What does the new operator do?

The new keyword is a four-step operation. Knowing the steps explains why constructors behave the way they do and why returning a value from one can be surprising.

• Create a new empty object
• Set its prototype to Constructor.prototype
• Call the constructor with this set to the new object
• Return the object, unless the constructor returns a different object

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

User.prototype.greet = function () {
  return "Hi, " + this.name;
};

const user = new User("Asha");
console.log(user.greet()); // "Hi, Asha"
console.log(user instanceof User); // true

What if the constructor returns something?

function Primitive() {
  this.value = 1;
  return 42;
}

function ObjectReturn() {
  this.value = 2;
  return { value: 99 };
}

console.log(new Primitive().value); // 1
console.log(new ObjectReturn().value); // 99

Arrow functions cannot be constructors because they do not have their own this or a prototype property. Class constructors require new; calling one without it throws.