JavaScript - Adding / Deleting Properties

Adding properties to an object
You can add new properties to an existing object by simply giving it a value. Assume that the person object already exists - you can then give it new properties:

function person(first, last, age, eye)
{
    this.firstName = first;
    this.lastName = last;
}

let father = new person("John", "Doe");
father.Age = 50;

let msg = "Father name is " + father.firstName + " " + father.lastName + ". Age is " + father.Age + "
";
document.getElementById("demo1").innerHTML = msg;;

Deleting properties to an object
The delete keyword deletes a property from an object.

let mother = new person("Jane", "Doe");
mother.Age = 50;

delete mother.Age; // or delete person["age"];

let msg2 = "Mother name is " + mother.firstName + " " + mother.lastName + ". Age is " + mother.Age + "<br />";
document.getElementById("demo2").innerHTML = msg2;

Adding a Method to an object
Adding a new method to an existing object is also easy.

let brother = new person("Jack", "Doe");
brother.FullName = function () { return this.FirstName + " " + this.LastName; };

let msg3 = "Brother name is " + brother.firstName + " " + brother.lastName + ". Full Name is " + brother.FullName() + "<br />";
document.getElementById("demo3").innerHTML = msg3;