nginx - response based on the requested header
map lets you define a variable's value based on another variable. map should be declared at http level (i.e. outside of server):
map $http_x_header $file_suffix { default "2"; OK "1";};Then the following location should do the trick using your new variable $file_suffix
location ~ ^(/files_dir/.+)\.js$ { root html; try_files $1$file_suffix.js =404;}