這是 FB Developer Blog 在 2012/5/8 發表的。用來判斷當 User 已經登入 FB 的判斷，先註記起來，以後一定用得上的！
<!DOCTYPE html>
<html>
<head>
<title>Example</title>
</head>
<body>
<!-- Load the Facebook JavaScript SDK -->
<div id="fb-root"></div>
<script src="//connect.facebook.net/en_US/all.js"></script>
<script type="text/javascript">
// Initialize the Facebook JavaScript SDK
FB.init({
appId: 'APP_ID',
xfbml: true,
status: true,
cookie: true,
});
// Check if the current user is logged in and has authorized the app
FB.getLoginStatus(checkLoginStatus);
// Login in the current user via Facebook and ask for email permission
function authUser() {
FB.login(checkLoginStatus, {scope:'email'});
}
// Check the result of the user status and display login button if necessary
function checkLoginStatus(response) {
if(response && response.status == 'connected') {
alert('User is authorized');
// Hide the login button
document.getElementById('loginButton').style.display = 'none';
// Now Personalize the User Experience
console.log(‘Access Token: ‘ + response.authResponse.accessToken);
} else {
alert('User is not authorized');
// Display the login button
document.getElementById('loginButton').style.display = 'block';
}
}
</script>
<input id="loginButton" type="button" value="Login!" onclick="authUser();" />
</body>
</html>
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