How the code execute immediately without hit the button?
<p>I made a very simple Ajax test code snippet:</p>

<pre><code>&lt;!DOCTYPE html&gt;
&lt;html&gt;
    &lt;head&gt;
        &lt;script src='main.js'&gt;&lt;/script&gt;
    &lt;/head&gt;
    &lt;body&gt;
        &lt;button id='get-content'&gt;Click&lt;/button&gt;
        &lt;p&gt;&lt;/p&gt;
    &lt;/body&gt;
&lt;/html&gt;
</code></pre>

<p>main.js</p>

<pre><code>(function(){
    window.addEventListener('load', function(){
        var xml_request = new XMLHttpRequest();
        xml_request.open('GET', '/test.txt', true);
        xml_request.addEventListener('readystatechange', change_content);
        var button_element = document.getElementById('get-content'); 
        button_element.addEventListener('click', fireup(xml_request));
        function fireup(xml_request) {
            xml_request.send();
        }
        function change_content(){
            if (xml_request.readyState == 4 &amp;&amp; xml_request.status == 200) {
                document.getElementsByTagName('p')[0].textContent = xml_request.responseText;
            };  
        };
    });
})();
</code></pre>

<p>it works, but except, the ajax request send and change the p tag, without I hit the button,
any idea?</p>
