Javascript data type can hold may data type like number string arrays object and other.
To declare a variable we use var keyword
example

var length = 16;                               // Number
var lastName = "Johnson";                      // String
var cars = ["Saab", "Volvo", "BMW"];           // Array
var x = {firstName:"John", lastName:"Doe"};    // Object

The Concept of Data Types

var x = 16 + "Volvo";

It will consider both as String as the second operand is String.

var x = 16 + 4 + "Volvo";

It will consider first two operand as integer and third as a String.
and will give result 20Volvo

var x = "Volvo" + 16 + 4;

It will consider second and third operand as a String now.
\ and the result will be Volvo164.
JavaScript Has Dynamic Types

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

JavaScript Strings

 var carName = "Volvo XC60";   // Using double quotes
var carName = 'Volvo XC60';   // Using single quotes

JavaScript Numbers

var x1 = 34.00;     // Written with decimals
var x2 = 34;        // Written without decimals

JavaScript Booleans

var x = true;
var y = false;

JavaScript Arrays
var cars = ["Saab", "Volvo", "BMW"];
JavaScript Objects

print("code sample");

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};