location.pathname

I want to add a class to a menuepoint if one path is selected.

It would be easy if every site was its own .php/.html file, however everything is ruled in one .php file and everything is navigated over actions (?action=main, ?action=userinformation).

I am searching for an alternative to get this work with the path and the action.

if (location.pathname == "/index.php") {
    $("#main1").addClass("mainActive");
} else if (location.pathname == "/index.php?action=other") {
    $("#main2").addClass("mainActive");
} 

location.pathname doesn't include the query string. But you can combine it with location.search:

var path = location.pathname + location.search;
//            "/index.php"   + "?action=other"

if(path === "/index.php?action=other") {
  // ...
}

Maybe you can use like;

var s = location.search;
if (s === "" || s === "?" || s.indexOf("?action=main") > -1)
    // main active
else if (s.indexOf("?action=userinformation") > -1)
    // userinformation active
// ... and so on