The decorate behavior is quite powerful, allowing nearly unlimited (synchronous) post-processing of the response. Since it relies on JavaScript injection, the --allowInjection flag must be passed in to mb on startup.

Here are a few ideas of what to do with post-processing:

• Add the current time to a response
• Add something from the request to the response
• Add a custom header to a proxied response

Add the current time to a response

Many people store static imposter files and load them via the --configfile command line switch. The decorate behavior supports an elegant way of adding dynamic data to the responses. Let's add the current timestamp to each response. We'll pass in a stringified version of the following JavaScript function:

function (request, response) {
    var pad = function (number) {
            return (number < 10) ? '0' + number : number.toString();
        },
        now = new Date(),
        time = pad(now.getHours()) + ':' + pad(now.getMinutes()) + ':' + pad(now.getSeconds());

    response.body = response.body.replace('${TIME}', time);
}

POST /imposters HTTP/1.1
Host: localhost:<%= port %>
Accept: application/json
Content-Type: application/json

{
  "port": 5545,
  "protocol": "http",
  "stubs": [
    {
      "responses": [
        {
          "is": {
            "body": "The time is ${TIME}"
          },
          "_behaviors": {
            "decorate": "function (request, response) { var pad = function (number) { return (number < 10) ? '0' + number : number.toString(); }, now = new Date(), time = pad(now.getHours()) + ':' + pad(now.getMinutes()) + ':' + pad(now.getSeconds()); response.body = response.body.replace('${TIME}', time); }"
          }
        }
      ]
    }
  ]
}

Now we can call the imposter to get the current time.

GET / HTTP/1.1
Host: localhost:5545

HTTP/1.1 200 OK
Date: Thu, 01 Jan 2015 02:30:31 GMT
Connection: close
Transfer-Encoding: chunked

The time is 16:43:02

Add something from the request to the response

It is occasionally useful to add something to the response that came from the request. Using a static response, we can add the dynamic values through decoration. In this example, we'll add a token in the static response body and replace it with the request path. Here's the decorator function, passed in as a string to the behavior below:

function (request, response) {
    response.body = response.body.replace('${PATH}', request.path);
}

POST /imposters HTTP/1.1
Host: localhost:<%= port %>
Accept: application/json
Content-Type: application/json

{
  "port": 6545,
  "protocol": "http",
  "stubs": [
    {
      "responses": [
        {
          "is": {
            "body": "The path was ${PATH}"
          },
          "_behaviors": {
            "decorate": "function (request, response) { response.body = response.body.replace('${PATH}', request.path); }"
          }
        }
      ]
    }
  ]
}

Now we can call the imposter to get the current time.

GET /test HTTP/1.1
Host: localhost:6545

HTTP/1.1 200 OK
Date: Thu, 01 Jan 2015 02:30:31 GMT
Connection: close
Transfer-Encoding: chunked

The path was /test

Add a custom header to a proxied response

Proxying provides considerable power out of the box. However, there are times where you need to augment the proxied response with something else to properly simulate your testing scenario. In this example, we'll proxy to the example above (port 6545) that adds the request path to the response body. We'll augment the proxied response with a custom header. Here's the decorator function, passed into the imposter creation as a string:

function (request, response) {
    response.headers['X-Test'] = 'True';
}

POST /imposters HTTP/1.1
Host: localhost:<%= port %>
Accept: application/json
Content-Type: application/json

{
  "port": 7545,
  "protocol": "http",
  "stubs": [
    {
      "responses": [
        {
          "proxy": {
            "to": "http://localhost:6545"
          },
          "_behaviors": {
            "decorate": "function (request, response) { response.headers['X-Test'] = 'True'; }"
          }
        }
      ]
    }
  ]
}

Now we should get the custom header back:

GET /test HTTP/1.1
Host: localhost:7545

HTTP/1.1 200 OK
Date: Thu, 01 Jan 2015 02:30:31 GMT
Connection: close
Transfer-Encoding: chunked
X-Test: True

The path was /test