you can pass this url as variable from php controller:

OW::getDocument()->addScriptDeclarationBeforeIncludes("var base_url = '" . OW::getRouter()->getBaseUrl() . "';");

Why do you need this URL? For what technical purposes?

i am making plugin for my site......

Topic was moved from General Questions.

Here is an example of the code you should use in your plugin init.php file if you need this variable to be global, or in a specific component:

function foo_js_declarations( $e )
{
    OW::getDocument()->addOnloadScript("window.BASE_URL=" . json_encode(OW_URL_HOME));
}
OW::getEventManager()->bind(OW_EventManager::ON_FINALIZE, 'foo_js_declarations');