JavaScript Data Types

String, Number, Boolean, Array, Object.

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JavaScript Data Types

JavaScript variables can hold many data types: numbers, strings, arrays, objects and more:

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

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The Concept of Data Types

In programming, data types is an important concept.
To be able to operate on variables, it is important to know something about the type.
Without data types, a computer cannot safely solve this:

var x = 16 + "Volvo";

Does it make any sense to add "Volvo" to sixteen? Will produce a result? Will it produce an error?
JavaScript will treat the example above as:

var x = "16" + "Volvo";

	If the second operand is a string, JavaScript will also treat the first operand as a string.

Example:

var x = 16 + "Volvo";

 

JavaScript evaluates expressions from left to right. Different sequences can produce different results:

JavaScript:

var x = 16 + 4 + "Volvo";

Result:

20Volvo

  

JavaScript:

var x = "Volvo" + 16 + 4;

Result:

Volvo164 

 

In the first example, JavaScript treats 16 and 4 as numbers, until it reaches "Volvo".
In the second example, since the first operand is a string, all operands are treated as strings.

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

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JavaScript Strings

A string (or a text string) is 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 

 

You will learn 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

 

You will learn more about numbers later in this tutorial.

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JavaScript Booleans

Booleans can only have two values: true or false.

Example

var x = true;
var y = false;

Booleans are often used in conditional testing.
You will learn 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"];

  

Array indexes are zero-based, which means the first item is [0], second is [1], and so on.
You will learn 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"};

 

 

The object (person) in the example above has 4 properties: firstName, lastName, age, and eyeColor.
You will learn more about objects later in this tutorial.

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The typeof Operator

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

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 

 

Undefined

In JavaScript, a variable without a value, has the value undefined. The typeof is also undefined.

Example

var person;                  // Value is undefined, type is undefined

  

Any variable can be emptied, by setting the value to undefined. The type will also be undefined.

Example

person = undefined;          // Value is undefined, type is undefined

 

Empty Values

An empty value has nothing to do with undefined.
An empty string variable has both a value and a type.

Example

var car = "";                // The value is "", the typeof is string

 

Null

In JavaScript null is "nothing". It is supposed to be something that doesn't exist.
Unfortunately, in JavaScript, the data type of null is an object.

	You can consider it a bug in JavaScript that typeof null is an object. It should be null.

You can empty an object by setting it to null:

Example

var person = null;           // Value is null, but type is still an object

 

You can also empty an object by setting it to undefined:

Example

var person = undefined;     // Value is undefined, type is undefined

 

Difference Between Undefined and Null

typeof undefined             // undefinedtypeof null                  // objectnull === undefined           // falsenull == undefined            // true