installprofile.js

// Declaration globale.
window.MonHelper = {
  function1: function (context, settings) { alert("truc de base"); }
};

(function ($, Drupal) {
  "use strict";
  
  Drupal.behaviors.TrucDuCore = {
    attach: function (context, settings) {
      MonHelper.function1(context, settings);
    }
  };

})(jQuery, Drupal);

module.js

(function ($, Drupal) {

  // Surcharge de la fonction uniquement.
  MonHelper.function1 = function (context, settings) { alert("truc surchargé"); };
  
  // -OU- surcharge de behavior.
  Drupal.behaviors.TrucDuCore = {
    attach: function (context, settings) {
      autre_truc();
    }
  };
  
})(jQuery, Drupal);