试图在 html 页面中实现 jquery/ajax

Question

    echo "<form method='post' action='regprocess.php' id='registerform'>";
        echo '<fieldset class="register">';
        echo"<h2>Register</h2>";
            echo "<ul>";
                    echo '<li><label for="FirstName">First Name: </label> <input type="text" name="FirstName" id="FirstName"></li>';
                    echo '<li><label for="LastName">Last Name: </label> <input type="text" name="LastName" id="LastName"></li>';
                    echo '<li><label for="Email">Email: </label><input type="email" name="Email" id="Email"></li>';
                    echo '<li><label for="Username">Username: </label><input type="text" name="Username" id="Username"></li>';
                    echo '<li><input type="button" id="check_username_availability" value="Check Availability"></li>';  
                    echo '<div id="username_availability_result"></div>'; 
                    echo '<li><label for="Password">Password: </label><input type="password" name="Password" id="Password"></li>';
                    echo '<li><input type="submit" value="Register"></li>';
                    echo "</ul>";
        echo "</fieldset>";
        echo "</form>";
        }
        $username = mysql_real_escape_string($_POST['Username']);  

//mysql query to select field username if it's equal to the username that we check '  
$usernameresult = 'Select Username from User where Username = "'. $username .'"'; 
$uresult = $conn->query($usernameresult); 

//if number of rows fields is bigger them 0 that means it's NOT available '  
if($uresult->num_rows==1) {  
    //and we send 0 to the ajax request  
    echo 0;  
}else{  
    //else if it's not bigger then 0, then it's available '  
    //and we send 1 to the ajax request  
    echo 1;  
}  
?>
<script src="jquery-1.8.1.min.js" type="text/javascript"></script>
<SCRIPT LANGUAGE="JAVASCRIPT" TYPE="TEXT/JAVASCRIPT">
$(document).ready(function() {  

        var checking_html = 'Checking...';  

        //when button is clicked  
        $('#check_username_availability').click(function(){  
            //run the character number check  
            if($('#username').val().length < min_chars){   
                $('#username_availability_result').html(checking_html);  
                check_availability();  
            }  
        });  

  });  

//function to check username availability  
function check_availability(){  

        //get the username  
        var username = $('#username').val();  

        //use ajax to run the check  
        $.post("check_username.php", { username: username },  
            function(result){  
                //if the result is 1  
                if(result == 1){  
                    //show that the username is available  
                    $('#username_availability_result').html(username + ' is Available');  
                }else{  
                    //show that the username is NOT available  
                    $('#username_availability_result').html(username + ' is not Available');  
                }  
        });  

}
</script>

大家好，所以我正在尝试制作一个表单来验证我的数据库中是否有可用的用户名。我以前有点用过 javascript，但我对 Ajax 和 JQuery 做的不多。我在控制台中不断收到此错误消息，上面写着 ReferenceError：找不到变量：$ 响应这行脚本 $(document).ready(function() {
我不太清楚为什么，我只是想知道是否有人可以给我一些关于如何解决它的见解？谢谢。

Answer 1

代码看起来没问题。唯一的问题我认为您对 jquery 库的引用不正确。查看源代码并单击包含行的 jquery 库以测试路径是否正常。将jquery-1.8.1.min.js 保存在这些 php 文件所在的同一文件夹中，或者使用任何 CDN 库，例如

<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.1/jquery.min.js"></script>

希望对你有帮助！！