JavaScript变量在xmlhttprequest对象中丢失

时间：2015-05-27 02:27:20

标签： javascript

我有一个简单的登录表单，使用javascript，ajax和php，它会询问用户名和密码。它检查'message'变量的值以查看是否要提交。

test.php的 

<!DOCTYPE html>
<html>
<head>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel="stylesheet" href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">
    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
    <script src="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
    <script src="ecommercescript.js" defer="true"></script>
 </head>
<body>
    <button type="button" class="btn btn-primary" data-toggle="modal" data-target="#login">Sign In</button>
    <div id="login" class="modal fade" role="dialog">
        <div class="modal-dialog">
            <div class="modal-content">
                <div class="modal-header">
                    <button type="button" class="close" data-dismiss="modal">&times;</button>
                    <h4 class="modal-title">Sign In With Your Account</h4>
                </div>
                <div class="modal-body">
                    <form id="loginForm" class="form-horizontal" role="form" action="login.php" method="post" onsubmit="return validateForm()">
                    <div class="form-group">
                        <label class="control-label col-sm-2" for="usr">Username:</label>
                        <div class="col-sm-10">
                            <input type="text" class="form-control" id="usr" placeholder="Enter your username" name="usr">
                        </div>
                    </div>
                    <div class="form-group">
                        <label class="control-label col-sm-2" for="pwd">Password:</label>
                        <div class="col-sm-10">
                            <input type="password" class="form-control" id="pwd" name="pwd" placeholder="Enter your password">
                        </div>
                    </div>
                    <div class="form-group">
                        <div class="col-sm-offset-4">
                            <label id="prompt" class="control-label">&nbsp;</label>
                        </div>
                    </div>
                    <div class="form-group">
                        <div class="col-sm-offset-4 col-sm-10">
                            <button type="submit" class="btn btn-primary">Submit</button>
                            <button type="button" class="btn btn-primary" onclick="clearInput()">Cancel</button>
                       </div>
                    </div>
                    </form>
                </div>
            </div>
        </div>
    </div>
</body>
</html>

ecommercescript.js 

function validateForm() {
    var usr = document.forms["loginForm"]["usr"].value.trim();
    var pwd = document.forms["loginForm"]["pwd"].value.trim();
    var message = "";
    var validity = true;

    if (usr.length > 0 && pwd.length > 0) {
        var xmlhttp;
        if (window.XMLHttpRequest) {
            xmlhttp = new XMLHttpRequest();
        } else {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                message = xmlhttp.responseText;
                document.getElementById('prompt').innerHTML = message;
            }
        }
        //value of 'message' gets lost here
        xmlhttp.open("POST", "searchuser.php", true);
        xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xmlhttp.send("username="+usr+"&password="+pwd+"")
    } else {
        message = "Username and/or password must be filled out";
        document.getElementById('prompt').innerHTML = message;
        validity = false;
    }

    if (message.length > 0) {
        validity = false;
    }

    return validity;
}
function clearInput() {
    document.getElementById('usr').value = "";
    document.getElementById('pwd').value = "";
}

searchuser.php 

<?php
$servername = "localhost";
$username = "root";
$password = "";
$dbname = "ecommerce";

//Create connection
$conn = new mysqli($servername, $username, $password, $dbname);

//Check connection
if ($conn->connect_error) {
    die("Database connect failed: " . $conn->connect_error);
}

$username = $_REQUEST["username"];
$password = $_REQUEST["password"];
$message = "Account does not exist";

$sql = "SELECT * FROM tbl_users";
$result = $conn->query($sql);

if ($result->num_rows > 0) {
    while($row = $result->fetch_assoc()) {
        if ($row['username'] == $username and $row['password'] == $password) {
            $message = "";
            break;
        } else if ($row['username'] == $username and $row['password'] != $password) {
            $message = "Incorrect password";
        }
    }
} 

$conn->close();
echo $message;
?>

我的问题是xmlhttp.onreadystatechange中'message'的值丢失，因此脚本无法验证并始终提交表单。我已经评论了它丢失的部分。需要你的帮助。

3 个答案:

答案 0 :(得分：0)

你基本上有这个：

When done with HTTP request {
    Do a bunch of stuff;
    Set message variable;
}
// comment about lost variable
Code to send HTTP request;
if (some stuff about message){
   More stuff;
}

设置消息的代码将在发送请求后运行。此外，验证message长度的代码将在您的HTTP请求解决之前运行（我认为这总是正确的，但也许快速的网络可以击败JS的条件？如果您知道，可以随意编辑）。

解决此问题的方法是：

When done with HTTP request {
    Do a bunch of stuff;
    Set message variable
    if (some stuff about message){
       More stuff;
    }
}
// comment about sending request
Code to send HTTP request;

希望这有帮助！

答案 1 :(得分：0)

它不会迷路，你在设置之前尝试使用异步变量。如果你想这样做，你需要承诺对象。 q.js非常好。 

function validateForm() {
    var deferred = Q.defer();
    var usr = document.forms["loginForm"]["usr"].value.trim();
    var pwd = document.forms["loginForm"]["pwd"].value.trim();
    var message = "";
    var validity = true;

    if (usr.length > 0 && pwd.length > 0) {
        var xmlhttp;
        if (window.XMLHttpRequest) {
            xmlhttp = new XMLHttpRequest();
        } else {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                message = xmlhttp.responseText;
                document.getElementById('prompt').innerHTML = message;
                deferred.resolve(message);
            }
        }
        //value of 'message' gets lost here
        xmlhttp.open("POST", "searchuser.php", true);
        xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xmlhttp.send("username="+usr+"&password="+pwd+"")

    } else {
        message = "Username and/or password must be filled out";
        document.getElementById('prompt').innerHTML = message;
        deferred.resolve(message);
    }

    return deferred.promise;
}

或者在xmlhttprequest结算时发出自定义事件

function validateForm() {
    var customEvent = new Event('messageResolved');
    var usr = document.forms["loginForm"]["usr"].value.trim();
    var pwd = document.forms["loginForm"]["pwd"].value.trim();
    var message = "";
    var validity = true;

    if (usr.length > 0 && pwd.length > 0) {
        var xmlhttp;
        if (window.XMLHttpRequest) {
            xmlhttp = new XMLHttpRequest();
        } else {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                message = xmlhttp.responseText;
                document.getElementById('prompt').innerHTML = message;
                document.dispatchEvent('messageResolved', { message: message });
            }
        }
        //value of 'message' gets lost here
        xmlhttp.open("POST", "searchuser.php", true);
        xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xmlhttp.send("username="+usr+"&password="+pwd+"")

    } else {
        message = "Username and/or password must be filled out";
        document.getElementById('prompt').innerHTML = message;
        document.dispatchEvent('messageResolved', { message: message });
    }
}

或使用回调

function validateForm() {
    var usr = document.forms["loginForm"]["usr"].value.trim();
    var pwd = document.forms["loginForm"]["pwd"].value.trim();
    var message = "";
    var validity = true;

    if (usr.length > 0 && pwd.length > 0) {
        var xmlhttp;
        if (window.XMLHttpRequest) {
            xmlhttp = new XMLHttpRequest();
        } else {
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        xmlhttp.onreadystatechange = function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                message = xmlhttp.responseText;
                document.getElementById('prompt').innerHTML = message;
                callback(message);
            }
        }
        //value of 'message' gets lost here
        xmlhttp.open("POST", "searchuser.php", true);
        xmlhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        xmlhttp.send("username="+usr+"&password="+pwd+"")

    } else {
        message = "Username and/or password must be filled out";
        document.getElementById('prompt').innerHTML = message;
        callback(message);
    }
}

function callback(message){

}

答案 2 :(得分：0)

非常感谢，我设法通过将async参数设置为false来使其工作。无论如何，谢谢你的见解。

<强>前 

 //value of 'message' gets lost here
 xmlhttp.open("POST", "searchuser.php", true);

<强>后 

//value of 'message' gets lost here
 xmlhttp.open("POST", "searchuser.php", false);
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