The JavaScript ‘new’ keyword

In JavaScript, the ‘new’ keyword is used to create an instance object of a class (mock class). When an object is instantiated, it inherits the properties and methods of the class.

Execution Process

Create a new and empty object.
Set the new object prototype to point to class constructor.
Execute the constructor, binding the ‘this’ keyword.
If the constructor does not return an object, it returns this.

Simulate the ‘new’ keyword

use Object.create to simulate ‘new’ keyword

const newNew = function (Parent, ...args) {
  let newObj = Object.create(Parent.prototype);
  let res = Parent.apply(newObj, args);
  return typeof res === "object" ? res : newObj;
};

let object1 = new Base();
let object2 = newNew(Base);

// true
console.log(object1.__proto__ === object2.__proto__);

No Object.create to simulate ‘new’ keyword

// newNew
const newNew1 = function (Parent, ...args) {
  // let newObject = {};
  let newObject = Object();
  newObject.__proto__ = Parent.prototype;
  let res = Parent.apply(newObject, args);
  return typeof res === "object" ? res : newObject;
};

let object1 = new Base();
let object2 = newNew(Base);

// true
console.log(object1.__proto__ === object2.__proto__);

‘new’ keyword and Object.create()

Their __proto__ points to different things. The ‘new’ point to the constructor’s prototype object and create point to the Function.

// Object.create & new
function Base() {
  let num = 0;
  getNum = function () {
    console.log(num);
  };
}

let newObj = new Base();
let createObj = Object.create(Base);

console.log(newObj, createObj);

Use ‘new’ to simulate Object.create()

const create1 = function (Parent) {
  let res = function () {};
  res.prototype = Parent;
  return new res();
};

let object3 = Object.create(Base);
let object4 = create1(Base);

// true
console.log(object3.__proto__ === object4.__proto__);