2. Threads = complexity and risk

Multithreading is not free. It creates problems that JavaScript wanted to avoid:

Problem	What it is
Race conditions	when two threads read and write the same value at the same time
Deadlocks	threads "hang", waiting for each other
Complex synchronization	you have to manually manage locks, mutexes, and so on

These problems are typical of languages like C++, Java, or Go. JS, by contrast, needs to be safe for beginners and "free of surprises".

3. But JS still does several things "at once" - how?

JavaScript uses an asynchronous model with the Event Loop, not classic multithreading.

It does not execute code in parallel, but schedules tasks and processes them "one after another" - very quickly, creating the illusion of parallelism.

This is implemented via:

the Call Stack - where code executes;
the Event Loop - which decides when to pick up the next task;
Web APIs - which run long operations (timers, fetch, and so on) outside the main thread.

4. What if you actually need parallelism?

It is possible, but in a limited and safe way - via Web Workers (in the browser) or Worker Threads (in Node.js).

javascript

// main.js
const worker = new Worker('worker.js');

worker.postMessage('Hello!');

worker.onmessage = (event) => {
  console.log('Response from the worker:', event.data);
};

javascript

// worker.js
onmessage = (event) => {
  console.log('Main thread said:', event.data);
  postMessage('Hello back!');
};

Web Workers run in separate threads, but they have no access to the DOM or shared memory. They communicate with the main thread only through message passing.

This is a safe way to get "multithreading without data races".

5. In Node.js - the same idea

Node.js is also single-threaded from JavaScript's point of view, but under the hood (via libuv) there is a thread pool, which runs low-level tasks (I/O, DNS, crypto, and so on) asynchronously.

So the JS code stays single-threaded, while the system underneath uses multithreading transparently.

6. Why keep it single-threaded

Reason	Explanation
Simplicity	The developer doesn't need to think about thread synchronization
Safety	Data races in the DOM are ruled out
Predictability	Code executes sequentially
Asynchrony solves 90% of tasks	Through the event loop you can handle thousands of requests "at once"

7. When is real multithreading actually needed?

When you have CPU-intensive computations (for example, ML, encryption, image processing). Then you can use:

Web Workers (in the browser)
Worker Threads (in Node.js)
WebAssembly (for heavy computations)

SUMMARY

Question	Answer
Why is JS single-threaded?	To avoid conflicts when working with the DOM and to make the language safe
What replaces threads?	Asynchrony through the Event Loop
Can code run in parallel?	Yes, via Web Workers / Worker Threads
Why not "just use threads"?	Because multithreading would complicate the language and make it unsafe

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