Are HTML5 Web Workers threads or processes?

Question

From the Mozilla documentation:

Web Workers is a simple means for web content to run scripts in background threads.

Considering Javascript is single-threaded, are web workers separate threads or processes? Is there shared memory that classifies them as threads?

Answer 1

They run in background threads, but the API completely abstracts from the implementation, so you may come across a browser that just schedules them to run on the same thread as other events like Node does. Processes are too heavyweight to run background tasks.

Answer 2

Considering Javascript is single-threaded

JavaScript is not single-threaded.

The main part of a JavaScript program runs on an event loop.

Long-running processes (XMLHttpRequest being the classic example) are almost always farmed out to stuff that runs outside the event loop (often on different threads).

Web Workers are just a means to write JavaScript that runs outside the main event loop.

are web workers separate threads or processes? Is there shared memory that classifies them as threads?

That's an implementation detail of the particular JS engine.

Answer 3

As per the MDN:-

The Worker interface spawns real OS-level threads, and mindful programmers may be concerned that concurrency can cause “interesting” effects in your code if you aren't careful.

Reference:- https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API/Using_web_workers#about_thread_safety