Despite being javascript an object oriented language, it doesn't use classes. Infact JavaScript is prototype based, not class based.
With javascript you can define and create your custom objects in many ways.

Direct Instance:

var o = new Object();
o.fName = "John";
o.lName = "Smith";
o.getFullName = function(){ return this.fName + " " + this.lName; };

The example above creates a new object's instance and adds two properties and one method.

Constructor Function:

function Person(fName, lName)
{
    this.fName = fName;
    this.lName = lName;
    this.getFullName = function(){ return this.fName + " " + this.lName; };
}

var o = new Person("John", "Smith");
var s = o.getFullName();

In this example the "class" is defined by the constructor function "Person". The object's instance is created by the clause "new".

Prototyping:

In javascript an object inherits properties and methods from its prototype object.
We can add properties and methods to the prototype object to define our custom class.

function Person(fName, lName)
{
    this.fName = fName;
    this.lName = lName;
}

Person.prototype = {
    age: 0,
    getFullName: function(){ return this.fName + " " + this.lName; },
    constructor: Person
}

var o = new Person("John", "Smith");
o.age = 23;
var s = o.getFullName();

Subclasses:

As we have seen in the previous example, a javascript object inherits properties and methods of its prototype object. We can use the Object.Create() function to create a subclass of a specific super-class.
In the example below we create the Customer class that inherit from the Person class  of the previous example:

// default constructor
function Customer(fName, lName) {
    Person.call(this, fName, lName);
};

Customer.constructor = Customer;
Customer.prototype = Object.create(Person.prototype);
Customer.prototype.getFullName = function () { return this.fName + " " + this.lName + ": " + this.id; }; // ovverride the getFullName of the base class
Customer.prototype.id = 0; // add a new porperty

var c = new Customer("John", "Smith");
c.id = "XYZ0001";
var cname = c.getFullName();