lecture 6 javascript event and event handling.ppt

The JavaScript Language
1
CP 221: Internet programming and
applications I
(CIVE)

Objectives
• Learn about JavaScript
• Using JavaScript in HTML
• Language Elements
• Variables
• Operators
• Control Statements
• Functions
• Objects
• Exception handling.
2

About JavaScript
• JavaScript was designed to add interactivity to HTML
pages
• JavaScript is a scripting language (a scripting language is
a lightweight programming language)
• A JavaScript consists of lines of executable computer
code
• JavaScript is an interpreted language (means that scripts
execute without preliminary compilation)
• Everyone can use JavaScript without purchasing a license
3

When not to use JavaScript
• When you need to access other resources.
– Files
– Programs
– Databases
• When you are using sensitive or copyrighted data or
algorithms.
– Your JavaScript code is open to the public.
4

Dealing with old browsers
• Some old browsers do not recognize script tags
• These browsers will ignore the script tags but will display the
included JavaScript
• To get old browsers to ignore the whole thing, use:
<script type="text/javascript">

</script>
• The , the //
starts a JavaScript comment, which extends to the end of the line
5

Objects in JavaScript
• Objects have attributes and methods.
• Many pre-defined objects and object types.
• Using objects follows the syntax of C++/Java:
objectname.attributename
objectname.methodname()
• JavaScript is not Object Oriented – but Object-
Based
6

Three ways to create an object
• You can use an object literal:
– var course = { number: "CIT597", teacher="Dr. Dave" }
• You can use new to create a “blank” object, and add fields
to it later:
– var course = new Object();
course.number = "CIT597";
course.teacher = "Dr. Dave";
• You can write and use a constructor:
– function Course(n, t) { // best placed in <head>
this.number = n;
this.teacher = t;
}
– var course = new Course("CIT597", "Dr. Dave"); 7

The document object
• Many attributes of the current document are available via the
document object:
Title Referrer
URL Images
Forms Links
Colors
8

document Methods
• document.write() like a print statement – the output
goes into the HTML document.
• document.writeln() adds a newline after printing.
• document.getElementById() Returns the element that
has the ID attribute with the specified value
document.write("My title is " +
document.title);
9

Example
<HEAD>
<TITLE>JavaScript is Javalicious</TITLE>
</HEAD>
<BODY>
<h3>I am a web page and here is my
name:</h3>
<script type=”text/Javascript”>
document.write(document.title);
</script>
<hr>
10

The navigator Object
• Represents the browser. Read-only!
• Attributes include:
appName
appVersion
platform
11

navigator Example
if (navigator.appName ==
"Microsoft Internet Explorer") {
document.writeln("<H1>This page
requires Netscape!</H1>");
}
12

The window Object
• Represents the current window.
• There are possible many objects of type Window, the
predefined object window represents the current window.
• Access to, and control of, a number of properties including
position and size.
13

window attributes
• document
• name
• status the status line
• parent
14

some window methods
alert()
close()
prompt()
moveTo() moveBy()
open()
scroll() scrollTo()
resizeBy()resizeTo()
15

The Math Object
• Access to mathematical functions and constants.
• Constants: Math.PI
• Methods:
Math.abs(), Math.sin(), Math.log(), Math.max(),
Math.pow(), Math.random(), Math.sqrt(), …
16

Math object in use
// returns an integer between 1 and 6
function roll() {
var x = Math.random();
// convert to range [0,6.0)
x = x * 6;
// add 1 and convert to int
return parseInt(1+x );
}
document.writeln("Roll is “ + roll() ); 17

Math object in use
<script type ="text/javascript">

</script>
18

Array Objects
• Arrays are supported as objects.
• Attribute length
• Methods include:
concat join pop push reverse sort
19

Some similarity to C++
• Array indexes start at 0.
• Syntax for accessing an element is the same:
a[3]++;
blah[i] = i*72;
20

New Stuff (different from
C++)
• Arrays can grow dynamically – just add new elements at the
end.
• Arrays can have holes, elements that have no value.
• Array elements can be anything
• numbers, strings, or arrays!
21

Four ways to create an array
• You can use an array literal:
var colors = ["red", "green", "blue"];
• You can use new Array() to create an empty array:
– var colors = new Array();
• You can add elements to the array later:
colors[0] = "red"; colors[2] = "blue"; colors[1]="green";
• You can use new Array(n) with a single numeric
argument to create an array of that size
– var colors = new Array(3);
• You can use new Array(…) with two or more arguments
to create an array containing those values:
– var colors = new Array("red","green", "blue");
22

The length of an array
• If myArray is an array, its length is given by
myArray.length
• Array length can be changed by assignment beyond the
current length
• Example: var myArray = new Array(5); myArray[10] = 3;
• Arrays are sparse, that is, space is only allocated for
elements that have been assigned a value
• Example: myArray[50000] = 3; is perfectly OK
• But indices must be between 0 and 232-1
• As in C and Java, there are no two-dimensional arrays; but
you can have an array of arrays: board[3][3]
23

Arrays examples
• car = { myCar: "Saturn", 7: "Mazda" }
– car[7] is the same as car.7
– car.myCar is the same as car["myCar"]
• If you know the name of a property, you can use dot notation:
car.myCar
• If you don’t know the name of a property, but you have it in a
variable (or can compute it), you must use array notation:
car["my" + "Car"]
• var colors = [" blue ",
" green ",
" yellow "];
var x = window.prompt("enter a number ");
document.body.style.background = colors[x];
24

Array of Arrays Example
var board = [ [1,2,3],
[4,5,6],
[7,8,9] ];
for (i=0;i<3;i++)
for (j=0;j<3;j++)
board[i][j]++;
25

Array functions
• If myArray is an array,
– myArray.sort() sorts the array alphabetically
– myArray.sort(function(a, b) { return a - b; }) sorts numerically
– myArray.reverse() reverses the array elements
– myArray.push(…) adds any number of new elements to the end of
the array, and increases the array’s length
– myArray.pop() removes and returns the last element of the array,
and decrements the array’s length
– myArray.toString() returns a string containing the values of the
array elements, separated by commas
26

Passing Arrays to Functions
• To pass an array argument to a function, specify the name of
the array without brackets.
var a = [1,2,3,4,5];
outputArray(" Your array: ", a);
function outputArray(heading, theArray)
{
document.writeln(heading + theArray.join("
") + " <br /> ");
} 27

JavaScript HTML DOM
• With the HTML DOM, JavaScript can access and
change all the elements of an HTML document.
• The HTML DOM (Document Object Model)
• When a web page is loaded, the browser creates
a Document Object Model of the page.
• The HTML DOM model is constructed as a tree
of Objects:
12/3

The HTML DOM Tree of
Objects
12/3

The HTML DOM Tree of
Objects
12/3
• With the object model, JavaScript gets all the power it needs
to create dynamic HTML:
• JavaScript can change all the HTML elements in the page
• JavaScript can change all the HTML attributes in the page
• JavaScript can change all the CSS styles in the page
• JavaScript can remove existing HTML elements and attributes
• JavaScript can add new HTML elements and attributes
• JavaScript can react to all existing HTML events in the page
• JavaScript can create new HTML events in the page

What is the DOM?
• The DOM is a W3C (World Wide Web
Consortium) standard.
• The DOM defines a standard for accessing
documents:
• "The W3C Document Object Model (DOM) is a
platform and language-neutral interface that
allows programs and scripts to dynamically
access and update the content, structure, and
style of a document."
12/3

What is the DOM?
The W3C DOM standard is separated into 3
different parts:
• Core DOM - standard model for all document
types
• XML DOM - standard model for XML documents
• HTML DOM - standard model for HTML
documents
12/3

What is the HTML DOM?
• The HTML DOM is a standard object model and
programming interface for HTML. It defines:
 The HTML elements as objects
 The properties of all HTML elements
 The methods to access all HTML elements
 The events for all HTML elements
• In other words: The HTML DOM is a standard for
how to get, change, add, or delete HTML
elements.
12/3

JavaScript - HTML DOM
Methods
• HTML DOM methods are actions you can perform (on HTML
Elements).
• HTML DOM properties are values (of HTML Elements) that
you can set or change.
• The HTML DOM can be accessed with JavaScript (and with
other programming languages).
• In the DOM, all HTML elements are defined as objects.
• The programming interface is the properties and methods of
each object.
• A property is a value that you can get or set (like changing the
content of an HTML element).
• A method is an action you can do (like add or deleting an
HTML element).
12/3

Example
The following example changes the content (the
innerHTML) of the <p> element with id="demo":
<html>
<body>
<p id="demo"></p>
<script>
document.getElementById("demo").innerHTML =
"Hello World!";
</script>
</body>
</html>
12/3

Example..
• In the example above, getElementById is a method, while
innerHTML is a property.
12/3

The getElementById Method
• The most common way to access an HTML
element is to use the id of the element.
• In the example above the getElementById
method used id="demo" to find the element. 12/3

The innerHTML Property
• The easiest way to get the content of an element
is by using the innerHTML property.
• The innerHTML property is useful for getting or
replacing the content of HTML elements.
• The innerHTML property can be used to get or
change any HTML element, including <html> and
<body>
12/3

The HTML DOM Document
Object
• The document object represents your web page.
• If you want to access any element in an HTML
page, you always start with accessing the
document object.
• Below are some examples of how you can use
the document object to access and manipulate
HTML.
12/3

The HTML DOM Document
Object
 Finding HTML Elements
12/3

JavaScript HTML DOM
Elements
How to find and access HTML
elements in an HTML page?
12/3

JavaScript HTML DOM
Elements
 Finding HTML Elements
• Often, with JavaScript, you want to manipulate HTML
elements.
• To do so, you have to find the elements first. There are several
ways to do this:
 Finding HTML elements by id
 Finding HTML elements by tag name
 Finding HTML elements by class name
 Finding HTML elements by CSS selectors
 Finding HTML elements by HTML object collections
12/3

JavaScript HTML DOM
Elements
 Finding HTML Element by Id
 The easiest way to find an HTML element in the DOM, is by
using the element id.
 This example finds the element with id="intro":
12/3

JavaScript HTML DOM
Elements
!DOCTYPE html>
<html>
<body>
<h2>Finding HTML Elements by Id</h2>
<p id="intro">Hello World!</p>
<p>This example demonstrates the <b>getElementsById</b>
method.</p>
<p id="demo"></p>
<script>
var myElement = document.getElementById("intro");
"The text from the intro paragraph is " + myElement.innerHTML;
</script>
12/3

JavaScript HTML DOM
Elements
 If the element is found, the method will return the
element as an object (in myElement).
 If the element is not found, myElement will contain null.
12/3

JavaScript HTML DOM Elements(Finding HTML
Elements by Tag Name)
<!DOCTYPE html>
<html>
<body>
<h2>Finding HTML Elements by Tag Name</h2>
<p>Hello World!</p>
<p>This example demonstrates the <b>getElementsByTagName</b>
method.</p>
<p id="demo"></p>
<script>
var x = document.getElementsByTagName("p");
'The text in first paragraph (index 0) is: ' + x[0].innerHTML;
</script>
12/3

Changing HTML Content
• The easiest way to modify the content of an
HTML element is by using the innerHTML
property.
• To change the content of an HTML element, use
this syntax:
document.getElementById(id).innerHTML = new
HTML
• This example changes the content of a <p>
element:
12/3

Changing HTML Content
<html>
<body>
<p id="p1">Hello World!</p>
<script>
document.getElementById("p1").innerHTML =
"New text!";
</script>
</body>
</html>
12/3

Changing the Value of an
Attribute
• To change the value of an HTML attribute, use this
syntax:
document.getElementById(id).attribute = new value
• This example changes the value of the src attribute of an
<img> element: 12/3

Changing the Value of an
Attribute
<!DOCTYPE html>
<html>
<body>
<img id="myImage" src="smiley.gif">
<script>
document.getElementById("myImage").src = "landscape.jpg";
</script>
</body>
</html>
12/3

Changing HTML Style
• To change the style of an HTML element, use this
syntax:
document.getElementById(id).style.property =
new style
• The following example changes the style of a <p>
element:
12/3

Changing HTML Style
• <html>
<body>
<p id="p2">Hello World!</p>
<script>
document.getElementById("p2").style.color = "blue";
</script>
<p>The paragraph above was changed by a script.</p>
</body>
</html>
12/3

Using Events
• The HTML DOM allows you to execute code when an event
occurs.
• Events are generated by the browser when "things happen" to
HTML elements:
 An element is clicked on
 The page has loaded
 Input fields are changed
• This example changes the style of the HTML element with
id="id1", when the user clicks a button:
12/3

Using Events
<!DOCTYPE html>
<html>
<body>
<h1 id="id1">My Heading 1</h1>
<button type="button"
onclick="document.getElementById('id1').style.color = 'red'">
Click Me!</button>
</body>
</html>
12/3

lecture 6 javascript event and event handling.ppt

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