[henryw]

From https://developer.mozilla.org/en-US/docs/Web/JavaScript/Even...:

JavaScript has a concurrency model based on an "event loop". This model is quite different than the model in other languages like C or Java.

...

A very interesting property of the event loop model is that JavaScript, unlike a lot of other languages, never blocks. Handling I/O is typically performed via events and callbacks, so when the application is waiting for an IndexedDB query to return or an XHR request to return, it can still process other things like user input.

[tyingq]

>>JavaScript, unlike a lot of other languages, never blocks

I think that's a little strong. It's more like "The group controlling Javascript has mostly tried to discourage introduction of things that block".

You can, for example, do a blocking XMLHttpRequest. It's deprecated, but possible. https://jsfiddle.net/923d5sda/

[coldtea]

You can do blocking everything.

A 10.000 repetitions for look while block the whole interpreter for its duration.

Any JSON parsing does the same.

Processing strings.

Doing math work.

...

[tyingq]

Guess I should have said "blocking I/O"? I thought it was a given that a single threaded language wouldn't magically inject some kind of concurrency around tight loops or CPU intensive tasks. Node.js people don't really think that sort of thing is "non blocking", do they?

[coldtea]

>Guess I should have said "blocking I/O"? I thought it was a given that a single threaded language wouldn't magically inject some kind of concurrency around tight loops or CPU intensive tasks.

Well, Erlang (and Elixir) does just that -- it's preemptive, and implicitly yields under the covers even in loops.

>Node.js people don't really think that sort of thing is "non blocking", do they?

Judging from forum threads and blog posts, a lot of them do, especially web programmers not familiar with blocking and non-blocking that only know that "Node is webscale".

[Matthias247]

I think that quoted statements are not very good.

The other languages don't have a builtin concurrency model. For C, Java and others event loop libraries and applications that are built on top of them can be found (nginx, netty, ...). As well as there are libraries that build on top of synchronous IO.

The eventloop was also not tied to Javascript in the older standards. Only the introduction of Promises and other stuff required the existence of an eventloop in order to define when continuations should run.

"The event loop model never blocks" is also only true as long as you (and all the libraries that you use) do not block it. There is no automatic "does not block" guarantee.

[coldtea]

>A very interesting property of the event loop model is that JavaScript, unlike a lot of other languages, never blocks.

JavaScript actually always blocks. It's only external function calls that somebody took care to write in an evented style that don't block -- but anything written in pure Javascript (from for loops to text manipulation) blocks.

JS single-thread async without preemptiveness is not something to write home about...