[SalesForce] Sharing JavaScript between multiple Lightning Components

In addition to what everyone else has said...

The helper is designed to have shared code in it. So, you could have code shared by your renderer and your controller or by multiple controller functions. That should go in the helper JS file.

I think that a good practice is to let the controller respond to events, possibly perform some amount of control flow, and delegate business logic to the helper as a separation of concerns. It also sets you up to be able to reuse your business logic in other parts of the component in the future.

Helper code can be shared among components when they are related through inheritance. If one component extends another it inherits its super component's helper and can override it.

Sub components can call super component controller methods, but that may be deprecated in the future according to the documentation in the Lightning Components Developer's Guide.

Components can communicate with each other via events to "share" code. For example, if you have a component that handles writing business logic errors in a uniform way in your app, and handles some sort of error related event, you could fire an event to trigger it to handle an error.

you could attach your utility methods to the global window object(SecureWindow) and use it in your controller.

Static Resource: TestScript

(function(w){
    "use strict"; //Optional because LC on LockerService active runs in strict mode

    var utilMethods = {
        "method1":method1,
        "method2":method2
    };

    function method1(){
        console.log("method1 called");
    } 

    function method2(){
        console.log("method2 called");
    }

    w.myUtil = utilMethods;

})(window);

Component:

<ltng:require scripts="{!$Resource.TestScript}" afterScriptsLoaded="{!c.afterScriptsLoaded}" />
<ui:button label="Do job" press="{!c.doJob}"/>

Controller.js:

doJob : function(cmp){
     myUtil.method1(); // Print method1 called
     myUtil.method2(); // Print method2 called
}