在某些情况下，倒着做就足够了。使用javascript添加一个类:

// Jquery
$('body').addClass('js-enabled');

/* CSS */
.menu-mobile {display:none;}
body.js-enabled .menu-mobile {display:block;}

这可能会在任何复杂的事情上产生维护问题，但对于某些事情来说，这是一个简单的修复。而不是试图检测它什么时候没有加载，只是根据它加载的时候样式。

人们已经发布了一些例子，这些例子都是很好的检测选项，但是基于你的要求“给出警告，如果浏览器没有启用JS，站点将无法正常运行”。你可以添加一个元素，让它以某种方式出现在页面上，例如当你获得徽章时，在Stack Overflow上弹出相应的消息，然后用一些Javascript删除它，在页面加载时立即运行(我指的是DOM，而不是整个页面)。

在什么地方发现它?JavaScript ?那是不可能的。如果只是为了记录日志，可以使用某种跟踪方案，其中每个页面都有JavaScript，将对一个特殊资源(可能是一个非常小的gif或类似的文件)发出请求。这样，您就可以获取唯一页面请求和跟踪文件请求之间的差异。

Why don't you just put a hijacked onClick() event handler that will fire only when JS is enabled, and use this to append a parameter (js=true) to the clicked/selected URL (you could also detect a drop down list and change the value- of add a hidden form field). So now when the server sees this parameter (js=true) it knows that JS is enabled and then do your fancy logic server-side. The down side to this is that the first time a users comes to your site, bookmark, URL, search engine generated URL- you will need to detect that this is a new user so don't look for the NVP appended into the URL, and the server would have to wait for the next click to determine the user is JS enabled/disabled. Also, another downside is that the URL will end up on the browser URL and if this user then bookmarks this URL it will have the js=true NVP, even if the user does not have JS enabled, though on the next click the server would be wise to knowing whether the user still had JS enabled or not. Sigh.. this is fun...

如果javascript被禁用，您的客户端代码将无法运行，所以我假设您的意思是您希望该信息可用的服务器端。在这种情况下，noscript就没那么有用了。相反，我有一个隐藏的输入，并使用javascript填充一个值。在你的下一个请求或回发后，如果值在那里，你知道javascript是打开的。

注意像noscript这样的事情，第一次请求可能显示javascript被禁用，但以后的请求会打开它。

下面是一个PHP脚本，它可以在生成任何输出之前包含一次。它不是完美的，但在大多数情况下，它工作得足够好，以避免交付客户端不会使用的内容或代码。标题注释解释了它是如何工作的。

<?php
/*****************************************************************************
 * JAVASCRIPT DETECTION                                                      *
 *****************************************************************************/

// Progressive enhancement and graceful degradation are not sufficient if we
// want to avoid sending HTML or JavaScript code that won't be useful on the
// client side.  A normal HTTP request will not include any explicit indicator
// that JavaScript is enabled in the client.  So a "preflight response" is
// needed to prompt the client to provide an indicator in a follow-up request.
// Once the state of JavaScript availability has been received the state of
// data received in the original request must be restored before proceding.
// To the user, this handshake should be as invisible as possible.
// 
// The most convenient place to store the original data is in a PHP session.
// The PHP session extension will try to use a cookie to pass the session ID
// but if cookies are not enabled it will insert it into the query string.
// This violates our preference for invisibility.  When Javascript is not
// enabled the only way to effect a client side redirect is with a "meta"
// element with its "http-equiv" attribute set to "refresh".  In this case
// modifying the URL is the only way to pass the session ID back.
//
// But when cookies are disabled and JavaScript is enabled then a client side
// redirect can be effected by setting the "window.onload" method to a function
// which submits a form.  The form has a "method" attribute of "post" and an
// "action" attribute set to the original URL.  The form contains two hidden
// input elements, one in which the session ID is stored and one in which the
// state of JavaScript availability is stored.  Both values are thereby passed
// back to the server in a POST request while the URL remains unchanged.  The
// follow-up request will be a POST even if the original request was a GET, but
// since the original request data is restored, the containing script ought to
// process the request as though it were a GET.

// In order to ensure that the constant SID is defined as the caller of this
// script would expect, call session_start if it hasn't already been called.
$session = isset($_SESSION);
if (!$session) session_start();

// Use a separate session for Javascript detection.  Save the caller's session
// name and ID.  If this is the followup request then close the caller's
// session and reopen the Javascript detection session.  Otherwise, generate a
// new session ID, close the caller's session and create a new session for
// Javascript detection.
$session_name = session_name();
$session_id = session_id();
session_write_close();
session_name('JS_DETECT');
if (isset($_COOKIE['JS_DETECT'])) {
    session_id($_COOKIE['JS_DETECT']);
} elseif (isset($_REQUEST['JS_DETECT'])) {
    session_id($_REQUEST['JS_DETECT']);
} else {
    session_id(sha1(mt_rand()));
}
session_start();

if (isset($_SESSION['_SERVER'])) {
    // Preflight response already sent.
    // Store the JavaScript availability status in a constant.
    define('JS_ENABLED', 0+$_REQUEST['JS_ENABLED']);
    // Store the cookie availability status in a constant.
    define('COOKIES_ENABLED', isset($_COOKIE['JS_DETECT']));
    // Expire the cookies if they exist.
    setcookie('JS_DETECT', 0, time()-3600);
    setcookie('JS_ENABLED', 0, time()-3600);
    // Restore the original request data.
    $_GET = $_SESSION['_GET'];
    $_POST = $_SESSION['_POST'];
    $_FILES = $_SESSION['_FILES'];
    $_COOKIE = $_SESSION['_COOKIE'];
    $_SERVER = $_SESSION['_SERVER'];
    $_REQUEST = $_SESSION['_REQUEST'];
    // Ensure that uploaded files will be deleted if they are not moved or renamed.
    function unlink_uploaded_files () {
        foreach (array_keys($_FILES) as $k)
            if (file_exists($_FILES[$k]['tmp_name']))
                unlink($_FILES[$k]['tmp_name']);
    }
    register_shutdown_function('unlink_uploaded_files');
    // Reinitialize the superglobal.
    $_SESSION = array();
    // Destroy the Javascript detection session.
    session_destroy();
    // Reopen the caller's session.
    session_name($session_name);
    session_id($session_id);
    if ($session) session_start();
    unset($session, $session_name, $session_id, $tmp_name);
    // Complete the request.
} else {
    // Preflight response not sent so send it.
    // To cover the case where cookies are enabled but JavaScript is disabled,
    // initialize the cookie to indicate that JavaScript is disabled.
    setcookie('JS_ENABLED', 0);
    // Prepare the client side redirect used when JavaScript is disabled.
    $content = '0; url='.$_SERVER['REQUEST_URI'];
    if (!$_GET['JS_DETECT']) {
        $content .= empty($_SERVER['QUERY_STRING']) ? '?' : '&';
        $content .= 'JS_DETECT='.session_id();
    }
    // Remove request data which should only be used here.
    unset($_GET['JS_DETECT'],$_GET['JS_ENABLED'],
            $_POST['JS_DETECT'],$_POST['JS_ENABLED'],
            $_COOKIE['JS_DETECT'],$_COOKIE['JS_ENABLED'],
            $_REQUEST['JS_DETECT'],$_REQUEST['JS_ENABLED']);
    // Save all remaining request data in session data.
    $_SESSION['_GET'] = $_GET;
    $_SESSION['_POST'] = $_POST;
    $_SESSION['_FILES'] = $_FILES;
    $_SESSION['_COOKIE'] = $_COOKIE;
    $_SESSION['_SERVER'] = $_SERVER;
    $_SESSION['_REQUEST'] = $_REQUEST;
    // Rename any uploaded files so they won't be deleted by PHP.  When using
    // a clustered web server, upload_tmp_dir must point to shared storage.
    foreach (array_keys($_FILES) as $k) {
        $tmp_name = $_FILES[$k]['tmp_name'].'x';
        if (move_uploaded_file($_FILES[$k]['tmp_name'], $tmp_name))
            $_SESSION['_FILES'][$k]['tmp_name'] = $tmp_name;
    }
// Have the client inform the server as to the status of Javascript.
?>
<!DOCTYPE html>
<html>
<head>
    <script>
        document.cookie = 'JS_ENABLED=1';
// location.reload causes a confirm box in FireFox
//      if (document.cookie) { location.reload(true); }
        if (document.cookie) { location.href = location; }
    </script>
    <meta http-equiv="refresh" content="<?=$content?>" />
</head>
<body>
    <form id="formid" method="post" action="" >
        <input type="hidden" name="<?=$session_name?>" value="<?=$session_id?>" />
        <input type="hidden" name="JS_DETECT" value="<?=session_id()?>" />
        <input type="hidden" name="JS_ENABLED" value="1" />
    </form>
    <script>
        document.getElementById('formid').submit();
    </script>
</body>
</html>
<?php
    exit;
}
?>