node.js: while loop callback not working as expected

Question

Knowing that while Node.js is working asynchronously, writing something like this:

function sleep() {
    var stop = new Date().getTime();
    while(new Date().getTime < stop + 15000) {
        ;
    }
}

sleep();
console.log("done");

...would call the sleep(), block the server for the duration of the while loop (15secs) and just THEN print "done" to the console. As far as I understand, this is because Node.js is giving JavaScript only access to the main thread, and therefore this kidn of thing would halt further execution.

So I understand the solution to this is to use callbacks:

function sleep(callback) {
    var stop = new Date().getTime();
    while(new Date().getTime() < stop + 15000) {
        ;
    }
    callback();
}

sleep(function() {
    console.log("done sleeping");
});

console.log("DONE");

So I thought this would print 'DONE' and after 15 secs. 'done sleeping', since the sleep() function gets called and is handed a pointer to a callback function. While this function is working (the while loop), the last line would be executed (print 'done'). After 15 seconds, when the sleep() function finishes, it calls the given callback function, which then prints 'done sleeping'.

Apparently I understood something wrong here, because both of the above ways block. Can anybody clarify please?

Thanks in advance, Slagjoeyoco

Mahn · Accepted Answer · 2012-10-13 10:58:36Z

14

Javascript and node.js are single threaded, which means a simple while blocks; no requests/events can be processed until the while block is done. Callbacks don't magically solve this problem, they just help pass custom code to a function. Instead, iterate using process.nextTick, which will give you esentially the same results but leaves space for requests and events to be processed as well, ie, it doesn't block:

function doSleep(callback) {
    var stop = new Date().getTime();

    process.nextTick(function() {
        if(new Date().getTime() < stop + 15000) {
            //Done, run callback
            if(typeof callback == "function") {
                callback();
            }
        } else {
            //Not done, keep looping
            process.nextTick(arguments.callee);
        }
    });
}

doSleep(function() {
    console.log("done sleeping");
    console.log("DONE");
});

edited Oct 13, 2012 at 10:58

answered Oct 13, 2012 at 0:34

Mahn

16.7k17 gold badges66 silver badges82 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

Fabián Heredia Montiel Over a year ago

You are complicating yourself. A simple setTimeout(callback, delay) should be enough to produce the same as instructed asynchronously.

Mahn Over a year ago

@FabiánH.jr. the point is OP might be interested in understanding why his while isn't working and how can a while be written in a non-blocking way, which is imho kinda a better answer than "it doesn't matter your method doesn't work, do it this other way instead".

Fabián Heredia Montiel Over a year ago

Well true, great answer overall and now that you mention it it was quite insightful in the inner process of node.js

slagjoeyoco Over a year ago

Funny thing is I knew that a while loop alone would block the script from doing other things, since it is running in one thread. For some reason I thought node would recognize blocking code and if I called it with a callback, it would just let it work and get back to it once it's finished, letting the last line be execute in the meantime. Of course, that is nonesence since, again, there is only one thread to work in, and since of course node does not 'see' that there's a busy loop in place, it does not move this function into a seperate thread and therefore the main thread gets blocked.

slagjoeyoco Over a year ago

So process.nextTick executes the given function literally in the next tick of the event loop, letting other queued tasks, like the last line that should print 'done', be executed before the script is halted for the next tick of the 15-second-loop. So, assuming we would loop for 5 ms and outside that loop would be 5 other tasks to be done, I guess using process.nextTick would execute queued task #1, then one tick of the 5ms loop, then task #2 and so on, right?

|

Abdullah Jibaly · Accepted Answer · 2012-10-15 16:02:09Z

9

You are calling sleep right away, and the new sleep function blocks. It keeps iterating until the condition is met. You should use setTimeout() to avoid blocking:

setTimeout(function () {
    console.log('done sleeping');
}, 15000);

edited Oct 15, 2012 at 16:02

answered Oct 13, 2012 at 0:26

Abdullah Jibaly

55.2k45 gold badges129 silver badges201 bronze badges

Comments

Ry- · Accepted Answer · 2012-10-13 00:26:35Z

2

Callbacks aren't the same thing as asynchronicity, they're just helpful when you want to get a... callback... from an asynchronous operation. In your case, the method still executes synchronously; Node doesn't just magically detect that there's a callback and long-running operation, and make it return ahead of time.

The real solution is to use setTimeout instead of a busy loop on another thread.

answered Oct 13, 2012 at 0:26

Ry-♦

226k56 gold badges496 silver badges504 bronze badges

Comments

skovalyov · Accepted Answer · 2012-10-13 00:33:24Z

0

As already mentioned, asynchronous execution should be achieved by setTimeout() rather than while, because while will freeze in one "execution frame".

Also it seems you have syntax error in your example.

This one works fine: http://jsfiddle.net/6TP76/

answered Oct 13, 2012 at 0:33

skovalyov

2,0991 gold badge16 silver badges12 bronze badges

Collectives™ on Stack Overflow

node.js: while loop callback not working as expected

4 Answers 4

7 Comments

Comments

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

7 Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related