Internet-schemes

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29/12/2015

Object oriented programming
Objects

An object is a 'thing'. For example a number is an object. An array is an object. Your browser window is an object. A form in your document is an object. There are hundreds more, and in some cases you can create your own.

Objects can have objects in them. For example, your document can have a form in it. This form is a 'child object' of the document. Therefore the document is the 'parent object' of the form. To reference the child object, we have to go through the parent object, eg. document.myForm

An array can have numbers in its cells. As we have already discussed, the array is an object, and so would be a cell within it, and so is the content of the cell. We cannot refer to the cell itself, but we can refer to its contents: myArray['cell name or number'] for example.
Classes (or types)

A class is a group of objects that are similar in some way. For example, a number and a piece of text can both be stored as a variable (in a way, like the variables you would use in mathematical algebra). In essence, we can say that pieces of text and numbers are examples of class 'variable'.

Numbers can be sub divided into two groups, integers and floats (or doubles). Integers are whole numbers: 1, 2, 3, 4, 0, -1, -2, etc. Floats have decimal points: 1.1, -5.7, 0.5, etc. In this case, we can say that 3 is an instance of class variable, (sub)class number, (sub)class integer.

In fact, a variable is a type of object. All instances of class 'object' have a certain two methods: toString() and valueOf(). Therefore, as 3 is an instance of class object, (sub)class variable, (sub)class number, (sub)class integer, it will inherit the toString() and valueOf() methods provided by the class 'object'.

Classes are not so important in JavaScript as they are in many other object oriented programming languages. Classes can be created when you define your own classes of objects, but it is not usual to create your own 'sub'-classes.
Collections

If you need to know what arrays are, see the section on Variables.

There are many arrays that are inbuilt into each document. The document itself is an array in certain uses. The most obvious of these collections is the images collection. To refer to images in the document, we use
document.images['name of image']
This is a special kind of array, known as a collection.
Properties

Take, for example, an image. When we define images in HTML, we write:



The properties of the image would be src, name, height, width, alt and if we also used Style Sheets we might have included several more (like background-color for example). All properties are a type of object so to refer to the src of my image, I would write document.images['myImage'].src
Methods

There are always actions that are associated with objects. For example, a form may be submitted or reset. The actions are methods. To submit a form in non-object-oriented programs, we might write submit('name of form')

That would simply be a function. In object oriented programming like JavaScript, we would use document.nameOfForm.submit()

The reason this is a method and not just a function is that each form will have its own submit() function built in, which is referred to using the object oriented syntax shown above. You will never have to write methods in yourself unless you create your own objects, the browser does that for you.

You can think of it like this:

With the non-object-oriented way, we would tell a function to submit the form.
With the object oriented way, we would tell the form to submit itself.

If wanted, you can run several methods in turn on the same object by using:

referenceToTheObject.method1().method2().method3().method1again()

In this case, method1 must return a class of object that has the method 'method2' etc.

Operators

The most common operators are mathematical operators; +, -, /, * (add, subtract, divide, multiply) for example. Operators can be split into two groups, comparison operators and assignment or 'action' operators. Comparison operators test to see if two variables relate to each other in the specified way, for example, one variable is a higher number than the other. Other operators perform an action on a variable, such as increasing it by one.

The following table gives those that are most commonly used. The JavaScript 1.3 operators such as the identity operator === are supported by all current browsers, but are not used as often as the others. Old browsers that do not understand them will just produce errors.
JavaScript operators Operator Uses
+ adds two numbers or appends two strings - if more than one type of variable is appended, including a string appended to a number or vice-versa, the result will be a string
- subtracts the second number from the first
/ divides the first number by the second
* multiplies two numbers
% divide the first number by the second and return the remainder
= assigns the value on the right to the object on the left
+= the object on the left = the object on the left + the value on the right - this also works when appending strings
-= the object on the left = the object on the left - the value on the right
> number on the left must be greater than the number on the right - this also works with strings and values
< number on the left must be less than the number on the right - this also works with strings and values
>= number on the left must be greater than or equal to the number on the right - this also works with strings and values
bitwise rightshift
& bitwise AND
| bitwise OR
^ bitwise XOR
~ bitwise NOT
! logical NOT (the statement must not be true)
&& logical AND (both statements must be true)
|| logical OR (either statement must be true)
in object or array on the right must have the property or cell on the left
=== the numbers or objects or values must be equal, and must be the same variable type
!== the numbers or objects or values must not be equal, or must not be the same variable type

Note, if you do not set the language="javascript1.2" attribute in the tag, 0 == false == '' and undefined == null. If you do set the language attribute to 'javascript1.2', Mozilla/Firefox and other Gecko browsers (but none of the other major browsers) will change this so that none of these will equate to each other. Since the attribute is deprecated anyway, and the JavaScript versions were never standardised, you should not rely on this behaviour. If you need that behaviour, use the === and !== operators.

There are also a few operators that can also be used like functions:

void
void statement or void(statement) (see the section on writing functions)
typeof
typeof variable or typeof(variable) returns the type (or class) of a variable.
eval
eval(string) interprets a string as JavaScript code.

There are also the 'var', 'new' and 'delete' operators. See the section on variables for examples of their uses. Also see the section on the Math object operators below.

Note that JavaScript has no logical XOR operator. If you need that functionality, see my separate XOR article.
Operator precedence

If you ask JavaScript to perform a calculation using multiple operators, those operators will be evaluated in a specific order. For example 3 + 6 * 7 is calculated as ( 6 * 7 ) + 3 because the * is calculated before the +. The order in which these are evaluated is: * / % + - + (where the second + is appending strings). To change the order in which they are calculated, use parenthesis ( ) as the contents of parenthesis are calculated before the contents outside the parenthesis. For example, 3 + 6 * 7 = 45 but ( 3 + 6 ) * 7 = 63
The Math object methods

In reality, these are methods of the Math object but they are used in place of operators.

Math object methods Operator What it does
Math.abs(n) Returns the absolute value of n
Math.acos(n) Returns (in radians) cos-1 of n
Math.asin(n) Returns (in radians) sin-1 of n
Math.atan(n) Returns (in radians) tan-1 of n
Math.atan2(n,k) Returns the angle (rads) from cartesian coordinates 0,0 to n,k
Math.ceil(n) Returns n rounded up to the nearest whole number
Math.cos(n) Returns cos n (where n is in radians)
Math.exp(n) Returns en
Math.floor(n) Returns n rounded down to the nearest whole number
Math.log(n) Returns ln(n)
Note, to find log10(n), use Math.log(n) / Math.log(10)
Math.max(a,b,c,...) Returns the largest number
Math.min(a,b,c,...) Returns the smallest number
Math.pow(n,k) Returns nk
Math.random() Returns a random number between 0 and 1
Math.round(n) Returns n rounded up or down to the nearest whole number
Math.sin(n) Returns sin n (where n is in radians)
Math.sqrt(n) Returns the square root of n
Math.tan(n) Returns tan n (where n is in radians)

15/12/2015

The Internet is the global system of interconnected computer networks that use the Internet protocol suite (TCP/IP) to link billions of devices worldwide. It is a network of networks that consists of millions of private, public, academic, business, and government networks of local to global scope, linked by a broad array of electronic, wireless, and optical networking technologies. The Internet carries an extensive range of information resources and services, such as the inter-linked hypertext documents and applications of the World Wide Web (WWW), electronic mail, telephony, and peer-to-peer networks for file sharing.

Although the Internet protocol suite has been widely used by academia and the military industrial complex since the early 1980s, events of the late 1980s and 1990s such as more powerful and affordable computers, the advent of fiber optics, the popularization of HTTP and the Web browser, and a push towards opening the technology to commerce eventually incorporated its services and technologies into virtually every aspect of contemporary life.

The impact of the Internet has been so immense that it has been referred to as the "8th continent". [1] [2]

The origins of the Internet date back to research and development commissioned by the United States government, the Government of the UK and France in the 1960s to build robust, fault-tolerant communication via computer networks.[3] [4] [5] [6] This work, led to the primary precursor networks, the ARPANET, in the United States, Manchester Mark 1 in the United Kingdom and CYCLADES in France. The interconnection of regional academic networks in the 1980s marks the beginning of the transition to the modern Internet.[7] From the late 1980s onward, the network experienced sustained exponential growth as generations of institutional, personal, and mobile computers were connected to it.

Internet use grew rapidly in the West from the mid-1990s and from the late 1990s in the developing world. In the 20 years since 1995, Internet use has grown 100-times, measured for the period of one year, to over one third of the world population.[8][9]

Most traditional communications media, including telephony and television, are being reshaped or redefined by the Internet, giving birth to new services such as Internet telephony and Internet television. Newspaper, book, and other print publishing are adapting to website technology, or are reshaped into blogging and web feeds. The entertainment industry was initially the fastest growing segment on the Internet.[citation needed] The Internet has enabled and accelerated new forms of personal interactions through instant messaging, Internet forums, and social networking. Online shopping has grown exponentially both for major retailers and small artisans and traders. Business-to-business and financial services on the Internet affect supply chains across entire industries.

The Internet has no centralized governance in either technological implementation or policies for access and usage; each constituent network sets its own policies.[10] Only the overreaching definitions of the two principal name spaces in the Internet, the Internet Protocol address space and the Domain Name System (DNS), are directed by a maintainer organization, the Internet Corporation for Assigned Names and Numbers (ICANN). The technical underpinning and standardization of the core protocols is an activity of the Internet Engineering Task Force (IETF), a non-profit organization of loosely affiliated international participants that anyone may associate with by contributing technical expertise.[11]