The burpCallbacks object is automatically injected into the DOM every time a document.onload event is triggered. For the most part, the burpCallbacks object adheres to the IBurpExtenderCallbacks API detailed in BurpSuite's documentation. In short, the following differences can be observed:

1. All constants (i.e. PARAM_XML, CONTENT_TYPE_UNKNOWN, INS_HEADER, etc.) can be found as read-only properties within the burpCallbacks object (i.e. burpCallbacks.PARAM_XML).
2. GUI-based BurpSuite extensions that operate in the SWING thread have slightly modified APIs to avoid deadlocks caused by event loop inter-weaving.
3. Additional helper functions have been provided to ease and augment the interaction between JavaScript and BurpSuite.
4. Methods with the same name but multiple signatures (i.e. foo(String), foo(String, int), etc.) are numbered due to limitations within JavaScript (i.e. foo(String), foo2(String, int), etc.).

The following example demonstrates how one could easily create a simple proxy listener using JavaScript with the BurpKit extensions:

// inject an HTTP helper library for message parsing
burpKit.requireLib('httplib');

// Get our burp helpers
var helpers = burpCallbacks.getHelpers();

// Register our proxy listener
burpCallbacks.registerProxyListener(function(isRequest, message) {
  var messageInfo = message.getMessageInfo();
  if (isRequest) {
    var request = httplib.parseRequest(helpers.bytesToString(messageInfo.getRequest()));
    alert(">> " + request.method + " " + messageInfo.getUrl().toString());
  } else {            
    var response = httplib.parseResponse(helpers.bytesToString(messageInfo.getResponse()));
    alert("<< " + response.statusCode + " " + response.statusMessage);
  }
});

This is just a simple example of how powerful BurpKit is. Many more examples can be found in the bundled examples directory that comes with BurpKit. The following subsections detail the methods available within the burpCallbacks object.