JavaScript Variable Types

JavaScript supports five different types of variable. These variable types are outlined in the following table together with examples and a brief description of each type:

Type	Example	Description
boolean	true, false	The boolean variable is used to record a value of either true or false. Internally this is essentially stored as 1 for true and 0 for false.
number	1, -31, 0.023	The number variable holds any type of number, either an integer or a real number.
string	"Hello World!"	The string variable type stores a string of characters, typically making up either a word or sentence. string variables can also be assigned empty strings.
function	any built-in or user defined function	A function is one of the basic building blocks of most programming languages including JavaScript. Functions provide a way to organize functionality into clean, reusable modules that can be accessed from any other JavaScript to perform specific tasks. In addition to user created functions, JavaScript contains a number of built-in functions that provide pre-defined functionality. Functions are covered in a later chapter.
object	document, window	JavaScript provides a set of predefined objects to which the JavaScript programmer has access. For example the document object refers to the current web page and can be accessed to make changes to the page content.

JavaScript sets the variable type based on the value assignment. For example whne JavaScript encounters the following code it knows that myVariable should be of type number:

var myVariable = 10;

and that the following variable type is string.

var myVariable = "Hello World!";

JavaScript is also mcuh more flexible than many other programming languages. With languages such as Java a variable must be declared to be a particular type when it is created and once created, the type cannot be changed. JavaScript, on the other hand, allows the type of a variable to changed at any time simply by assigning a value of a different type. The following example is perfectly valid use of a variable in javaScript. At creation time, the variable is clearly of type number. A later assigment of a string to this variable changes the type from number to string.

var myVariable = 10;
myVariable = "This is now a string type variable";