JavaScript Data Types

String, Number, Boolean, Array, Object.

JavaScript Data Types

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

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:

Does it make any sense to add "Volvo" to sixteen? Will it produce a result? Will it produce an error?

JavaScript will treat the example above as:

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

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

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.

JavaScript Has Dynamic Types

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

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:

You can use quotes inside a string, as long as they don't match the quotes surrounding the string:

You will learn more about strings later in this tutorial.

JavaScript Numbers

JavaScript has only one type of numbers.

Numbers can be written with, or without decimals:

Extra large or extra small numbers can be written with scientific (exponential) notation:

You will learn more about numbers later in this tutorial.

JavaScript Booleans

Booleans can only have two values: true or false.

Booleans are often used in conditional testing.

You will learn more about conditional testing later in this tutorial.

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):

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.

JavaScript Objects

JavaScript objects are written with curly braces.

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

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.

The typeof Operator

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

	In JavaScript, an array is a special type of object. Therefore typeof [1,2,3,4] returns object.

Undefined

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

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

Empty Values

An empty value has nothing to do with undefined.

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

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:

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

Difference Between Undefined and Null

source - http://www.w3schools.com/js/js_datatypes.asp

Have you ever taken a look under the hood at the JQuery 1.4 source code and noticed how it's encapsulated in the following way:

(function( window, undefined ) {

  //All the JQuery code here 
  ...

})(window);

I've read an article on JavaScript Namespacing and another one called "An Important Pair of Parens," so I know some about what's going on here.

But I've never seen this particular syntax before. What is that undefined doing there? And why does window need to be passed and then appear at the end again?

A.

The undefined is a normal variable and can be changed simply with undefined = "new value";. So jQuery creates a local "undefined" variable that is REALLY undefined.

The window variable is made local for performance reasons. Because when JavaScript looks up a variable, it first goes through the local variables until it finds the variable name. When it's not found, JavaScript goes through the next scope etc. until it filters through the global variables. So if the window variable is made local, JavaScript can look it up quicker. Further information: Speed Up Your JavaScript - Nicholas C. Zakas

source - http://stackoverflow.com/questions/2716069/how-does-this-javascript-jquery-syntax-work-function-window-undefined

JavaScript Objects

	In JavaScript, objects are king. If you understand objects, you understand JavaScript.

In JavaScript, almost "everything" is an object.

• Booleans can be objects (or primitive data treated as objects)
• Numbers can be objects (or primitive data treated as objects)
• Strings can be objects (or primitive data treated as objects)
• Dates are always objects
• Maths are always objects
• Regular expressions are always objects
• Arrays are always objects
• Functions are always objects
• Objects are objects

In JavaScript, all values, except primitive values, are objects.

Primitive values are: strings ("John Doe"), numbers (3.14), true, false, null, and undefined.  

Objects are Variables Containing Variables

JavaScript variables can contain single values:

Objects are variables too. But objects can contain many values.

The values are written as name : value pairs (name and value separated by a colon).

	A JavaScript object is an unordered collection of variables called named values.

Object Properties

The named values, in JavaScript objects, are called properties.

Objects written as name value pairs are similar to:

• Associative arrays in PHP
• Dictionaries in Python
• Hash tables in C
• Hash maps in Java
• Hashes in Ruby and Perl

Object Methods

Methods are actions that can be performed on objects.

Object properties can be both primitive values, other objects, and functions.

An object method is an object property containing a function definition.

	JavaScript objects are containers for named values, called properties and methods.

You will learn more about methods in the next chapters.

Creating a JavaScript Object

With JavaScript, you can define and create your own objects.

There are different ways to create new objects:

• Define and create a single object, using an object literal.
• Define and create a single object, with the keyword new.
• Define an object constructor, and then create objects of the constructed type.

	In ECMAScript 5, an object can also be created with the function Object.create().

Using an Object Literal

This is the easiest way to create a JavaScript Object.

Using an object literal, you both define and create an object in one statement.

An object literal is a list of name:value pairs (like age:50) inside curly braces {}.

The following example creates a new JavaScript object with four properties:

Spaces and line breaks are not important. An object definition can span multiple lines:

Using the JavaScript Keyword new

The following example also creates a new JavaScript object with four properties:

	The two examples above do exactly the same. There is no need to use new Object(). For simplicity, readability and execution speed, use the first one (the object literal method).

Using an Object Constructor

The examples above are limited in many situations. They only create a single object.

Sometimes we like to have an "object type" that can be used to create many objects of one type.

The standard way to create an "object type" is to use an object constructor function:

The above function (person) is an object constructor.

Once you have an object constructor, you can create new objects of the same type:

The this Keyword

In JavaScript, the thing called this, is the object that "owns" the JavaScript code.

The value of this, when used in a function, is the object that "owns" the function.

The value of this, when used in an object, is the object itself.

The this keyword in an object constructor does not have a value. It is only a substitute for the new object.

The value of this will become the new object when the constructor is used to create an object.

	Note that this is not a variable. It is a keyword. You cannot change the value of this.

Built-in JavaScript Constructors

JavaScript has built-in constructors for native objects:

The Math() object is not in the list. Math is a global object. The new keyword cannot be used on Math.

	Did You Know?

As you can see, JavaScript has object versions of the primitive data types String, Number, and Boolean.

There is no reason to create complex objects. Primitive values execute much faster.

And there is no reason to use new Array(). Use array literals instead: []

And there is no reason to use new RegExp(). Use pattern literals instead: /()/

And there is no reason to use new Function(). Use function expressions instead: function () {}.

And there is no reason to use new Object(). Use object literals instead: {}

JavaScript Objects are Mutable

Objects are mutable: They are addressed by reference, not by value.

If y is an object, the following statement will not create a copy of y:

The object x is not a copy of y. It is y. Both x and y points to the same object.

Any changes to y will also change x, because x and y are the same object.

source - http://www.w3schools.com/js/js_object_definition.asp