JavaScript Data Types

Value types (basic types): String, Number, Boolean, Null, Undefined, Symbol.

Reference data types (object types): Object, Array, Function, and two special objects: RegExp and Date.

Note:Symbol is a new primitive data type introduced by ES6, representing a unique value.

JavaScript has dynamic types

JavaScript has dynamic types. This means the same variable can be used as different types:

Example

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

The data type of a variable can be checked using thetypeofoperator:

Example

typeof "John"                // returns string
typeof 3.14                  // returns number
typeof false                 // returns boolean
typeof [1,2,3,4]             // returns object
typeof {name:'John', age:34} // returns object

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typeof [1,2,3,4] returns "object", which is a "flaw" in JavaScript's early design; arrays are essentially a special type of object.

The correct way to detect an array:

Array.isArray([1,2,3]); // true
[1,2,3] instanceof Array; // true

JavaScript Strings

Strings are variables that store characters (such as "Bill Gates").

Strings can be any text inside quotes. You can use single or double quotes:

Example

var carname="Volvo XC60";
var carname='Volvo XC60';

You can use quotes inside a string, as long as they don't match the quotes surrounding the string:

Example

var answer="It's alright";
var answer="He is called 'Johnny'";
var answer='He is called "Johnny"';

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You will learn more about strings in the advanced section of this tutorial.

JavaScript Numbers

JavaScript has only one type of number. Numbers can be written with or without decimal points:

Example

var x1=34.00; // written with a decimal point
var x2=34; // written without a decimal point

Very large or very small numbers can be written using scientific (exponential) notation:

Example

var y=123e5;      // 12300000
var z=123e-5;     // 0.00123

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You will learn more about numbers in the advanced section of this tutorial.

JavaScript Booleans

Booleans (logical) can only have two values: true or false.

var x=true;
var y=false;

Booleans are often used in conditional tests. You will learn more about conditional tests in a later chapter of this tutorial.

JavaScript Arrays

The following code creates an array named cars:

var cars=new Array();
cars[0]="Saab";
cars[1]="Volvo";
cars[2]="BMW";

Or (condensed array):

var cars=new Array("Saab","Volvo","BMW");

Or (literal array):

Example

var cars=["Saab","Volvo","BMW"];

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Array indexes are zero-based, so the first item is

You will learn more about arrays later in this tutorial.

JavaScript Objects

Objects are separated by curly braces. Inside the braces, object properties are defined in the form of name and value pairs (name : value). Properties are separated by commas:

var person={firstname:"John", lastname:"Doe", id:5566};

The object (person) in the example above has three properties: firstname, lastname, and id.

Spaces and line breaks are not important. A declaration can span multiple lines:

var person={
firstname : "John",
lastname  : "Doe",
id        :  5566
};

There are two ways to address object properties:

Example

name=person.lastname;
name=person["lastname"];

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You will learn more about objects later in this tutorial.

Undefined and Null

The value Undefined indicates that a variable does not contain a value.

You can clear a variable by setting its value to null.

Example

cars=null;
person=null;

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Declare Variable Types

When you declare a new variable, you can use the keyword "new" to declare its type:

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

lamp JavaScript variables are all objects. When you declare a variable, you create a new object.
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