11

Javascript is single threaded. So does the callback function and it's containing function execute on the same thread as the main loop/event loop?

database.query("SELECT * FROM hugetable", function(rows) {  // anonymous callback function
   var result = rows;
   console.log(result.length);
});
console.log("I am going without waiting...");

If query() method and its callback function executes on the same thread as the event loop does then the blocking will occur. If not why Javascript is called single-threaded?

Can anyone help verify that javascript(browser/node.js) is using multiple threads behind the scene in order to achieve non-blocking?

Friends, I saw your comments and answers. Sorry I am very new to Javascript. I am confused in that single thread async call won't block. If there are 100 users request data from hugeTable which may take one minute each concurrently and the event loop dispatch these tasks in to a queue and execute them in turn how can the query() method execution not blocking the event loop since they are all on one single thread?

Brad answered this part.

Brian Tompsett - 汤莱恩
  • 5,753
  • 72
  • 57
  • 129
Shawn
  • 5,130
  • 13
  • 66
  • 109
  • 1
    You can look into webworkers, which allow you to execute code on a seperate thread – megawac Dec 05 '13 at 04:03
  • 3
    *"If query() method and its callback function executes on the same thread as the event loop does then the blocking will occur."* no. Consider setTimeout in browsers. It runs on the same thread as any other code, but it is not blocking. The callback is just put on a queue to be executed later. Now, if you think about Ajax, then yes, I believe the request itself is made in a different thread. However, the callback function will be executed in the UI thread again. – Felix Kling Dec 05 '13 at 04:04
  • javascript is called evented and not I/O blocking – Fikri Marhan Dec 05 '13 at 05:05

4 Answers4

3

Node.js native libraries (not the JavaScript portions) use separate threads all the time, but all of the data for your code gets shuffled back to a single JavaScript execution thread.

It's impossible to tell you if other threads in the background are working on this query, since you didn't specify what DB library you're using. However, that doesn't matter as much as you think.

Let's suppose you were to create a thread for handling your database connection. You fire off a query, and that thread takes care of getting the query to the database server. Then what? That thread sits around doing absolutely nothing until there's data to come back. You've effectively wasted resources keeping a thread around that doesn't do a whole lot. Node.js doesn't work this way. You have one thread for JavaScript execution.

If you are sending or receiving data (which is mostly what your DB connector is going to do), then this is handled out of a background thread pool for you automatically. It's also possible whatever DB connector you're using has a native extension that can do whatever it wants with threads.

See my post here for a more full explanation: https://stackoverflow.com/a/19324665/362536

Community
  • 1
  • 1
Brad
  • 159,648
  • 54
  • 349
  • 530
  • Thank you much, Brad, I read your comments on the link you provided. That answer my question perfectly. Also thanks to other friends who answered and/or made comments. – Shawn Dec 05 '13 at 05:01
2

Javascript is single threaded. So does the callback function and it's containing function execute on the same thread as the main loop/event loop?

Yes, you kind of answered your own question.

You are correct, JavaScript is single threaded. There is no other thread for a callback to execute on.

That said, external libraries written in C or C++ are free to spawn threads all they want.

user229044
  • 232,980
  • 40
  • 330
  • 338
0

Good question, javaScript is single-threaded yet asynchronous, that means for each asynchronous call it will perform a small pause on the main thread to execute the callback, at least that's for browsers and even there you will have web-workers that will spawn a new thread per worker.

Charlie Affumigato
  • 1,017
  • 1
  • 7
  • 10
0

What you're doing in your code is deferring the evaluation of the query (execution of the callback) until after the query has run and rows have been retrieved.

In modern browsers you can create two threads if desired using webworkers.

Node.js is a seperate story - you can create child processes which will run on a seperate thread when desired. See this post for further information.

Community
  • 1
  • 1
megawac
  • 10,953
  • 5
  • 40
  • 61