有没有一种方法可以将JS属性的设置器/获取器抽象到对象的原型？

Question

I am trying to create a very simple class in Node. The class is called Student and it contains a few variables which I wish to keep private. Thus, I have not labeled them as

this.field = "";

but rather

let field = "";

I read that properties of an object could be created inside the constructor like so:

exports.student = function Student(firstName, lastName, ID, courses){

if(courses.length < 2) throw "Not enough courses supplied!";

let _firstName= firstName || "";
let _lastName = lastName || "";
let _ID = ID || 0;
let _courses = courses || [];

Object.defineProperties(this, {
  firstName: {
      get: () => {return _firstName;},
      set: (name) => {_firstName = name; return this}
  },
  lastName: {
      get: () => {return _lastName},
      set: (name) => {_lastName = name; return this}
  },
  ID: {
      get: () => {return _ID},
      set: (name) => {_ID = name; return this}
  },
  courses: {
      get: () => {return _courses}
  }
});
};

Which works fine on a basic level. However, it came to my attention that every instance of Student will generate entirely new setters and getters in memory, something highly inefficient when many instances are created.

I have also read functions attributed to the Prototype Object are created once only; however, whenever I attempt to set these to the prototype like so (by replacing this):

function Student(){...
    Object.defineProperties(Student.prototype, {...});
}

It fails when 2 or more instances are created, citing "TypeError: Cannot redefine property". What is going wrong, and what can I do to make this code as efficient as possible? It would be preferable to have all the code inside the constructor function.

Edit: The simple tests that are causing it to fail.

let a = new Student("A", "S", 1, [1, 2]);
let b = new Student("G", "X", 0, [3, 2]);
console.log(a.firstName);

Answer 1

Local variables created in the constructor for purposes of privacy cannot be accessed by shared methods on the prototype. Those variables are not in the scope of methods added to the prototype.

And, as you have figure out, adding things to the prototype in the constructor is not going to work either. The best that would do (if it doesn't fail with an error) is create a single method that all object use that has access to only the last object created (which is really bad and misleading).

Private properties in the constructor can only be accessed by methods added individually to the object within the scope of the constructor (this is just normal Javascript scoping rules for declared variables) which means making duplicate copies of the methods.

So, you have to pick what you want to optimize for. Either you have private variables declared in the constructor or you use methods on the prototype that only access regular properties of the object, but you can't have methods on the prototype that can access those private variables in the constructor directly.