为什么当计时器为0ms时setTimeout回调要等待循环结束？

Question

The thing I don't understand about closures is the fact that the setTimeout callbacks don't fire until the loop has finished. It looks to me that clearly the function is being executed during each iteration.

I am not concerned why they are all "6"'s I understand that part. I just need an explanation as to why it waits till the loop has to finish before they actually run.

for (var i = 1; i <= 5; i++) 
{
    setTimeout(function timer() 
    {
        console.log(i);
    }, 0);
}

Answer 1

The setTimeouts are queued to start running at the end of the current event loop. So, the setTimeouts can fire much later than the end of the for loop.

MDN has a great description about the event loop with regard to setTimeout:

 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/EventLoop

Specifically this part:

Calling setTimeout will add a message to the queue after the time passed as second argument. If there is no other message in the queue, the message is processed right away; however, if there are messages, the setTimeout message will have to wait for other messages to be processed. For that reason the second argument indicates a minimum time and not a guaranteed time.

Answer 2

JavaScript runs with a single event loop .

The event loop is busy running the function that is calling setTimeout so it isn't looking to see if any events are on the timeout queue waiting to run.

Even if that wasn't the case, most JS implementations implement a minimum timeout (see Minimum delay and timeout nesting ) value and raise whatever you pass ( 0 ) to it if it falls below that value.