js
        <script type="text/javascript">
        $(function(){
            $('#submit').click(function(){
                var username = $('#username').val();
                var password = $('#password').val();
                var xhr = null;
                if (window.XMLHttpRequest) {
                    xhr = new
 
 XMLHttpRequest();
                } else {
                    xhr = new ActiveXObject('Microsoft.XMLHTTP');
                }
                var url = 'http://localhost/php_pro/ci/user/login';
                var data = "username=" + username + "&password=" + password;     //注意等號左右邊不要留有空格！！！
                xhr.open('post'
 
, url, true);
                xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded"); 
                xhr.send(data);

                xhr.onreadystatechange = function() {
                    if (xhr.readystate == 4) {
                        if (xhr.status == 200) {
                            alert(responseText);
                        }
                    }
                }
            });
        });
        
 
</script>

    Jquery
        <script type="text/javascript">
        $(function(){
            $('#submit').click(function(){
                var username = $('#username').val();
                var password = $('#password').val();
                $.ajax({
                     data:{"username":username,"password":password,"first":false},       //要傳送的資料  
                     type:"POST", //傳送的方式  
                     url:"login", //url地址  
                     error:function(msg){ //處理出錯的資訊  
                          var errormessage="再試一次";  
                          $(".loginerror").html(errormessage);  
                      },  
                     success:function(msg){  //處理正確時的資訊  
                          //alert("success" + msg)  
                          alert(msg);
                          if(msg=='登入成功'){  
                              var errormessage="登入成功";  
                              $(".loginerror").html(errormessage);  

                              location.href = "user_center"  
                          }else{  
                              var errormessage="使用者名稱或密碼錯誤";  
                              $(".loginerror").html(errormessage);  
                          }  
                      }  
                });
        });
        </script>

使用html5 FormData物件提交表單及上傳圖片
把以上2種方式中的資料替換成formdata即可！

var formdata = new FormData();  
formdata.append('name','fdipzone');  
formdata.append('gender','male'); 
或者
var form = document.getElementById('form1'); 
var formdata = new FormData(form); 
     ps:<form id='form1'>...</form>