Finally I get a fully worked solution on each site in SharePoint.
It needs to be pasted in default.aspx under the tag:
<asp:Content ContentPlaceHolderId="PlaceHolderAdditionalPageHead" runat="server">
Here is the js code:
<script type = "text/javascript" >
var webCollection = null;
function runCode() {
var clientContext = new SP.ClientContext.get_current();
if (clientContext != undefined && clientContext != null) {
var web = clientContext.get_web();
this.webCollection = web.getSubwebsForCurrentUser(null);
clientContext.load(this.webCollection);
clientContext.executeQueryAsync(Function.createDelegate(this, this.onQuerySucceeded), Function.createDelegate(this, this.onQueryFailed));
}
}
function onQuerySucceeded() {
var webName = "";
var webEnumerator = this.webCollection.getEnumerator();
webEnumerator.moveNext();
var web = webEnumerator.get_current();
webName += web.get_url();
window.location.href = webName;
}
function onQueryFailed(sender, args) {
alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
}
window.setTimeout(function(){runCode()},0);
</script>
IMPORTANT!
I must notice that first subsite is the first created subsite.
If somebody moves the hyperlinks in top navigation bar, it doesn't matter for above redirection method - it always moves to the first created (by admin or developer) subsite!