How can I add header conditionally in nginx configuration?

I used lua nginx module to solve a similar problem:

localtion / {
  header_filter_by_lua_block {
    ngx.header["X-TEST-0"] = "always-0";
    # example of conditional header
    if ngx.req.get_method() == ngx.HTTP_POST {
      ngx.header["X-TEST-1"] = "only if POST request";
    }
    ngx.header["X-TEST-2"] = "always-2";
  }
}    

https://github.com/openresty/lua-nginx-module#header_filter_by_lua_block ```

This is simple if you think differently and use a map.

map $variable  $headervalue {
    1        only-true;
    default  '';
}

# ...later...
location / {
    add_header X-TEST-1   $headervalue;
}

If $variable is 1, $headervalue will be set and hence the header will be added. By default $headervalue will be empty and the header will not be added.

This is efficient because Nginx only evaluates the map when it's needed (when it gets to a point where $headervalue is referenced).

See similar: https://serverfault.com/questions/598085/nginx-add-header-conditional-on-an-upstream-http-variable