jQuery get函数返回true / false

$(document).ready(function(){
 //global vars
 var name = $("#username");
    var email = $("#email");


 function usernameExists() {
  $.get("register.PHP",{ check: 1,username: name.val(),email: email.val() },function(m) {
      if(m==1) {
     return false;
    } else { 
     return true;
    }
  });
 }
});

$(document).ready(function(){
    //global vars
    var form = $("#register");
    var name = $("#username");
    var email = $("#email");

     $.get("register.PHP",// Have this callBACk take care of the rest of the submit()
             function(m) {
                if(m==1) {
                     form.submit(function(){ return false; });
                } else {
                    form.submit(function(){
        if(validatename() & validateEmail() & validatePass1() & validatePass2())
            return true
        else
            return false;
                });
             }

         }
      );

function validatename(){
        // some check here
    }

// and other functions

});

您正在调用的函数不会返回任何内容.

即使它确实尝试从$.get()返回响应,它也不会起作用,因为调用是异步的,所以当收到响应时,任何使用返回值的代码都可能已经执行.

您需要做的是从$.get()回调中调用您的代码.

function usernameExists() {
    $.get("register.PHP",function(m) {
            someOtherFunction(m==1);
    });
}

function someOtherFunction(parameter) {
    // The parameter will be true or false
    //    depending on the value of m==1
}

根据您的评论更新.

可能更好的只是将$.get()带入submit(),但保持原始想法,这就是它的外观.

form.submit(function(){
       // After usernameExists() is called,we need to hand off
       //    the rest of the execution to that function since
       //    this one will be done execuTing before the get()
       //    response is received
    usernameExists();
    return false;
}); 

function usernameExists() {
    $.get("register.PHP",// Have this callBACk take care of the rest of the submit()
             function(m) {
                if(m==1) {
                     // do something if true
                } else {
                     // do something if false
                }
             }
      );
}

解释同步与异步javascript的乐趣

Javascript代码通常是同步执行的.这只意味着它一次执行一行,或者一行必须在下一行可以触发之前完成执行.

var greeTing = "hi there";  // set the greeTing variable

alert( greeTing );   // the alert won't fire,//    until the prevIoUs line finished successfully

这使事情变得非常好和可预测.但该规则有一些例外.一个值得注意的例外是AJAX调用.

你的$.get()是一个AJAX调用的例子. AJAX中的“A”代表异步,这意味着它不会阻止下一行代码执行.

分支是当你执行一个$.get()需要花费(例如)1秒来完成时,$.get()之后的任何代码都会在$.get()收到它之后很久完成.响应.

以前的问候语为例,但这次使用的是AJAX.

var greeTing;  // will hold the response from our AJAX call

$.get('some/path/to/data.PHP',function( m ) {
             greeTing = m;  // populate the greeTing variable with the data returned
         }
);

alert( greeTing );   // Will alert "undefined" instead of the data that was returned
                     //   because the $.get() code above is asynchronous,which allows
                     //   the code below it (the alert in this casE) to conTinue 
                     //   execuTing.

正如您所看到的,警报(问候语)将在收到$.get()响应之前很久就已执行,因为$.get()是异步的,并且在等待其数据时不会暂停执行链.

要解决此问题,您可以将alert()放在$.get()的回调中,以便在收到响应之前它不会运行.

var greeTing;  // will hold the response from our AJAX call

$.get('some/path/to/data.PHP',function( m ) {
             greeTing = m;  // populate the greeTing variable with the data returned
             alert( greeTing );  // Now the alert will give the expected result
                                 //    because it is in the callBACk.
         }
);

结果是,在您的代码中,一旦您调用$.get(),任何依赖于收到的响应的剩余代码都应该在回调内部进行.

将代码放在回调之外的唯一方法是将它放在自己的函数中,该函数从回调内部调用(就像我用我的原始答案做的那样).

代码运行方式的基本布局：

请记住,您不一定需要usernameExists()的单独函数.您可以将所有代码放在submit()中

form.submit(function() {
       // check to make sure input is valid **before** you send the AJAX
    if(validatename() & validateEmail() & validatePass1() & validatePass2()) {
        usernameExists();  // If valid,conTinue with the usernameExists()
    }
    return false; // We return false whether or not the content was valid,//   in order to prevent the form from submitTing prematurely
}); 

function usernameExists() {
    $.get("register.PHP",// Have this callBACk take care of the rest of the submit()
             function(m) {
                   // If "m" is less than one,there were no exisTing users
                   //    so we can go ahead and post the data to the server
                if( parseInt(m) < 1 ) {
                     // Here,you would need to manually do a post to 
                     //   submit the data to the server
                     $.post(url,data,callBACk,datatype );
                }
             }
     );
}

http://api.jquery.com/jquery.post/