JavaScript dynamic data types are explained as follows:

var x;               // Now x is undefined
var x = 5;           // Now x is a Number
var x = "Bill";      // Now x is a String

For elaboration of javascript data types, explained as follows:

• String:  A string can be any text inside quotes. You can use single or double quotes

var str="It's alright";
var str ="He is called 'Akon'";
var str ='He is called "Akon"';

• Number:

var num=33.00;      // Written with decimals
var num=33;         // Written without decimals
var exp=333e5;      // 33300000
var exp=333e-5;     // 0.00333

• Boolean:

var x=true;
var y=false;

• Array:

//First Way of Declaration
var friendsName=new Array();
friendsName[0]="John";
friendsName[1]="Sachin";
friendsName[2]="Naren";

//Second Way of Declaration
 var friendsName =new Array("John","Sachin","Naren");
 
//Third Way of Declaration
var friendsName =["John","Sachin","Naren"];

• Object:

var friends={
firstname : "Sachin",
lastname  : "Tendulkar"
};

//Access the object value as follows: 
document.write(friends.lastname + "<br>");
document.write(friends["lastname"] + "<br>");

• Undefined and Null

Var frineds;  //undefined variable
Var frinds=null;  // null variable

When you declare a new variable, you can declare its type using the “new” keyword:

var carname=new String;
var x=      new Number;
var y=      new Boolean;
var cars=   new Array;
var person= new Object;