cors missing allow origin code example

Example 1: allow cross origin

Access-Control-Allow-Origin: *
Access-Control-Allow-Origin: <origin>
Access-Control-Allow-Origin: null

Example 2: javascript cors error

$.ajax({
            headers: { "Accept": "application/json"},
            type: 'GET',
            url: 'http://cl.ly/2wr4',
            crossDomain: true,
            beforeSend: function(xhr){
                xhr.withCredentials = true;
          },
            success: function(data, textStatus, request){
                console.log(data);
            }
 });

Example 3: cross origin even with allow header

header('Access-Control-Allow-Origin: http://localhost:8100');
header ("Access-Control-Expose-Headers: Content-Length, X-JSON");
header ("Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE, OPTIONS");
header ("Access-Control-Allow-Headers: Content-Type, Authorization, Accept, Accept-Language, X-Authorization");
header('Access-Control-Max-Age: 86400');

if ($_SERVER['REQUEST_METHOD'] === 'OPTIONS') {
    // The request is using the POST method
    header("HTTP/1.1 200 OK");
    return;

}