The order of execution of a piece of code outside of the Jquery $.ajax function _ajax Related

Today, I met a very painful problem. The asynchronous function $.ajax of jquery is called in one function, and then there is a section of jquery code outside the $.ajax function. Each time the code after the $.ajax is executed first.

Search on the internet for a long time finally found the reason. Take it and share it with everyone before you forget it.

Async:false

var flag=true; 

The Async:false representative only executes the 
$.ajax ({url: "Emailcheck.ashx", Async:false,data: {"Email": $ ("#email") after waiting for the Ajax execution to complete. Val () }). Done (function (data) { 
if (data== "Fail") 
{ 
flag=false; 
} 
); 

if (flag) 
{ 
$ ("#showInfo"). Text (""); 
return true; 
} 
else 
{ 
alert ("This mailbox already exists!"); 
$ ("#showInfo"). Text ("This mailbox already exists"); 
return false; 
} </span>

Because jquery has not been very proficient, it seems that the future will take time to learn.