Using nginx map function can be a powerful way to add headers if they don’t already exist. Let’s start with pseudo code example:

map $ice_cream_flavor $toppings {
  default '';
  '~choc' 'sprinkles';
  '~vanil' 'strawberries';
}

server {
  add_toppings $toppings;
}

Add strawberries to vanilla and vanillatastic ice cream, add sprinkles to chocolate and chocoloco ice cream, add nothing to other flavors. add_topings does nothing if $toppings is empty. Tilde starts a regular expression. $ice_cream_flavor would be an nginx internal variable.

nginx add headers if not already set

Using a regular expression that will match 1 or more characters (i.e. the header is set) along with the $upstream_http_* internal variables to see response headers from an upstream service, we can perform add_header ONLY if the proxied upstream service (like php fpm) has not set it already. This avoids the problem of nginx sending duplicate headers if the upstream service has already set them. add_header will not have any effect if what it is supposed to add is an empty string.

map $upstream_http_access_control_allow_origin $proxy_header_acao {
    default '*';
    '~.' "";
}

map $upstream_http_access_control_allow_headers $proxy_header_acah {
    default 'Authorization,Accept,Content-Type,Origin,X-API-VERSION,X-Visitor-Token,X-Agent-Token,X-Auth-Token';
    '~.' "";
}

map $upstream_http_access_control_allow_methods $proxy_header_acam {
    default 'GET, PUT, PATCH, POST, DELETE, OPTIONS, HEAD';
    '~.' "";
}

server {
...
        add_header 'Access-Control-Allow-Origin' $proxy_header_acao always;
        add_header 'Access-Control-Allow-Headers' $proxy_header_acah always;
        add_header 'Access-Control-Allow-Methods' $proxy_header_acam always;

...
}