Tyk Gateway JSVM middleware to inspect the Content-Type Header, and if matches, transform it to something else.

api_definition_preTransformContentType.json

{
    "id": "5b6973533f03d3608d46cf8c",
    "name": "httpbin",
    "slug": "httpbin",
    "api_id": "e69126ef8100421c4c81b9982b51eb80",
    "org_id": "5b296ceb3f03d310fffc9b9d",
    "use_keyless": true,
    "use_oauth2": false,
    "use_openid": false,
    "openid_options": {
        "providers": [],
        "segregate_by_client": false
    },
    "oauth_meta": {
        "allowed_access_types": [],
        "allowed_authorize_types": [],
        "auth_login_redirect": ""
    },
    "auth": {
        "use_param": false,
        "param_name": "",
        "use_cookie": false,
        "cookie_name": "",
        "auth_header_name": "",
        "use_certificate": false
    },
    "use_basic_auth": false,
    "use_mutual_tls_auth": false,
    "client_certificates": [],
    "upstream_certificates": {},
    "pinned_public_keys": {},
    "enable_jwt": false,
    "use_standard_auth": false,
    "enable_coprocess_auth": false,
    "jwt_signing_method": "",
    "jwt_source": "",
    "jwt_identity_base_field": "",
    "jwt_client_base_field": "",
    "jwt_policy_field_name": "",
    "jwt_disable_issued_at_validation": false,
    "jwt_disable_expires_at_validation": false,
    "jwt_disable_not_before_validation": false,
    "notifications": {
        "shared_secret": "",
        "oauth_on_keychange_url": ""
    },
    "enable_signature_checking": false,
    "hmac_allowed_clock_skew": -1,
    "base_identity_provided_by": "",
    "definition": {
        "location": "header",
        "key": "x-api-version",
        "strip_path": false
    },
    "version_data": {
        "not_versioned": true,
        "default_version": "",
        "versions": {
            "Default": {
                "name": "Default",
                "expires": "",
                "paths": {
                    "ignored": [],
                    "white_list": [],
                    "black_list": []
                },
                "use_extended_paths": true,
                "extended_paths": {},
                "global_headers": {},
                "global_headers_remove": [],
                "global_size_limit": 0,
                "override_target": ""
            }
        }
    },
    "uptime_tests": {
        "check_list": [],
        "config": {
            "expire_utime_after": 0,
            "service_discovery": {
                "use_discovery_service": false,
                "query_endpoint": "",
                "use_nested_query": false,
                "parent_data_path": "",
                "data_path": "",
                "port_data_path": "",
                "target_path": "",
                "use_target_list": false,
                "cache_timeout": 60,
                "endpoint_returns_list": false
            },
            "recheck_wait": 0
        }
    },
    "proxy": {
        "preserve_host_header": false,
        "listen_path": "/httpbin/",
        "target_url": "http://httpbin.org/",
        "strip_listen_path": true,
        "enable_load_balancing": false,
        "target_list": [],
        "check_host_against_uptime_tests": false,
        "service_discovery": {
            "use_discovery_service": false,
            "query_endpoint": "",
            "use_nested_query": false,
            "parent_data_path": "",
            "data_path": "",
            "port_data_path": "",
            "target_path": "",
            "use_target_list": false,
            "cache_timeout": 0,
            "endpoint_returns_list": false
        },
        "transport": {
            "ssl_ciphers": [],
            "ssl_min_version": 0,
            "proxy_url": ""
        }
    },
    "disable_rate_limit": false,
    "disable_quota": false,
    "custom_middleware": {
        "pre": [
            {
                "name": "preTransformContentType",
                "path": "middleware/preTransformContentType.js",
                "require_session": false
            }
        ],
        "post": [],
        "post_key_auth": [],
        "auth_check": {
            "name": "",
            "path": "",
            "require_session": false
        },
        "response": [],
        "driver": "",
        "id_extractor": {
            "extract_from": "",
            "extract_with": "",
            "extractor_config": {}
        }
    },
    "custom_middleware_bundle": "",
    "cache_options": {
        "cache_timeout": 60,
        "enable_cache": true,
        "cache_all_safe_requests": false,
        "cache_response_codes": [],
        "enable_upstream_cache_control": false,
        "cache_control_ttl_header": ""
    },
    "session_lifetime": 0,
    "active": true,
    "auth_provider": {
        "name": "",
        "storage_engine": "",
        "meta": {}
    },
    "session_provider": {
        "name": "",
        "storage_engine": "",
        "meta": {}
    },
    "event_handlers": {
        "events": {}
    },
    "enable_batch_request_support": false,
    "enable_ip_whitelisting": false,
    "allowed_ips": [],
    "enable_ip_blacklisting": false,
    "blacklisted_ips": [],
    "dont_set_quota_on_create": false,
    "expire_analytics_after": 0,
    "response_processors": [],
    "CORS": {
        "enable": false,
        "allowed_origins": [],
        "allowed_methods": [],
        "allowed_headers": [],
        "exposed_headers": [],
        "allow_credentials": false,
        "max_age": 24,
        "options_passthrough": false,
        "debug": false
    },
    "domain": "",
    "do_not_track": false,
    "tags": [],
    "enable_context_vars": false,
    "config_data": {},
    "tag_headers": [],
    "global_rate_limit": {
        "rate": 0,
        "per": 0
    },
    "strip_auth_data": false
}

preTransformContentType.js

var preTransformContentType = new TykJS.TykMiddleware.NewMiddleware({});

preTransformContentType.NewProcessRequest(function(request, session) {
  log("Running sample PRE PROCESSOR preTransformContentType middleware");

  if (request.Headers["Content-Type"] === undefined) {
    log("no content-type set");

    return preTransformContentType.ReturnData(request, {});
  }

  log("Content-Type: " + request.Headers["Content-Type"][0]);

  switch (request.Headers["Content-Type"][0]) {
    case "application/vnd.aaa.xxx+json":
      request.SetHeaders["Content-Type"] = "application/vnd.bbb.xxx+json";
      break;
    case "application/vnd.aaa.yyy+json":
      request.SetHeaders["Content-Type"] = "application/vnd.bbb.yyy+json";
      break;
    default:
      log("no need to transform headers as no match");
  }

  return preTransformContentType.ReturnData(request, {});
});

log("Sample PRE middleware initialised");

To get this working

1. ensure Tyk gateway has enable_jsvm: true in the tyk.conf
2. save contents of snippet above as new file as middleware/preTransformContentType.js
3. Restart / hot reload gateway to ensure middleware is loaded
4. Add the following object to the custom_middleware.pre array in the raw API definition via dashboard.

{
  "name": "preTransformContentType",
  "path": "middleware/preTransformContentType.js",
  "require_session": false
}

Example sending normal curl with no Content-Type header

$ curl http://gateway.ahmet:8080/httpbin/get
{
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip",
    "Connection": "close",
    "Host": "httpbin.org",
    "User-Agent": "curl/7.54.0"
  },
  "origin": "127.0.0.1, 5.70.19.189",
  "url": "http://httpbin.org/get"
}

Example sending curl with non-matching Content-Type header

$ curl -H 'Content-Type: application/json' http://gateway.ahmet:8080/httpbin/get
{
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip",
    "Connection": "close",
    "Content-Type": "application/json",
    "Host": "httpbin.org",
    "User-Agent": "curl/7.54.0"
  },
  "origin": "127.0.0.1, 5.70.19.189",
  "url": "http://httpbin.org/get"
}

Example sending curl with Content-Type: application/vnd.aaa.xxx+json

$ curl -H 'Content-Type: application/vnd.aaa.xxx+json' http://gateway.ahmet:8080/httpbin/get
{
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip",
    "Connection": "close",
    "Content-Type": "application/vnd.bbb.xxx+json",
    "Host": "httpbin.org",
    "User-Agent": "curl/7.54.0"
  },
  "origin": "127.0.0.1, 5.70.19.189",
  "url": "http://httpbin.org/get"
}

Example sending curl with Content-Type: application/vnd.aaa.yyy+json

$ curl -H 'Content-Type: application/vnd.aaa.yyy+json' http://gateway.ahmet:8080/httpbin/get
{
  "args": {},
  "headers": {
    "Accept": "*/*",
    "Accept-Encoding": "gzip",
    "Connection": "close",
    "Content-Type": "application/vnd.bbb.yyy+json",
    "Host": "httpbin.org",
    "User-Agent": "curl/7.54.0"
  },
  "origin": "127.0.0.1, 5.70.19.189",
  "url": "http://httpbin.org/get"
}

Works like a charm

To make this work with tyk gateway 4.3.3 (community edition) it is necessary to set the driver too. I.e:

...
  "custom_middleware": {
    "pre": [
      {
                "name": "preTransformContentType",
                "path": "middleware/preTransformContentType.js",
      }
    ],
    "driver": "otto"
  },
...