JavaScript – Data Types and Variables

JavaScript Data Types

JavaScript provides set of data types to hold data such as String values, Decimal values and Boolean values. The data types depend on the values which are hold by the variable. The JavaScript data types acts as dynamically means same variable can be used as different types.

JavaScript contain following three primitive data types :


We can write numbers in two types i.e. integer and float. Numbers can be written with or without decimals. They represent numeric values. The simplest type of number is an integer with no fractional component. Another type is floating point number which is assumed to have decimal point.


<script langauge="javascript">
    var a=2;
    var b=3;
    var c=a+b;
    document.write ("Result :");


For string type, a value must be assigned in within single or double quotes. It is sequence of characters within given character set. Normally it is used to represent text. If we create a string with no value in it, then it is called as empty string.


<script langauge="javascript">
   var a="hello world";


It can have two boolean values: true and false. It’s evaluates whether condition meets or doesn’t meet specified criteria. In JavaScript, if need to use boolean values then it will convert automatically true to 1 and false to 0.While comparing the result, JavaScript will treat non zero value as true and zero value as false.

<script langauge="javascript">
   var a=true;
      document.write("result is true");

Composite Data Types

A composite data type for the purposes of JavaScript. It can contain multiple values grouped together in some way. It is same as object in the class. We can define attributes and methods to composite types. JavaScript contain following composite data types :

  • Object: An object is collection of values, which is referred to as properties of objects and functions associated with the object are referred to as methods of that object. The properties and methods are denoted with dot notation which starts with object name and ends with property name. For instance: emp.emp_name.
  • Array: It is ordered collection of data values which is used to store multiple data values into a single variable. In JavaScript, array is an object which contains index to refer to its contents. The array index is included in square brackets after array name and array index starts with zero. For instance: stud [0], stud [1] and so on.

Special Data Types

JavaScript contain following special data types :


It is empty value. The null keyword cannot be used as name of the variable. In JavaScript null is not same as 0. It is absence of any value. If we try to reference a value for variable which is not defined and has no value then it returns a null value.
For instance:

    var myTest=null;
    alert(myTest);//shows null value


A variable declared without a value can be called as undefined. The undefined value is assigned to a variable to which value has never assigned to it.


if (a==undefined)
     document.write ("comparing a to undefined ");

JavaScript Variables

JavaScript variables are containers which hold some values or information. JavaScript variables are case-sensitive. We use variables to store, retrieve and manipulating values that appear in the code. We can place the data into container and refer to the data by naming container.

Variables are declared by using “var” keyword.


var a=5;		 --> Integer type variable
var str="javabeat";     --> String type variable
var b=true;		 -->Boolean type variable

JavaScript Variable Scope

JavaScript variable has two scopes:

  • Global variables: A variable can be declared outside a function is called global variable.
  • Local variables: A variable which is defined within function definition is called local variable. It is created and destroyed every time when function is executed and it cannot be accessed by outside any of the function.

For Example:

var myTest; //global declaration
function result(){
     var myTest1="hello world!!" ; //local declaration

JavaScript Variable Names

  • JavaScript variable names should begin with a letter.
  • JavaScript variable names begin with $ or _ character. E.g. $123 or _123java
  • JavaScript variable names are case sensitive. E.g. Java and java are different variables.
  • We should not use reserved keyword as variable name.

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