xhttp is not defined when using ajax calls into an iframe
<p>Good day all, I'm developing a php page in which there is an iframe, that opens another php page with a checkbox in it, this second page, when the user click on the checkbox, has to make an ajax call to confirm the "click".</p>

<p>so there is pageA.php, with a iframe in it that points to pageB.php, in this one, there is only a form with a checkbox and a javascript (vanilla javascript), that sould call a third page on click.</p>

<p>this is the javascript I'm using to send the "click":</p>

<pre><code>document.getElementById("checkboxMe").onclick = function() {
    xhttp.open("POST", "pageC.php", true);
    xhttp.send("foo=bar");  
    };
</code></pre>

<p>when clicking on the checkbox, this is what I see on the console:</p>

<pre><code>Uncaught ReferenceError: xhttp is not defined
</code></pre>

<p>it never happen something like this, infact I can't find this error easily on google, does anyone has some clues? 
maybe is the fact I'm into an iframe?
how could I solve this issue?</p>

<p>thanks in advance ppl.</p>