JavaScript Data Types

String, Number, Boolean, Array, Object, Null, Undefined.

JavaScript Has Dynamic Types

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

A string is a variable which stores a series of characters like "John Doe".

Strings are written with quotes. You can use single or double quotes:


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

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


var answer = "It's alright";             // Single quote inside double quotes
var answer = "He is called 'Johnny'";    // Single quotes inside double quotes
var answer = 'He is called "Johnny"';    // Double quotes inside single quotes

Try it yourself »

You will learn a lot more about strings later in this tutorial.

JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:


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

Extra large or extra small numbers can be written with scientific (exponential) notation:


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

Try it yourself »

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

JavaScript Booleans

Booleans can only have two values: true or false.

var x = true;
var y = false;

Booleans are often used in conditional testing.

You will learn a lot more about conditional testing later in this tutorial.

JavaScript Arrays

JavaScript arrays are written with square brackets.

Array items are separated by commas.

The following code declares (creates) an array called cars, containing three items (car names):


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

Try it Yourself »

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.

You will learn a lot more about arrays later in this tutorial.

JavaScript Objects

JavaScript objects are written with curly braces.

Object properties are written as name:value pairs, separated by commas.


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

Try it Yourself »

The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.

You will learn a lot more about objects later in this tutorial.

Undefined and Null

The value of a variable with no value is undefined.

Variables can be emptied by setting the value to null.


var cars;              // Value is undefined
var person = null;     // Value is null

Try it Yourself »

The typeof Operator

You can use the JavaScript typeof operator to find the type of a JavaScript variable.


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

Try it yourself »

Note  In JavaScript, an array is a special type of object. Therefore typeof [1,2,3,4] returns object. 

Do Not Declare String, Number, and Boolean as Objects!

When a JavaScript variable is declared with the keyword "new", the variable is created as an object:

var x = new String();          // Declares x as a String object
var y = new Number();          // Declares y as a Number object
var z = new Boolean();         // Declares z as a Boolean object

Note Avoid String, Number, and Boolean objects. They complicate your code and slow down execution speed.

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