Php联系人表单引用错误

我在youtube上学习了一个关于建立联系页面的教程，由于某种原因，我在控制台得到一个错误消息，当我按下提交时，说submitForm没有定义。我的问题是我得到了相同的代码在另一个网站上工作，但当我复制完全相同的代码它不工作。下面是我的js代码:

function _(id){ return document.getElementById(id); }
function submitForm(){
    _("mybtn").disabled = true;
    _("status").innerHTML = 'please wait ...';
    var formdata = new FormData();
    formdata.append( "n", _("n").value );
    formdata.append( "e", _("e").value );
    formdata.append( "m", _("m").value );
    var ajax = new XMLHttpRequest();
    ajax.open( "POST", "example_parser.php" );
    ajax.onreadystatechange = function() {
        if(ajax.readyState == 4 && ajax.status == 200) {
            if(ajax.responseText == "success"){
                _("my_form").innerHTML = '<h2>Thanks '+_("n").value+', your message has been sent.</h2>';
            } else {
                _("status").innerHTML = ajax.responseText;
                _("mybtn").disabled = false;
            }
        }
    }
    ajax.send( formdata );
}

<?php
if( isset($_POST['n']) && isset($_POST['e']) && isset($_POST['m']) ){
    $n = $_POST['n']; // HINT: use preg_replace() to filter the data
    $e = $_POST['e'];
    $m = nl2br($_POST['m']);
    $to = "skoolboi434@gmail.com";  
    $from = $e;
    $subject = 'Contact Form Message';
    $message = '<b>Name:</b> '.$n.' <br><b>Email:</b> '.$e.' <p>'.$m.'</p>';
    $headers = "From: $from'n";
    $headers .= "MIME-Version: 1.0'n";
    $headers .= "Content-type: text/html; charset=iso-8859-1'n";
    if( mail($to, $subject, $message, $headers) ){
        echo "success";
    } else {
        echo "The server failed to send the message. Please try again later.";
    }
}
?>

正如你所看到的，我定义了这个函数，但是得到了这个错误。