Prototype discussion

main.js

const log = console.log;

function List(_values) {
  //create a List object
  if (!Array.isArray(_values)) return;
  //exit the function if not given an array
  this.items = [..._values]; //copy the array values into a property called items
  this.size = this.items.length;
  this.first = this.items.length > 0 ? this.items[0] : undefined;
  this.last = this.items.length > 1 ? this.items[this.items.length - 1] : undefined;
  Object.defineProperty(this, 'mid', {
    get: function () {
      return this.getMiddle();
      //we could put the functionality from getMiddle here instead
    },
  });
}
Object.setPrototypeOf(List.prototype, Array.prototype); //extend the prototype chain
// List.prototype.__proto__ = Array.prototype; //same as the above line

List.prototype.getMiddle = function () {
  let middleIndex = Math.floor(this.size / 2);
  return this.items[middleIndex];
};
List.prototype.map = function () {
  //intercept map() before the map inside Array
  return {
    map: 'mappity, mappity mappity',
  };
};
List.prototype.blah = function () {
  return this.items.map((item) => 'blah');
};

let arr = new Array(1, 2, 3, 4, 5);
let myList = new List(arr);
log(myList.size, myList.first, myList.last); //access the props from this List instance
log(myList.getMiddle(), myList.mid); //same value twice
log(myList); //calls the toString() method in Object.prototype
log(myList instanceof List); //true
log(typeof myList); //object
log(myList.constructor.name); // List
log(myList.blah()); //calls the blah method in List.prototype
log(myList.map()); //calls the map method in List.prototype
log(Object.hasOwn(myList.__proto__, 'blah')); //true
log(Object.hasOwn(List.prototype, 'blah')); //true
log(Object.hasOwn(Array.prototype, 'blah')); //false
log(Object.getOwnPropertyNames(myList)); //first, last, mid, items, size
log(Object.getOwnPropertyNames(List)); // length, name, prototype
log(Object.getOwnPropertyNames(List.prototype)); // 'constructor', 'getMiddle', 'map', 'blah'

log(Object.getPrototypeOf(myList)); //  'getMiddle', 'map', 'blah'
log(List.prototype); //  'getMiddle', 'map', 'blah'
log(myList.__proto__); //  'getMiddle', 'map', 'blah'
log(Array.prototype);

List.steve = function () {
  log('Hey Steve!');
};

List.steve(); //works
// myList.__proto__.steve(); //ERROR
// myList.prototype.steve(); //ERROR
myList.constructor.steve(); // works

log(Object.getOwnPropertyDescriptor(myList, 'first')); //data property
log(Object.getOwnPropertyDescriptor(myList, 'mid')); //accessor property