Technical background

We use our callbacks syntax to put it into the global namespace because ActionTag loads asynchronously. Also, we wish to support multiple callback sources, hence the instructions to not redeclare it over and over again.

A warning on the power of these abilities

Since our callbacks allow you to alter internals of our system, it is possible to make quite a mess of things, sometimes even in potentially dangerous/breaking ways. With great power comes great responsibility. For example, using the alterFormDefinition() callback:

var alterFormDefinition = function(args) {
  // This will blank out the form_elements array thereby effectively
  // breaking this form definition!
  args.form_definition.form_elements = [];
  return args;

So always test your callbacks carefully on a page before putting them live.

A note on the examples below

For the examples given below with each callback we are omitting the following bit:

var nvtag_callbacks = nvtag_callbacks || {};
nvtag_callbacks.postRender = nvtag_callbacks.postRender || [];

Note that this must happen for each callback in order for them to be registered and called by ActionTag.