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

SharedLibrary.js

/* SharedLibrary.js, upload as a static resource */
window.sum = function(a, b) {
  return ( a + b );
}

YourComponent.html

<!-- YourComponent.cmp -->
<aura:component>

  <ltng:require scripts="{!$Resource.SharedLibrary}"
                afterScriptsLoaded="{!c.getSum}"/>

</aura:component>

YourComponentController.js

/* YourComponentController.js */
({
  getSum: function(component, event, helper) {
    let sum = window.sum(2, 3); // use shared function on `window` object
    console.log(sum); // 5
  }
})