1. 5
JavaScript

2. Overview
A "scripting" language for HTML pages - a scripting language is a lightweight programming language
Embed code in HTML pages so they are downloaded directly to browser
The browser interprets and executes the script (it is not compiled)
Was designed to add interactivity to HTML pages
Everyone can use JavaScript without purchasing a license
Supported by all major browsers

3. … Overview Do not declare data types for variables (loose typing)
Dynamic binding – object references checked at runtime
Scripts can manipulate "browser objects:"
HTML form elements
Images
Frames
etc.
For security – cannot write to disk (when run on a client)

4. Abilities Generating HTML content dynamically
Monitoring and responding to user events
Validate forms before submission
Manipulate HTTP cookies
Interact with the frames and windows of the browser
Customize pages to suit users

5. It is not Java JavaScript is not Java, or even related to Java
The original name for JavaScript was “LiveScript”
The name was changed when Java became popular
Released in the Fall of 1995
Statements in JavaScript resemble statements in Java, because both languages borrowed heavily from the C language
JavaScript should be fairly easy for Java programmers to learn
JavaScript is seldom used to write complete “programs”
Instead, small bits of JavaScript are used to add functionality to HTML pages
JavaScript is often used in conjunction with HTML “forms”
JavaScript is reasonably platform-independent

6. … It is not Java
JavaScript has some features that resemble features in Java:
JavaScript has Objects and primitive data types
JavaScript has qualified names; for example, document.write("Hello World");
JavaScript has Events and event handlers
Exception handling in JavaScript is almost the same as in Java
JavaScript has some features unlike anything in Java:
Variable names are untyped: the type of a variable depends on the value it is currently holding
Objects and arrays are defined in quite a different way
JavaScript has with statements and a new kind of for statement

7. Scripting The entire script is stored in memory as plain text
When requested by the user the applicable portion of the script is executed by fetching the associated machine instructions from a library
Tends to be a bit slower than compiling programs
However, there is no burden on the author to compile anything
Errors are not obvious when scripting; only rigorous testing will find errors

8. History Built into Netscape Navigator since v2.0 (early 1996)
Developed independently of Java
Proprietary, but submitted as standard and built into Microsoft IE 3.0 and later
Standardized by ECMA (European Computer Manufacture’s Association) into ECMAscript
EMCAscript joins JavaScript and Jscript to one standard

9. Javascript has many names
In Netscape it’s Javascript
In Internet Explorer it’s JScript
IE also supports it’s own VB Script, a Visual Basic scripting language
VB Script won’t work in Netscape
There is also ECMAscript
A variation of Javascript 1.1
Open Standard
Promoted by European Computer Manufacturers Association (ECMA)
JScript is essentially ECMAscript in IE 4.0+

10. Javascript Versions 1.0 – Original version, largely obsolete 1.1 1.2
Supported in Navigator 2.0
Buggy version of it supported in IE 3.0 as JScript
1.1
Improved array processing
Supported in Navigator 3.0 as JScript, some discrepancies
1.2
Supports regular expressions, new statements
Supported in Navigator 4.0
1.3
Fixed some problems with dates, introduced in Navigator 4.06
ECMAScript
First supported in IE 4.0, also in Navigator 4.06
Largely the same as Javascript 1.1

11. Dynamic HTML
HTML
CSS
Java
Script
Java
Script
HTML
HTML
CSS
HTML

12. Web Architecture for JavaScript
"CLIENT"
"SERVER"
Desktop access
Remote host
Web browser
Web
(HTTP)
Server
HTML Page:
<SCRIPT>
…code..…
</SCRIPT>
Internet
HTML/HTTP
TCP/IP
HTML/HTTP
TCP/IP
built-in
JavaScript
interpreter
HTML
pages w/
embedded
script

13. Client and Server JavaScript can be used
On the client side
On the server
More lightweight and reliable on clients than Java (Applets)
Useful for developing interactive interface (Dynamic HTML)

14. Example JavaScript code is included within <script> tags: Notes:
<script type="text/javascript"> document.write("<h1>Hello World!</h1>") ; </script>
Notes:
The type attribute is to allow you to use other scripting languages (but JavaScript is the default)
This simple code does the same thing as just putting <h1>Hello World!</h1> in the same place in the HTML document
The semicolon at the end of the JavaScript statement is optional
You need semicolons if you put two or more statements on the same line
It’s probably a good idea to keep using semicolons

15. 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"> <! document.write("Hello World!") //--> </script>
The <!-- introduces an HTML comment
To get JavaScript to ignore the HTML close comment, -->, the // starts a JavaScript comment, which extends to the end of the line

16. Where to put JavaScript
JavaScript can be put in the <head> or in the <body> of an HTML document
JavaScript functions should be defined in the <head>
This ensures that the function is loaded before it is needed
JavaScript in the <body> will be executed as the page loads
JavaScript can be put in a separate .js file
<script src="myJavaScriptFile.js"></script>
Put this HTML wherever you would put the actual JavaScript code
An external .js file lets you use the same JavaScript on multiple HTML pages
The external .js file cannot itself contain a <script> tag
JavaScript can be put in HTML form object, such as a button
This JavaScript will be executed when the form object is used

17. Primitive data types
JavaScript has three “primitive” types: number, string, and boolean
Everything else is an object
Numbers are always stored as floating-point values
Hexadecimal numbers begin with 0x
Some platforms treat 0123 as octal, others treat it as decimal
Strings may be enclosed in single quotes or double quotes
Strings can contains \n (newline), \" (double quote), etc.
Booleans are either true or false
0, "0", empty strings, undefined, null, and NaN are false, other values are true

18. Variables Variables are declared with a var statement:
var pi = , x, y, name = "Dr. ABC" ;
Variables names must begin with a letter or underscore
Variable names are case-sensitive
Variables are untyped (they can hold values of any type)
The word var is optional (but it’s good style to use it)
Variables declared within a function are local to that function (accessible only within that function)
Variables declared outside a function are global (accessible from anywhere on the page)

19. Operators, I
Because most JavaScript syntax is borrowed from C (and is therefore just like Java), we won’t spend much time on it
Arithmetic operators: * / %
Comparison operators: < <= == != >= >
Logical operators: && || ! (&& and || are short-circuit operators)
Bitwise operators: & | ^ ~ << >> >>>
Assignment operators: = -= *= /= %= <<= >>= >>>= &= ^= |=

20. Operators, II String operator: +
The conditional operator: condition ? value_if_true : value_if_false
Special equality tests:
== and != try to convert their operands to the same type before performing the test
=== and !== consider their operands unequal if they are of different types
Additional operators: new typeof void delete

21. Comments Comments are as in C or Java:
Between // and the end of the line
Between /* and */

22. Statements, I Most JavaScript statements are also borrowed from C
Assignment: greeting = "Hello, " + name;
Compound statement: { statement; ...; statement }
If statements: if (condition) statement; if (condition) statement; else statement;
Familiar loop statements: while (condition) statement; do statement while (condition); for (initialization; condition; increment) statement;

23. Statements, II
The switch statement: switch (expression){ case label : statement; break; case label : statement; break; default : statement; }
Other familiar statements:
break;
continue;
The empty statement, as in ;; or { }

24. Exception handling, I
Exception handling in JavaScript is almost the same as in Java
throw expression creates and throws an exception
The expression is the value of the exception, and can be of any type (often, it's a literal String)
try { statements to try } catch (e) { // Notice: no type declaration for e exception-handling statements } finally { // optional, as usual code that is always executed }
With this form, there is only one catch clause

25. Exception handling, II
try { statements to try } catch (e if test1) { exception-handling for the case that test1 is true } catch (e if test2) { exception-handling for when test1 is false and test2 is true } catch (e) { exception-handling for when both test1 and test2 are false } finally { // optional, as usual code that is always executed }
Typically, the test would be something like e == "InvalidNameException"

26. Object literals You don’t declare the types of variables in JavaScript
JavaScript has object literals, written with this syntax:
{ name1 : value1 , ... , nameN : valueN }
Example:
car = {myCar: "Toyota", 7: "Mazda", getCar: CarTypes("Honda"), special: Sales}
The fields are myCar, getCar, 7 (this is a legal field name) , and special
"Toyota" and "Mazda" are Strings
CarTypes is a function call
Sales is a variable you defined earlier
Example use: document.write("I own a " + car.myCar);

27. Three ways to create an object
You can use an object literal:
var course = { number: “CS450", teacher="Dr. ABC" }
You can use new to create a “blank” object, and add fields to it later:
var course = new Object(); course.number = “CS450"; course.teacher = "Dr. ABC";
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(“CS450", "Dr. ABC");

28. Array literals You don’t declare the types of variables in JavaScript
JavaScript has array literals, written with brackets and commas
Example: color = ["red", "yellow", "green", "blue"];
Arrays are zero-based: color[0] is "red"
If you put two commas in a row, the array has an “empty” element in that location
Example: color = ["red", , , "green", "blue"];
color has 5 elements
However, a single comma at the end is ignored
Example: color = ["red", , , "green", "blue”,]; still has 5 elements

29. 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");

30. 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: myArray[5][3]

31. Arrays and objects Arrays are objects
car = { myCar: “Toyota", 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"]

32. 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

33. The for…in statement
You can loop through all the properties of an object with for (variable in object) statement;
Example: for (var prop in course) { document.write(prop + ": " + course[prop]); }
Possible output: teacher: Dr. ABC number: CS450
The properties are accessed in an undefined order
If you add or delete properties of the object within the loop, it is undefined whether the loop will visit those properties
Arrays are objects; applied to an array, for…in will visit the “properties” 0, 1, 2, …
Notice that course["teacher"] is equivalent to course.teacher
You must use brackets if the property name is in a variable

34. Functions
Functions should be defined in the <head> of an HTML page, to ensure that they are loaded first
The syntax for defining a function is: function name(arg1, …, argN) { statements }
The function may contain return value; statements
Any variables declared within the function are local to it
The syntax for calling a function is just name(arg1, …, argN)
Simple parameters are passed by value, objects are passed by reference

35. Regular expressions
A regular expression can be written in either of two ways:
Within slashes, such as re = /ab+c/
With a constructor, such as re = new RegExp("ab+c")
Regular expressions are almost the same as in Perl or Java (only a few unusual features are missing)
string.match(regexp) searches string for an occurrence of regexp
It returns null if nothing is found
If regexp has the g (global search) flag set, match returns an array of matched substrings
If g is not set, match returns an array whose 0th element is the matched text, extra elements are the parenthesized subexpressions, and the index property is the start position of the matched substring

36. Debugging If you mess up on the syntax you will get a Javascript Error
Netscape
You will see a notification of an error on the status bar in the bottom left corner
You type “javascript:” in the URL field to pinpoint the error
Internet Explorer
By default a tiny little Javascript error message appears at the bottom left corner of the browser in yellow. Usually you won’t see it.
Can be explicitly disabled under Tools/Internet Options
Recommend under Tools/Internet Options/Advanced/Browsing to uncheck “Disable Script Debugging” and to check “Display a Notification about every script error” while doing development

37. Fixing Javascript Errors
If possible use the debugging tool to locate the line containing the error
Errors can be hard to find and fix
“code a little, test a little” strategy
Often errors are due to things that are easy to overlook, like not closing a quote

38. Numbers In JavaScript, all numbers are floating point
Special predefined numbers:
Infinity, Number.POSITIVE_INFINITY -- the result of dividing a positive number by zero
Number.NEGATIVE_INFINITY -- the result of dividing a negative number by zero
NaN, Number.NaN (Not a Number) -- the result of dividing 0/0
NaN is unequal to everything, even itself
There is a global isNaN() function
Number.MAX_VALUE -- the largest representable number
Number.MIN_VALUE -- the smallest (closest to zero) representable number

39. Strings and characters
In JavaScript, string is a primitive type
Strings are surrounded by either single quotes or double quotes
There is no “character” type
Special characters are:
\0 NUL
\b backspace
\f form feed
\n newline
\r carriage return
\t horizontal tab
\v vertical tab
\' single quote
\" double quote
\\ backslash
\xDD Unicode hex DD
\xDDDD Unicode hex DDDD

40. Some string methods charAt(n) concat(string1, ..., stringN)
Returns the nth character of a string
concat(string1, ..., stringN)
Concatenates the string arguments to the recipient string
indexOf(substring)
Returns the position of the first character of substring in the recipient string, or -1 if not found
indexOf(substring, start)
Returns the position of the first character of substring in the given string that begins at or after position start, or -1 if not found
lastIndexOf(substring), lastIndexOf(substring, start)
Like indexOf, but searching starts from the end of the recipient string

41. More string methods match(regexp) replace(regexp, replacement)
Returns an array containing the results, or null if no match is found
On a successful match:
If g (global) is set, the array contains the matched substrings
If g is not set:
Array location 0 contains the matched text
Locations 1... contain text matched by parenthesized groups
The array index property gives the first matched position
replace(regexp, replacement)
Returns a new string that has the matched substring replaced with the replacement
search(regexp)
Returns the position of the first matched substring in the given string, or -1 if not found.

42. boolean The boolean values are true and false
When converted to a boolean, the following values are also false:
"0" and '0'
the empty string, '' or ""
undefined
null
NaN

43. Arrays As in C and Java, there are no “true” multidimensional arrays
However, an array can contain arrays
The syntax for array reference is as in C and Java
Example:
var a = [ ["red", 255], ["green", 128] ];
var b = a[1][0]; // b is now "green"
var c = a[1]; // c is now ["green", 128]
var d = c[1]; // d is now 128

44. Input
Programming languages need to start with some data and manipulate it
Confirm asks a yes or no question in a dialog box
Prompt prompts the user to type in some information into a text field inside the dialog box
Sources of data can include:
Files
Databases
User (keyboard & mouse typically)
Variable assignments (ex: pi= )
Javascript objects
Example: date object
Example:
User_name = prompt(“What is your name?”, “Enter your name here”);

45. Output
After a program manipulates the input data with various statements it usually creates an output of some kind
Source of output may include:
Files
Database
Display or Printer
Devices (sound card, modems etc)
Javascript Objects
Via Object Methods

46. Simple Output
document is an object (not a class) representing the current document
write is a method on the document object that let’s you write any text to the browser window at the current location of the cursor
Warning: if invoked as part of a form action output will appear in a new window
Example:
document.write(“Hello world!”);

47. alert method
A dialog box containing information can be written by using the window.alert method
Example:
alert(“This brings up an annoying non-modal dialog box. The user can’t do anything until they click OK.”);

48. HTML names in JavaScript
In HTML the window is the global object
It is assumed that all variables are properties of this object, or of some object decended from this object
The most important window property is document
HTML form elements can be referred to by document.forms[formNumber].elements[elementNumber]
Every HTML form element has a name attribute
The name can be used in place of the array reference
Hence, if
<form name="myForm"> <input type="button" name="myButton" ...>
Then instead of document.forms[0].elements[0]
you can say document.myForm.myButton

49. Document Object Model (DOM)
The Dynamic HTML Document Object Model in this case
There are other DOM’s
It allows a document (in this case the web browser) to be manipulated as multiple objects
For example, the document with focus in the browser is the document object
You manipulate properties of the objects by setting object properties and calling methods of objects

50. DOM Object Hierarchy

51. Navigating through the DOM
5 objects have predefined names that can’t be changed and are available to any page
window, document, history, location, navigator
Any object of a type that can be replicated is accessed by the HTML name or ID attribute.
Ex: <form name=“form1”>
Accessed by: window.document.form1
If an object is named uniquely it is not necessary to give a fully qualified name
Ex: use form1 instead of window.document.form1
Objects that are not named may be accessed indirectly through an array. Element 0 indicates the number of objects
Ex: document.forms[1] or forms[1]

52. Javascript object example
Document
Properties
fgcolor: specifies color of document text
same as <body text=“color”>
Methods
clear: erases contents of current document
write: sends text to the browser

53. Math Object Can be accessed as Math.property, ex:
x=Math.pow(3,3); // x=27
Allows many common mathematical calculations including:
abs(x) : absolute value
ceil(x) and floor(x) : smallest integer not less than x and largest integer not greater than x
cos(x), exp(x), log(x), sin(x), tan(x) : trigonometric and log rhythmic functions
min(x,y) or max(x,y) : returns the minimum or maximum of values x and y
pow(x,y) : raises x to the power y
round(x) : rounds to nearest integer
sqrt(x) : Square root

54. Date Object
By default creates an object with the computer’s current date and time, ex:
now = new Date(); // variable now contains current date and time
Note: months are expressed 0-11, 0 being January, 11 being December
Dates are actually stored as an integer representing the number of milliseconds since January 1st, 1970
Negative values indicate dates before this date
Once you have a date object you can set the date, or read the date in a number of useful formats
now.setFullYear(2003, 0, 31); /* Jan 31st, 2003 */
Now.setHours(13, 13, 13); /* 1:13:13 PM, local time zone */

55. Date Properties
Some of the more useful properties of the date class include:
now.getDay(); /* returns 0-6 for the day of the week, 0=Sunday, 6=Saturday */
now.getFullYear(); /* year of this date object */
now.getMonth(); /* 0-11 returned for the current month */
now.getDate(); /* 1-31 for the day in the month */
now.getHours(); /* 0-23, may need to translate to PM */
now.getMinutes(); /* 1-60 */
Now.getSeconds(); /* 1-60 */

56. Window Object
The window object is the “master” DOM object at the top of the DOM hierarchy
Useful properties:
length: number of frames in window
frames: an array of window objects, one for each frame
parent: Since frames are window objects, sometimes parent window is needed
Examples:
window.document : if frameless, accesses the top level document. If frames, accesses the top frame’s document
window.frame[1].document : Access the document contained in the first frame
frame[1].parent.document : Access the document contained in the parent frame

57. Window Object Methods
alert, confirm and prompt are actually methods of the window object, ex: window.alert
window.open(); /* opens a window */
window.close(); /* closes window */

58. Navigator Object Contains information about the browser
Can be accessed as window.navigator or just navigator
Useful properties:
appName: name of browser used (can be deceiving; more on this in a later class)
appVersion: version of browser used (can be deceiving; more on this in a later class)
platform: operating system in use
cookieEnabled: can the browser store cookies?

59. Location Object Contains information about the current URL
Can be accessed as window.location or just location
Useful properties:
href: retrieves entire URL
host: retrieves just the domain name (ex: yahoo.com)
pathname: retrieves just the path inside the domain (page name is at end)
hash: retrieves the anchor

60. History Object
Contains information on the URLs that the browser has visited in this session within a window
Can be accessed as window.history or just history
Useful properties: next, previous (tells you the URL, but won’t direct you there)
Useful methods:
back: same as pressing the back arrow button
forward: same as pressing the forward arrow button
go: go back or forward a given number of pages; to go back 3 pages:
history.go(-3);

61. Document Object This is the typically the most accessed object
You can access all items in the document window through the document object
Forms, tables, paragraphs, lists, images, etc.
Consult a reference for properties and methods
Frameless document: Access as window.document or document
Document contained in a frame: window.frame[x].document, where x is the number or name of the frame

62. DOM Collections
Inside objects may be one or more collections of child objects
Syntax: object.collection
object is either a standard object (like document) or the HTML ID attribute that uniquely identifies an item on a page
collection is a group of objects all of the same type
Ex: document.links
document is the DOM object
links is a collection of link objects inside the document object
The document object may have many links embedded inside of it (just as a page may have many links in it)
The collection of links can be thought of as an array of link objects, i.e. links[0], links[1] etc.

63. Addressing DOM Collections
Collection objects: anchors, applets, embeds, forms, frames, images, links, plugins, scripts, styleSheets
Addressing an element in a collection can be done in a number of ways. Pick the way easiest for you:
document.collection[i]
i is a number, 0 thru n where n is the last element in the collection array and 0 the first
Ex: document.links[0] references the first link on the page
Use this if there was no HTML ID attribute assigned to an element
document.collection.id
id is the HTML ID attribute of the object
HTML: <a href=“a.htm” id=“B”>Link</a>
DOM reference (using IE): document.all.B
document.collection[“id”]
DOM reference: document.all[“B”]

64. Addressing blocks by ID
Assume tag <div id=“X”> … </div>
IE:
document.all.id
Ex: document.all.X
or just id
Ex: X
Mozilla/Netscape 6+: find by ID, ex:
handle = document.getElementById("X");
// Now use the handle variable to access properties

65. Cross platform code
Sometimes you have to write one set of code that will work for both browsers
Here is a sample technique:
var isNS = false;
var isIE = false;
if (!(document.all)) isNS=true;
if (document.all) isIE=true;
if (isNS) {
el = document.getElementById("X");
el.setAttribute("style", "background-color:pink;"); }
if (isIE)
document.all.X.style.backgroundColor="pink";

66. Events Most browser DOM objects have “events” associated with them
Recall Javascript objects include windows, frames, forms, form fields, links, etc.
An event is Javascript code that can be triggered when something happens to a Javascript object
Example: clicking on a hyperlink
Example: leaving a form field

67. Events
Event code is usually invoked through HTML “Javascript only attributes”
They are not “HTML” attributes
They are not in the HTML specification
However, if Javascript is enabled and the event is valid for the Javascript object then associated code is executed.
Example:
<form name=“test” onsubmit=“return validate();”>

68. Events
Some events will behave differently depending on whether the code associated with the event returns a true or a false value
Example: If the form onsubmit event returns false, the form is not submitted
All events start with “on”, ex: onclick, onsubmit, etc. This makes them easy to distinguish

69. this
“this” is a shortcut that can be used to refer to the current object
It is useful in event handlers to pass properties of the object to generic functions
This example passes the entire text object named “month” to the function checkrange()
<input type="text" name="month" size="2" onblur="checkrange(this, 1, 12);” />
Otherwise would have to code something
like document.form1.month!

70. this Example
This function embedded in the <head> tag processes the example on the previous page. Note that field can be any Javascript object which has a numeric value attribute
function checkrange (field, min_value, max_value) {
if ((field.value < min_value) || (field.value > max_value)) {
alert("The field value must be at least " + min_value + " and may not be greater than " + max_value + ".");
return false; }
else
return true;

71. onabort Event Action to occur when the user aborts loading an image
Occurs when the browser STOP button is pressed or clicks on an image before it is loaded
By default nothing happens which is out of the ordinary
Example:
<img src=“myimage.gif” onabort=“alert(‘User decided not to wait for image to load!’);” />

72. onblur Event
Occurs when the user leaves a form field (either by clicking outside the form field or pressing the tab key)
onblur and its cousin onchange are very popular for validating form fields
Example:
<input type=“text” name=“LastName” size=“30” onblur=“checklength(this, 5, 30);” />

73. onchange Event
Occurs when the value of a form field is changed by the user and loses focus, or when a new choice is made in a select element
Example:
<input type=“text” name=“lastname” size=“30” onchange=‘confirm(“Are you SURE you want to change your last name?”);’ />

74. onclick Event
Occurs when user clicks on a clickable form control or hyperlink with a mouse
Example:
<a href=“ onclick=‘alert(“You are about to go to Yahoo!”;)’>Go to Yahoo!</a>

75. onerror Event Occurs when a document or image fails to load properly
Example:
<img src=“myimage.gif” onerror=“alert(‘The image is corrupt! Please notify the Webmaster!’)” />

76. onfocus Event
Occurs when a window or form field is made active by moving the cursor into the field or clicking on the object
Example:
<input type=“text” name=“lastname” size=“30” onfocus=‘alert(“You have selected the field named “ + this.name);’ />

77. onload Event Action to take when the page has finished loading
onunload is similar and occurs when a document is unloaded
Example:
<body onload=‘alert(“The document has finished loading. You may begin working on this page now.”);’ />

78. onmouseover Event
Occurs once each time the mouse pointer moves over an object or area from outside that object or area
Example:
<a href="    onmouseover="window.status='Click this if you dare!'; return true;"> Click me</a>

79. onmouseout Event
Occurs each time the mouse pointer leaves an area (client-side image map) or link from inside that area or link
Example:
<a href="    onmouseout="window.status=‘You missed a fabulous opportunity for savings!’; return true;"> Click me</a>

80. onsubmit Event Can be used to prevent a form from being submitted
Commonly used with a form validation logic
Must put the word “return” in front of your code. Anything other than a false value will let the form submit
A common mistake is to instead attach logic to the onClick event of a submit button. This won’t keep the form from submitting!
Example:
<form name=“myform" action=“ onsubmit=“return validate_form(this);”>

81. onreset Event Occurs when a reset button on the form is pressed
Event only works in the form tag
A common mistake is to attach logic to the onclick event of a reset button. This won’t keep the form from resetting!
Example:
<form name="form1" action=“ onsubmit=“return validate_form(this);”> onreset="alert(‘Defaults have been restored.');”>

82. onselect Event
Occurs when a user selects some of the text within a text or textarea field
Example:
<input type="text" value="" name="valuefield" onselect=“alert(‘onSelect event was triggered!’);” />

83. Object Event Handlers Button - onclick Check Box - onclick
Document - onload, onunload, onerror
Form - onsubmit, onreset
Frames - onblur, onfocus
Hyperlink - onclick, onmouseover, onmouseout
Image - onload, onerror, onabort

84. Object Event Handlers Image Hot Spot - onmouseover, onmouseout
Input Box - onblur, onchange, onfocus, onselect
Radio Button - onclick
Reset Button - onclick
Selection List - onblur, onchange, onfocus
Submit Button - onclick
Text Area Box - onblur, onchange, onfocus, onselect
Window - onload, onunload, onblur, onfocus

85. Emulating Events
There are a number of methods for Javascript objects that let you pretend to be executing an event:
click(), clear(), reset(), submit(), blur(), close(), focus() and select()
These can be useful, for example, to force the focus back into a particular field, use the focus() event
Use with care, particularly focus() events because they can cause loops that will hang your browser!

86. Warnings JavaScript is a big, complex language
We’ve only scratched the surface
It’s easy to get started in JavaScript, but if you need to use it heavily, plan to invest time in learning it well
Write and test your programs a little bit at a time
JavaScript is not totally platform independent
Expect different browsers to behave differently
Browsers aren’t designed to report errors
Don’t expect to get any helpful error messages

87. References W3 Schools JavaScript Tutorial Several Online Presentations
Several Online Presentations

88. Reading List W3 Schools JavaScript Tutorial
Voodoo's Introduction to JavaScript