new Keyword

Suppose you want to create an object type for cars. You want this type of object to be called Car, and you want it to have properties for make, model, and year. To do this, you would write the following function:

function Car(make, model, year) {
  this.make = make;
  this.model = model;
  this.year = year;
}

const myCar = new Car('Eagle', 'Talon TSi', 1993);

This statement creates myCar and assigns it the specified values for its properties. Then the value of myCar.make is the string "Eagle", myCar.year is the integer 1993, and so on.

You can create any number of car objects by calls to new. For example:

const kensCar = new Car('Nissan', '300ZX', 1992);

Object property that is itself another object

function Person(name, age, sex) {
  this.name = name;
  this.age = age;
  this.sex = sex;
}

And then instantiate two new Person objects as follows:

const rand = new Person('Rand McNally', 33, 'M');
const ken = new Person('Ken Jones', 39, 'M');

Then you can rewrite the definition of Car to include an owner property that takes a Person object, as follows:

function Car(make, model, year, owner) {
  this.make = make;
  this.model = model;
  this.year = year;
  this.owner = owner;
}

To instantiate the new objects, you then use the following:

const car1 = new Car('Eagle', 'Talon TSi', 1993, rand);
const car2 = new Car('Nissan', '300ZX', 1992, ken);

Instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects rand and ken as the parameters for the owners. To find out the name of the owner of car2, you can access the following property:

car2.owner.name

Using new with classes

class Person {
  constructor(name) {
    this.name = name;
  }
  greet() {

    console.log(`Hello, my name is ${this.name}`);
  }
}

const p = new Person("Caroline");
p.greet(); // Hello, my name is Caroline