[Javascript] Task queue & Event loop.

Posted 2020-06-09

javascript with Chorme v8 engine works like this :

For Chorme engine, v8, it has call stack. 

 

And all the async opreations functions are stay in webapis. So everytime  you call ‘setTimeout()‘ or http call, it will always call webapis. 

 

So, in the picture, 

we call the main() function, and push into the call stack;

then console.log("Hi"); put into call stack and finish then poped up from the call stack;

third, setTimeout() function run into the call stack, then it call webapis with callback function, so push into webapis. And setTimeout() popup from the call stack.

fourth, console.log(‘JSC‘); push into the call stack and poped up.

Finally main() finished and poped up:

So what left here is just webapis have a callback function, waiting 5 second, when the time out, it was push to the task queue, NOT to the call stack.

 

 

Now the ‘Event loop‘ come in, what event loop does is: it check whether there is any task in the call stack.... if yes, finish those functions in the calll stack first. If not, then event loop checks the task queue then push the callback from the task queue to call stack.

 

Then console.log() function run and poped up from the call stack.

 

-----------------------------------------

 

You might have met the problem that you run setTimeout(fn(), 0); it wait no time, but the fn() come at last:

function one(){
  console.log("One");
}

function two(){
  console.log("Two");
}

function delay(){
  console.log("Delay");
}

one();
setTimeout(delay, 0);
two();

/*
"One"
"Two"
"Delay"
*/

 

That is because, setTimeout is webapis, so the callback function delay() will be pushed into the ‘task queue‘. It will wait all the functions in ‘call stack‘ finish, then even loop will check the task queue and push the callback to the call stack. So the "Delay" console log out at the bottom.

Link: https://www.youtube.com/watch?v=8aGhZQkoFbQ