JavaScript Data Type ES5

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

var length = 16;                               // Number
var lastName = "Johnson";                      // String
var x = {firstName:"John", lastName:"Doe"};    // Object


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 it produce an error or will it produce a result?

JavaScript will treat the example above as:

var x = "16" + "Volvo";

JavaScript Types are Dynamic

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


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


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:


var carName1 = "Volvo XC60";   // Using double quotes
var carName2 = 'Volvo XC60';   // Using single quotes


var answer1 = "It's alright";             // Single quote inside double quotes
var answer2 = "He is called 'Johnny'";    // Single quotes inside double quotes
var answer3 = 'He is called "Johnny"';    // Double quotes inside single quotes


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


JavaScript Booleans

Booleans can only have two values: true or false.


var x = 5;
var y = 5;
var z = 6;
(x == y)       // Returns true
(x == z)       // Returns false


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"];


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"};


The typeof Operator

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

The typeof operator returns the type of a variable or an expression:


typeof ""             // Returns "string"
typeof "John"         // Returns "string"
typeof "John Doe"     // Returns "string"



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


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


Empty Values

An empty value has nothing to do with undefined.

An empty string has both a legal value and a type.


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



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:


var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};
person = null;    // Now value is null, but type is still an object


Difference Between Undefined and Null

undefined and null are equal in value but different in type:

typeof undefined           // undefined
typeof null                // object

null === undefined         // false
null == undefined          // true


Primitive Data

A primitive data value is a single simple data value with no additional properties and methods.

The typeof operator can return one of these primitive types:

  • string
  • number
  • boolean
  • undefined


typeof "John"              // Returns "string" 
typeof 3.14                // Returns "number"
typeof true                // Returns "boolean"
typeof false               // Returns "boolean"
typeof x                   // Returns "undefined" (if x has no value)


Complex Data

The typeof operator can return one of two complex types:

  • function
  • object

The typeof operator returns an object for both objects, arrays, and null.

The typeof operator does not return object for functions.


typeof {name:'John', age:34} // Returns "object"
typeof [1,2,3,4]             // Returns "object" (not "array", see note below)
typeof null                  // Returns "object"
typeof function myFunc(){}   // Returns "function"