mountebank predicate proxy with CORS

Question

I set up a mountebank predicate to proxy a downstream server. The response from the server does not have the Access-Control-Allow-Origin set to *. I can definitely record the responses from the downstream server and then spin up a new mountebank instance with the allowCORS option that allows my browser to consume from this test double without CORS issue.

I was wondering if it is possible to directly use the proxy predicate to modify the response headers from the downstream servers to add "Access-Control-Allow-Origin": "*",. This way I can only use one instance of mountebank with the proxyOnce option and allow my browser to interact with this test double. For my use case this helps me move from a record and replay to just use proxy to downstream.

I tried to also stub the OPTIONS method but it doesn't work.

curl --location --request POST 'http://localhost:2525/imposters' \
--header 'Accept: application/json' \
--header 'Content-Type: application/json' \
--data-raw '{
    "port": 4500,
    "protocol": "http",
    "name": "Proxy Request",  
    "allowCORS": true,
    "stubs": [
        {
            "predicates": [
                {
                    "equals": {
                        "method": "OPTIONS"
                    }
                }
            ],
            "responses": [
                {
                    "is": {
                        "headers": {
                            "Access-Control-Allow-Origin": "*",
                            "Access-Control-Allow-Methods": "GET, POST, PUT, PATCH, DELETE",
                            "Access-Control-Allow-Headers": "${ALLOW-HEADERS}"
                        }
                    },
                    "_behaviors": {
                        "copy": [
                            {
                                "from": {
                                    "headers": "Access-Control-Request-Headers"
                                },
                                "into": "${ALLOW-HEADERS}",
                                "using": {
                                    "method": "regex",
                                    "selector": ".+"
                                }
                            }
                        ]
                    }
                }
            ]
        },        
        {
            "responses": [
                {
                    "proxy": {
                        "to": "https://downstream.api.com",
                        "mode": "proxyOnce",
                        "predicateGenerators": [
                            {
                                "matches": {
                                    "method": true,
                                    "path": true,
                                    "query": true
                                }
                            }
                        ]
                    }
                }
            ]
        }
    ]
}'

Any suggestions?

Answer 1

For anyone looking into this for reference, I was able to achieve this by using the decorate behaviors.

My stub was setup as:

{
  "responses": [
    {
      "proxy": {
        "to": "https://downstream.api.com",
        "mode": "proxyOnce",
        "predicateGenerators": [
          {
            "matches": {
              "method": true,
              "path": true,
              "query": true
            }
          }
        ]
      },
      "behaviors": [
        {
          "decorate": "<%- stringify('../templates/add_access_control_allow_origin_header.ejs') %>"
        }
      ]
    }
  ]
}

And the decorate behaviour was as simple as this:

config => {
  config.response.headers['Access-Control-Allow-Origin'] = '*';
}