点击submit按钮，执行ajax，验证输入的用户名是否被占用。这个该怎么实现？

Question

点击submit按钮，执行ajax，验证输入的用户名是否被占用。这个该怎么实现？

Answer 1

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$(function (){     $("#submit").click(function (){         var username=$("#username").val()         $.ajax({             type: "get",             url:"post.asp",             async: true,             timeout: 15000,             data:{username:username},             beforeSend: function(xhr) {   //beforeSend定义全局变量                 xhr.setRequestHeader("If-Modified-Since", "0");  //If-Modified-Since HTTP请求头标签,即比较浏览器缓存页面时间             },             success: function(xmlDoc, textStatus, xhr)              {                 if(xhr.readyState==4)                     {                     if(xhr.status == 200)                     {                         var str=xhr.responseText; //获取返回值                         if (str==0){ //查询返回0表示已注册                             alert("已被注册")                             return;                             }                         else{                             //运行正确的代码                             alert("可以正常注册")                             }                     }                 }             },         });                     });     });