All JavaScript objects inherit properties and methods from a prototype.

• Date objects inherit from Date.prototype
• Array objects inherit from Array.prototype
• Person objects inherit from Person.prototype

The Object.prototype is on the top of the prototype inheritance chain:

Date objects, Array objects, and Person objects inherit from Object.prototype

Example

function Person(first, last, age, eyecolor) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eyecolor;
}

Person.prototype.nationality = "English";

Person.prototype.name = function() {
  return this.firstName + " " + this.lastName;
};

var myFather = new Person("John", "Doe", 50, "blue");
var myMother = new Person("Sally", "Rally", 48, "green");

❗Please do not add a new property to an existing object constructor as below. That is a wrong way.

Person.nationality = "English";