How can an object have a `.constructor` property if I reset its prototype to an empty object (`{}`)?

Question

In this case prototype object doesn't have constructor property anymore. Here is just an empty object. But, despite this, somehow new obj.constructor still creates an empty object. How?

function Constructor() { 
    this.name = 'Kira';
    this.age = 35;
    this.city = 'New-York';
}; 
Constructor.prototype = {};

let obj = new Constructor;
let obj2 = new obj.constructor;
console.log(obj2); //{}

And here object is also created. In the console of Chrome browser, it's displayed as a String.

function User(name) {
  this.name = name;
}
User.prototype = {};

let user = new User('John');
let user2 = new user.constructor('Pete');

console.log(user2);

How can it be? How are objects created here?

Answer 1

When you set the prototype to {} that does not mean there are no properties. It means that the only properties are the ones you would find on a new object. In particular, the constructor property is coming from the prototype and points to Object

const o = {};

console.log("has constructor property:",
  "constructor" in o);
console.log("constructor property is own:", 
  o.hasOwnProperty("constructor"));
console.log("constructor property comes from prototype:",
  Object.getPrototypeOf(o).hasOwnProperty("constructor"));
console.log("constructor property is Object():",
  o.constructor === Object);

So, when you call new obj.constructor that is equivalent to new Object.

function Foo() {}; 
Foo.prototype = {};

let obj = new Foo;
console.log(obj.constructor === Object);
console.log(obj.constructor === ({}).constructor);
let obj2 = new obj.constructor;
console.log(obj2); //{}

When calling new user.constructor('Pete'); that is again just calling the global Object, so it's identical to new Object('Pete'). And if you pass a value that has an object wrapper, you get that wrapper from the constructor:

const pete = new Object('Pete');
console.log(typeof pete, pete instanceof String);

const answer = new Object(42);
console.log(typeof answer, answer instanceof Number);