"JavaScript: The Good Parts"-way to implement prototype methods?

Having read this article https://www.toptal.com/javascript/es6-class-chaos-keeps-js-developer-up and subsequently "JavaScript: The Good Parts", I shall henceforth commit to becoming a better JavaScript developer. However, one question remained for me. I usually implemented methods like this:

function MyClass(){
    this.myData = 43;
    this.getDataFromObject = function(){
        return this.myData;
    }
}

MyClass.prototype.getDataFromPrototype = function(){
    return this.myData;
}

var myObject = new MyClass();
console.log(myObject.getDataFromObject());
console.log(myObject.getDataFromPrototype());

My assumption that underlies this whole post is that getDataFromObject is faster (during call, not during object creation) because it saves an indirection to the prototype but it is also less memory-efficient because every object gets an own instance of the function object. If that is already wrong, please correct me and you can probably stop reading here.

Else: Both article and book recommend a style like this:

function secretFactory() {
    const secret = "Favor composition over inheritance [...]!"
    const spillTheBeans = () => console.log(secret)

    return {
        spillTheBeans
    }
}

const leaker = secretFactory()
leaker.spillTheBeans()

(quote from the article, the book didn't have ES6 yet but the ideas are similar)

My issue is this:

const leaker1 = secretFactory()
const leaker2 = secretFactory()

console.log(leaker1.spillTheBeans === leaker2.spillTheBeans) // false

Do I not mostly want to avoid that every object gets an own instance of every method? It might be insignificant here but if spillTheBeans is more complicated and I create a bazillion objects, each with twelvetythousand other methods?

If so, what is the "goot parts"-solution? My assumption would be:

const spillStaticBeans = () => console.log("Tabs rule!")
const spillInstanceBeans = (beans) => console.log(beans)

function secretFactory() {
    const secret = "Favor composition over inheritance [...]!"

    return{
        spillStaticBeans,
        spillInstanceBeans: () => spillInstanceBeans(secret)
    }
}

const leaker1 = secretFactory()
const leaker2 = secretFactory()

leaker1.spillStaticBeans()
leaker2.spillInstanceBeans()

console.log(leaker1.spillStaticBeans === leaker2.spillStaticBeans) // true
console.log(leaker1.spillInstanceBeans === leaker2.spillInstanceBeans) // false

The spillInstanceBeans method is still different because each instance needs its own closure but at least they just wrap a reference to the same function object which contains all the expensiveness.

But now I have to write every method name two to three times. Worse, I clutter the namespace with public spillStaticBeans and spillInstanceBeans functions. In order to mitigate the latter, I could write a meta factory module:

const secretFactory = (function(){

    const spillStaticBeans = () => console.log("Tabs rule!")
    const spillInstanceBeans = (beans) => console.log(beans)

    return function() {
        const secret = "Favor composition over inheritance [...]!"

        return{
            spillStaticBeans,
            spillInstanceBeans: () => spillInstanceBeans(secret)
        }
    }

}())

This can be used the same way as before but now the methods are hidden in a closure. However, it gets a bit confusing. Using ES6 modules, I could also leave them in module scope and just not export them. But is this the way to go?

Or am I mistaken in general and JavaScript's internal function representation takes care of all this and there is not actually a problem?