snippet.js

/////////////////////////////////////
// In the promotion rule application:

rule = new Rule();

rule.callbacks.get_condition_options = function(type, property, operator, value) {
  // Make AJAX call to services:
  // GET /promos/rules/options?type=type&property=property&operator=operator&value=value
  // Parse the server response into whatever return format is expected of this type of
  // callback function (as defined by the general purpose rule system)
  return {
    types: [...]
    properties: [...]
    operators: [...]
    value_options: [...]
  }
});

/////////////////////////////////////
// In the general purpose rule system:

Rule = function() {
  this.callbacks = {};
}

Condition = function(rule) {
  this.rule = rule;
  this.to_html= function() {
    options = rule.callbacks.get_condition_options();
    // Do whatever you processing we need with those options to render html
    return html;
  }
}