Every javascript object has a prototype by default. And the prototypes is also an objects.
All objects in JavaScript inherit methods and properties from  their prototype  and we can override it, but we can not override an Object with a null prototype, i.e. Object.create(null)).

Lets create a prototype:

function Employee(first, last, age, dept) {
    this.firstName = first;
    this.lastName = last;
    this.age = age;
    this.dept_name = dept;
}

New keyword is used to create an object using prototype:

var emp1 = new Employee("john", "doe", 35, "Accounts")
var emp2 = new Employee("tom", "paul", 40, "Sales")

Now add a new property to this object

emp1.salary = 50000

This property(salary) is added to the object emp1 only not to the emp2 or any other object. But you can not add a property like this to a prototype

Employee.salary = 500000 // error

Add method to a prototype

function Employee(first, last, age, dept) {
    this.firstName = first;
    this.lastName = last;
    this.age = age;
    this.dept_name = dept;
    this.fullname= function (){return this.firstName + "" + "lastName";};
}