JavaScript Data Types

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

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JavaScript Has Dynamic Types

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

Example

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

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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:

Example

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:

Example

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.

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JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:

Example

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:

Example

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.

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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.

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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):

Example

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.

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JavaScript Objects

JavaScript objects are written with curly braces.

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

Example

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.

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Undefined and Null

Undefined is the value of a variable with no value.

Variables can be emptied by setting the value to null;

Example

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

Try it Yourself »

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The typeof() Function

You can use the global JavaScript function typeof() to find the type of a variable.

Example

typeof("john");                 // Returns "string"
typeof(3.14);                   // Returns "number"
typeof(false);                  // Returns "boolean"
typeof({name:'john', age:34});  // Returns "object"

Try it yourself »

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Declaring Variables as Objects

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

var name = new String;
var x =    new Number;
var y =    new Boolean;

	With JavaScript, all variables can be created as objects, but you should avoid it. Unnecessary objects complicate the code, and slow down the execution speed.