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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定義全局變量

                xhr.setRequestHeader("If-Modified-Since", "0");  //If-Modified-Since HTTP請求頭標簽,即比較瀏覽器緩存頁面時間

            },

            success: function(xmlDoc, textStatus, xhr) 

            {

                if(xhr.readyState==4)

                    {

                    if(xhr.status == 200)

                    {

                        var str=xhr.responseText; //獲取返回值

                        if (str==0){ //查詢返回0表示已注冊

                            alert("已被注冊")

                            return;

                            }

                        else{

                            //運行正確的代碼

                            alert("可以正常注冊")

                            }

                    }

                }

            },

        });                

    });    

});