SetTimeout and asynchronous thinking in JavaScript

The following code, how long will the ' end ' pop up? Why?

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var t = true; settimeout (function () {t = false;}, 1000); while (t) {} Alert (' End ');

This is an implementation that was previously thought of when you could not implement a blocking JavaScript thread (that is, to implement the Sleep method).
It's simple, isn't it?
Is it?

Re-recognize JavaScript settimeout and Asynchrony
Tonight I saw a JavaScript question (settimeout) of Qleelulu, a little thought, and finally took a guess at the answer. But what is the reason? I also said that I was not very clear, anyway, the feeling is a death cycle caused. And then look at the comments below the article, found on the 5 floor (typical death cycle ...) JS is single-threaded, while inside the settimeout function is not a chance to execute. And the 6 floor (settimeout just hangs a timed task, but JS itself is single-threaded, While there must have died. The answer is very reasonable, the main meaning is that the JavaScript engine is single-threaded execution, while loop where the execution, settimeout inside the function does not have the opportunity to execute, so while there is always true, resulting in a dead loop. But the simple look is still not steadfast, finally play the practical spirit, do-it-yourself did two experiments:
1. Simple settimeout

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settimeout (function () {while (true) {}},1000); settimeout (function () {alert (' End 2 ');},2000); settimeout (function () {alert (' End 1 ');},100); Alert (' End ');

The result of the execution is to eject ' end ' 1′, and then the browser to suspend animation, is not to eject the 2′. That is, the first settimeout execution is a dead loop, which leads directly to a function in the second settimeout that is theoretically more than a second later, and this is not consistent with the asynchronous function multithreading we normally understand.

2. Ajax Request Callback
Then we'll test the Ajax asynchronous request invocation via XMLHttpRequest, the main code is as follows:

The code is as follows

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var xmlreq = Createxmlhttp ();//Create a XMLHttpRequest object function Testasynrequest () { var url = "/asynchandler.ashx?action=ajax"; Xmlreq.open ("Post", url, True); Xmlreq.setrequestheader ("Content-type", "application/x-www-form-urlencoded"); Xmlreq.onreadystatechange = function () { if (xmlreq.readystate = = 4) { if (Xmlreq.status = = 200) { var jsondata = eval (' (' + xmlreq.responsetext + ') '); alert (jsondata.message); }else if (xmlreq.status = = 404) { Alert ("Requested URL is not found."); }else if (xmlreq.status = = 403) { Alert ("Access denied."); }else{ Alert ("Status is" + Xmlreq.status); } } }; Xmlreq.send (NULL); } Testasynrequest ();//1 seconds after calling callback function while (true) {}

Implement simple output on the server side:

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private void Processajaxrequest (HttpContext context) { String action = context. Request["Ajax"]; Thread.Sleep (1000);//wait 1 seconds String jsonobject = "{" Message ":" "+ Action +" "}"; Context. Response.Write (Jsonobject); }

Theoretically, if the Ajax asynchronous request, its asynchronous callback function is in a separate thread, then the callback function must not be "blocked" by other threads to execute smoothly, that is, 1 seconds later, it callback execution pop-up ' Ajax ', but this is not the case, the callback function can not be executed, Because the browser again because of dead loop suspended animation.