Why does a while loop block the event loop?

Question

The following example is given in a Node.js book:

var open = false;  setTimeout(function() {   open = true }, 1000)  while (!open) {   console.log('wait'); }  console.log('open sesame'); 

Explaining why the while loop blocks execution, the author says:

Node will never execute the timeout callback because the event loop is stuck on this while loop started on line 7, never giving it a chance to process the timeout event!

However, the author doesn't explain why this happens in the context of the event loop or what is really going on under the hood.

Can someone elaborate on this? Why does node get stuck? And how would one change the above code, whilst retaining the while control structure so that the event loop is not blocked and the code will behave as one might reasonably expect; wait will be logged for only 1 second before the setTimeout fires and the process then exits after logging 'open sesame'.

Generic explanations such as the answers to this question about IO and event loops and callbacks do not really help me rationalise this. I'm hoping an answer which directly references the above code will help.

Answer 1

It's fairly simple really. Internally, node.js consists of this type of loop:

• Get something from the event queue
• Run whatever task is indicated and run it until it returns
• When the above task is done, get the next item from the event queue
• Run whatever task is indicated and run it until it returns
• Rinse, lather, repeat - over and over

If at some point, there is nothing in the event queue, then go to sleep until something is placed in the event queue.

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So, if a piece of Javascript is sitting in a while() loop, then that task is not finishing and per the above sequence, nothing new will be picked out of the event queue until that prior task is completely done. So, a very long or forever running while() loop just gums up the works. Because Javascript only runs one task at a time (single threaded for JS execution), if that one task is spinning in a while loop, then nothing else can ever execute.

Here's a simple example that might help explain it:

 var done = false;   // set a timer for 1 second from now to set done to true  setTimeout(function() {       done = true;  }, 1000);   // spin wait for the done value to change  while (!done) { /* do nothing */}   console.log("finally, the done value changed!"); 

Some might logically think that the while loop will spin until the timer fires and then the timer will change the value of done to true and then the while loop will finish and the console.log() at the end will execute. That is NOT what will happen. This will actually be an infinite loop and the console.log() statement will never be executed.

The issue is that once you go into the spin wait in the while() loop, NO other Javascript can execute. So, the timer that wants to change the value of the done variable cannot execute. Thus, the while loop condition can never change and thus it is an infinite loop.

Here's what happens internally inside the JS engine:

1. done variable initialized to false
2. setTimeout() schedules a timer event for 1 second from now
3. The while loop starts spinning
4. 1 second into the while loop spinning, the timer fires internally to the JS engine and the timer callback is added to the event queue. This likely occurs on a different thread, internal to the JS engine.
5. The while loop keeps spinning because the done variable never changes. Because it continues to spin, the JS engine never finishes this thread of execution and never gets to pull the next item from the event queue.

node.js is an event driven environment. To solve this problem in a real world application, the done flag would get changed on some future event. So, rather than a spinning while loop, you would register an event handler for some relevant event in the future and do your work there. In the absolute worst case, you could set a recurring timer and "poll" to check the flag ever so often, but in nearly every single case, you can register an event handler for the actual event that will cause the done flag to change and do your work in that. Properly designed code that knows other code wants to know when something has changed may even offer its own event listener and its own notification events that one can register an interest in or even just a simple callback.