I know it is possible to edit php and css file in the editor in dashboard [Appearance -> Editor].. But i was wondering why it is not possible to edit all the folders [including js files] in the editor..
I searched for this and found a plugin to make all files editable inside wordpress dashboard.. https://wordpress.org/plugins/solid-code-theme-editor/
Unfortunately there isn't a way to extend the types of files that Appearance -> Editor lets you edit (for example, adding files in subfolders, or JS files). I've tried to find a way to do this before, but - despite almost everything else being hook'able in WordPress - there is no way to hook into the file editor to change the way it works.
This could be for security reasons (having the ability to edit files like this can be a problem in the first place anyway), but in the end, the only way to add the ability to edit every file is to use an outside-of-WordPress editor, which is what the plugin you referred to does.
I usually disable the Editor entirely (by adding
wp-config.php), and use another method of editing the theme instead (usually FTP, or occasionally the file manager accessible through the web host's control panel). This way you'll have access to everything you need.