JavaScript Data Types

Primitiv data types

In JavaScript we find a number of primitiv data types:

  • Boolean
  • Null
  • Undefined
  • Number
  • String
  • Symbol (this is new in ECMAScript 6)

In addition we have:

  • Object

Learn more about JavaScript Booleans Learn more about JavaScript Strings Learn more about JavaScript Numbers Learn more about JavaScript Dates

Composite Data Types

The existing composite (reference) data types in JavaScript are:

  • Object
  • Array

Special Data Types

The existing special data types in JavaScript are:

  • Null
  • Undefined

String Data Type

The existing special data types in JavaScript are:

  • Null
  • Undefined

A string is a combination of Unicode characters (letters, digits, and punctuation marks). A string usually represent a text in JavaScript and it's single or double quotation marks. Double quotation marks can be used in strings surrounded by single quotation marks. Single quotation marks can be used in strings surrounded by double quotation marks:

"Hallo there; today is friday!"
'"This is a nice car!" said the old man.'

Learn more about JavaScript Objects and Arrays

JavaScript does not have a type for just one single character. This is solved by using a string of only one character.

Number Data Type

JavaScript number can be either an integer or a floating-point values. In JavaScript, there is no distinction between them.

Integer Values

Integer values can be represented as positive whole numbers, negative whole numbers, in addition to 0. They can be in base 10 (decimal), base 16 (hexadecimal), and base 8 (octal). Usually the numbers in JavaScript are written in decimal.

Floating-point Values

Floating-point values can represented as whole numbers with decimals. They can also be expressed in scientific notation and that means as an uppercase or lowercase "e" is used as "ten to the power of".

Number Description Decimal Equivalent
.0001, 0.0001, 1e-4, 1.0e-4 Four equivalent floating-point numbers. 0.0001
3.45e2 A floating-point number. 345
45 An integer. 45
0378 An integer. Although this looks like an octal number (it begins with a zero), 8 is not a valid octal digit, so the number is treated as a decimal. 378
0377 An octal integer. Notice that although it only appears to be one less than the number above, its actual value is quite different. 255
0.0001 A floating point number. Even though this begins with a zero, it is not an octal number because it has a decimal point. 0.0001
00.0001 This is an error. The two leading zeros mark the number as an octal, but octals are not allowed a decimal component. N/A (compiler error)
0Xff A hexadecimal integer. 255
0x37CF A hexadecimal integer. 14287
0x3e7 A hexadecimal integer. Notice that the 'e' is not treated as exponentiation. 999
0x3.45e2 This is an error. Hexadecimal numbers cannot have decimal parts. N/A (compiler error)

JavaScript also have numbers with special values. These are:

  • NaN (not a number). 
  • Positive Infinity. 
  • Negative Infinity. 
  • Positive and Negative 0. 

Learn more about JavaScript Numbers

Boolean Data Type

The string and number data types can have an unlimited number of different values. The Boolean data type can only have two different values and they are the literals true and false. A Boolean value will specify if a condition is true or false.

Comparisons you make in your scripts always have a Boolean outcome. Consider the following line of JavaScript code.

//How to use boolean values in a control structure.
if (x == 100) {
    z = z + 1;
else {
    x = x + 1;

Learn more about JavaScript Booleans

The null Data Type

In JavaScript the data type null has only one value and that is: null. 

A variable that contains null has no valid Number, String, Boolean, Array, or Object. It is possible to erase the content of a variable by assigning it to null without erasing the variable.

As in C and C++ null is the same as 0.

Undefined data type

Example of the undefined data type in JavaScript
JS JavaScript - Undefined data type

var x;
// This method works.
if (x == undefined) {
    document.write ( "comparing x to undefined <br/>");
// This method doesn't work - you must check for the string "undefined".
if ( typeof (x) == undefined) {
    document.write ( "comparing the type of x to undefined <br/>" );
// This method does work.
if ( typeof (x) == "undefined") {
    document.write ( "comparing the type of x to the string 'undefined'");
// Output:  
// comparing x to undefined
// comparing the type of x to the string 'undefined' 


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JavaScript is one of the 3 core language technologies of the World Wide Web that all web developers must learn:

1.      HTML to define the content of web pages.

2.      CSS to specify the layout of web pages.

3.      JavaScript to program the behavior of web pages.

JavaScript is a scripting or programming language that allows you to implement complex things on web pages. Every time a web page does more than just sit there and display static information for you to look at, displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. − you can bet that JavaScript is probably involved.

The benefits of using JavaScript are:

1.      You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

2.      Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.

3.      Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

4.      Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.


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