My website on server is with 755 permission for all files .
Is there any problem with this permission ? Is this permission vulnearable ? Or should i decrease the permission ?
How can i stop seeing javascript and css files of that website ? Is that possible ?
From page view source one can easily see javascript and css files and can download it.
644 is probably better than 755 because website resource files are usually not directly executable.
Aside from that, you cannot (well, should not) stop someone from seeing Javascript and CSS files. A user’s browser needs to be able to download those files in order to render the website correctly.