AJAX Change [Лаб. 6.1]

<!DOCTYPE html>
<html>
    <head>
        <meta charset="utf-8">
        <title>AJAX One</title>
        <style rel="stylesheet" type="text/css">
            .game-changer {padding: 10px 0;}
        </style>
        <script type="text/javascript">
            // This code works just fine on Edge and Mozilla, in order to work on Chrome you need to disable CORS on Chrome,
            // by typing the following command in command prompt: "C:\Program Files (x86)\Google\Chrome\Application\chrome.exe" --disable-web-security --user-data-dir="c:/chromedev OR you install CORS extension on your Chrome browser.
            // If you work on Chrome besides disabling CORS don't check if request status is 200, because for requesting local files it will
            // always return 0 so if statement will always fail, although for requesting online files it will still return 200.

            function get(success) {
                var request = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject("Microsoft.XMLHTTP");
                request.open("GET", "plain_text.txt");
                request.onreadystatechange = function() {
                    if(this.readyState === 4 && this.status === 200)
                        success(this.responseText);
                }

                request.send();
            }

            document.addEventListener("DOMContentLoaded", function() {
                document.getElementsByClassName("change")[0].addEventListener("click", function() {
                    get(function(data) {
                        document.getElementsByClassName("game-changer")[0].innerHTML = data;
                    })
                });
            });
        </script>
    </head>
    <body>
        <div class="game-changer">
            <h1>Let AJAX change this text</h1>
        </div>
        <button class="change">Change Content</button>
    </body>
</html>