Nginx return custom json

custom-error-page

  error_page 400 404 405 =200 @40*_json;

  location @40*_json {
    default_type application/json;
    return 200 '{"code":"1", "message": "Not Found"}';
  }

  error_page 500 502 503 504 =200 @50*_json;

  location @50*_json {
    default_type application/json;
    return 200 '{"code":"1", "message": "Unknown Error"}';
  }

Returning 200 means that anything that isn't a straightforward web browser will not know it has an error. Things like search engines may assume that the page is correct and useful and try to index it. archive.org may try to store it. Assistive technology for disabled people may tell it's users the wrong thing.

Very useful for error-handling on API reverse proxy.
"error_page" directive can be put in specific "location" only, as it supports multiple contexts: http://nginx.org/en/docs/http/ngx_http_core_module.html#error_page