Arrays and Objects In Prototypes - Not Treated as References

Question

I have a prototype object in Javascript, when I initialise a new instance of the prototype and update properties in the prototype, it updates for all elements. I understand that

Answer 1

I understand that arrays and objects are passed by reference

No, they are not. But they're referenced by object references, which is an entirely different thing,¹ and is indeed the issue you're running into.

and was wondering of a solution that would get you around this?

Do exactly what you did in the ES6 approach: Put it on the object itself, not the prototype:

let Test = function () {
    this.array = [];
};

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¹ (All that the concepts "pass by reference" and "object reference" have in common is that they both use the word "reference." In the former, it's a reference to a variable [and a concept JavaScript doesn't have]. In the latter, it's a reference to an object.)