求帮忙解释一段验证

<script type="text/javascript" reload="1">
var pagescroll = new pagescroll_class('floatlayout_register', 600, 400);
$('floatlayout_register').scrollLeft = 600;
document.body.focus();
try {
$('username').focus();
} catch(e) {}

var profile_username_toolong = '用户名超过 15 个字符';
var profile_username_tooshort = '用户名小于3个字符';
var doublee = parseInt('1');
var lastusername = lastpassword = lastemail = lastinvitecode = '';

function messagehandle_register(key, msg) {
$('returnmessage4').className = key == 1 ? 'onerror' : '';
$('returnmessage4').innerHTML = msg;
}

function checkusername() {
var username = trim($('username').value);
if(username == '' || username == lastusername) {
return;
} else {
lastusername = username;
}
var unlen = username.replace(/[^\x00-\xff]/g, "**").length;
if(unlen < 3 || unlen > 15) {
messagehandle_register(1, unlen < 3 ? profile_username_tooshort : profile_username_toolong);
return;
}
        ajaxget('ajax.php?infloat=register&handlekey=register&action=checkusername&username=' + (is_ie && document.charset == 'utf-8' ? encodeURIComponent(username) : username), 'returnmessage4');
}

function checkemail() {
var email = trim($('email').value);
if(email == '' || email == lastemail) {
return;
} else {
lastemail = email;
}
ajaxget('ajax.php?infloat=register&handlekey=register&action=checkemail&email=' + email, 'returnmessage4');

}
function checkinvitecode() {
var invitecode = trim($('invitecode').value);
if(invitecode == lastinvitecode) {
return;
} else {
lastinvitecode = invitecode;
}
        ajaxget('ajax.php?infloat=register&handlekey=register&action=checkinvitecode&invitecode=' + invitecode, 'returnmessage4');
}
function trim(str) {
return str.replace(/^\s*(.*?)[\s\n]*$/g, '$1');
}
</script>