Sharing JavaScript Code in a Component Bundle, https://douglascayers.com/2018/12/29/sharing-javascript-code-in-lightning-aura-components-and-freezing-javascript-prototypes/

controller.js

/* controller.js */
({
  onActionConfirmed: function(component, event, helper) {
    helper.callServer();
})

helper.js

/* helper.js */
({
  callServer: function(component) {
    var action = component.get("c.someApexMethod");
    $A.enqueueAction(action);
  }
})

renderer.js

/* renderer.js */
({
  afterRender: function(component, helper) {
    this.superAfterRender();
    helper.callServer();
  }
})