Here are the steps to do so:
1) You need to use this step by step guide to connect user to your facebook page in order to fetch user basic information
https://developers.facebook.com/docs/facebook-login/getting-started-web/
FB.api('/me', function(response) {
console.log('Good to see you, ' + response.name + '.');
}
2) After knowing that user is connected using facebook you need to issue the graph GET request to find out about this user that he liked your page or not
FB.api('/me/likes/YOUR_APP_ID', function(response) {
console.log(response.data);
}
3) And then run your business logic (whether to take user to download page or other page)
Code from the tutorial above is given below too
<body>
<div id="fb-root"></div>
<script>
window.fbAsyncInit = function() {
FB.init({
appId : 'YOUR_APP_ID', // App ID
channelUrl : '//www.example.com/channel.html', // Channel File
status : true, // check login status
cookie : true, // enable cookies to allow the server to access the session
xfbml : true // parse XFBML
});
FB.Event.subscribe('auth.authResponseChange', function(response) {
if (response.status === 'connected') {
testAPI();
} else if (response.status === 'not_authorized') {
$("#btnFB").show();
FB.login();
} else {
$("#btnFB").show();
FB.login();
}
});
};
// Load the SDK asynchronously
(function(d){
var js, id = 'facebook-jssdk', ref = d.getElementsByTagName('script')[0];
if (d.getElementById(id)) {return;}
js = d.createElement('script'); js.id = id; js.async = true;
js.src = "//connect.facebook.net/en_US/all.js";
ref.parentNode.insertBefore(js, ref);
}(document));
function testAPI() {
FB.api('/me', function(response) {
console.log('Good to see you, ' + response.name + '.');
});
FB.api('/me/likes/PAGE_ID', function(response) {
console.log(response.data);
});
}
How to get PAGE_ID?
Goto http://developers.facebook.com/tools/explorer/?method=GET&path=me%2Flikes%2F
This worked for me! :)
signed_request
parameter. In any other situation you will have to have the user login to your app and give you permission to read their likes. – Lowdown