JavaScript: Browser Programming

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JavaScript: Browser Programming

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Title: JavaScript: Browser Programming

1
JavaScript Browser Programming

Marek Podgorny, EECS, Syracuse University, and
CollabWorx

2
Outline

Java vs. JavaScript, Interpreters vs. Compilers
JavaScript Basics and Syntax
JavaScript and Document Object Model
JavaScript Methods
JavaScript and HTML reflection

3
General Remarks

JavaScript is a web scripting language for
browsers
Invented by Netscape and innovated by Microsoft
Standardized as ECMA language
Netscape server-side JavaScript exists (Livewire)
but obsolete
JavaScript facilitates rapid prototyping of Web
pages and it takes load off the the server
Particularly useful for processing HTML forms
JavaScript is also enabler of DHTML
References JavaScript, The Definitive Guide by
David Flanagan (OReilly, 1998) The JavaScript
Bible by Danny Goodman (IDG, 1998) Dynamic HTML,
The Definitive Reference by Danny Goodman
(OReilly, 1998).

4
Why would you use JavaScript?

Typically JavaScript is invoked from HTML
HTML text and JavaScript text are intertwined
JavaScript allows you to modify Web page after it
was downloaded form server
If you need to make a dynamic page from C,
Perl, or Java you must write the HTML from these
languages
You can invoke applets from HTML but HTML and
applet actions are (typically) not linked
HTML and JavaScript are closely linked
JavaScript can be seen as API to the browser
This is since JavaScript can interact with
browser objects

5
Java vs. JavaScript I

Suns Java is fast becoming a broad industry
standard
Java is well designed and documented
Java is object-oriented with (single) inheritance
Java is class-based

JavaScript is primarily supported by Netscape and
Microsoft
JavaScript started quite roughly but is
improving in design -- however some issues
connected to browser are totally ill defined as
browser ill defined -- e.g. what happens in what
order when frames loaded .
JavaScript is object-based with no class
structure
JavaScript is prototype-based

6
Java vs. JavaScript (2)

Java classes and instances are distinct
A Java class has zero or more constructors
Java property inheritance follows class hierarchy
In Java, no way to add properties at run-time

JS object definition and constructor are
identical and defined like methods
JavaScript property inheritance follows prototype
chain
JavaScript properties may be added or removed at
run-time
So essentially in JS, class structure is totally
dynamic (a.k.a. Ill defined)

7
Java vs. JavaScript (3)

Java applets are distinct from HTML
Java is strongly typed with static (compile-time)
binding
Java bytecode is interpreted (or Just-In-Time
compiled) on the client

JavaScript is tightly coupled with HTML
JavaScript is loosely typed with dynamic
(run-time) binding
High-level JavaScript source code is interpreted
on the client but is often MUCH faster than Java
as integrated into browser

8
Java vs. JavaScript (4)

Java applet deployment cycle
Java source --gt javac compiler --gt JavaVM
Universal Machine code in .class file
Store JavaVM .class files on Web Server
Download JavaVM from Server to Client
Interpreter built into browser, reads JavaVM and
executes on client
Java event handlers interpret events within
applet
JavaScript cycle
JavaScript Source is included in HTML text or
special .js files included in HTML files
Combined JavaScript and HTML is downloaded and
interpreted by browser on client to produce HTML
Page
Events in HTML Page (mouse clicks etc.) are
either interpreted by browser default or
overridden by user JavaScript code

9
JavaScript Version History

JavaScript 1.0 debuted in NN 2.0 (Feb 96)
JavaScript 1.1 appeared in NN 3.0
NN 4.0 (a.k.a. Communicator) supports JavaScript
1.2
MSIE 3.0 introduced JScript 2.0, a subset of
JavaScript 1.1
JScript 3.x is supported in MSIE 4.0
JScript 3.0 and later from IE are ECMA-compliant
Standard ECMA-262, June 1997 as is JavaScript
1.1 and following versions from Netscape
JavaScript 1.3 and 1.4 released with modest
changes including better exception handling and
Java-JavaScript linkage

10
JavaScript 1.2

Version 1.2 is the first one worth using
New features in JavaScript 1.2 (NN4.0)
Event object and event capturing
Ten new event handlers
Nested functions new Number() and String()
functions
New statements switch and dowhile
Many new methods and properties (especially for
Window, String, and Array objects)
Perl4-compatible regular expressions
Signed script security model

11
JScript 3.1

JScript 3.1 was released with MS IE v. 4.0
Conditional execution (meta-statements)
New event handlers (39 in all!)
New statements switch and dowhile
Two new identity operators ( and !) which
insist on variable types matching (default is to
convert types in and !)
Perl4-compatible regular expressions
Debugger support
Adequate documentation is lacking, however

12
ECMAScript

ECMAScript refers to standard ECMA-262 released
in June 1997
Netscape site has ECMAScript standard document
ECMAScript is essentially a definition of
JavaScript as programming language, without
reference to DOM
Meaning is the language and not the browser AWT
handler
Both Netscape and Microsoft versions are
ECMA-compliant but nevertheless mutually
incompatible
The HTML and emerging XML Document Object Models
(DOMs) will be bound initially to ECMAScript
Hence, one can use ECMAScript in several
different domains (HTML, XML, VRML ) by adding
support for appropriate domain specific DOMs

13
JavaScript Basics
14
JavaScript and HTML

ltSCRIPT language"javascript"gt
document.write("This document was last modified
on ")
document.write(document.lastModified)
lt/SCRIPTgt
This example illustrates a few key points
JavaScript source code can be placed inline with
HTML code, contained within the ltSCRIPTgt
and lt/SCRIPTgt tags
JavaScript uses the dot (.) character for object
structures
An object may have properties (document.lastModifi
ed) and methods (document.write)
JavaScript has many built-in variables and
functions for interacting with web page content.

15
JavaScript and HTML

Besides scripts, JavaScript code can be placed in
HTML tags to define event handlers
ltFORMgt
ltINPUT type"button" value"Push Me"
onClick"alert(Thank you very much)"gt
lt/FORMgt
This HTML creates a button in a form
The mouse click event handler for the button is a
single JavaScript statement that displays an
alert box with a message
The statement could also be a call to a function
you wrote to provide a more useful response

16
JavaScript URLs

ltA HREF"JavaScriptJumpTo(dest)"gt Go now! lt/Agt
This HTML creates a link that looks like a normal
hyperlink
Clicking it runs a JavaScript function called
JumpTo, and passes it an argument dest which is
presumably a variable that was set by a previous
button click or other interaction with the user
The JavaScriptsomeStatement notation can be used
anywhere a normal URL can go
Try interactive console!

17
JavaScript URLs

A trivial example of a JavaScript URL
isjavascriptalert("Hello World!")
A JavaScript URL may appear anywhere an ordinary
URL is expectedltA HREF"javascripthistory.back(
)"gtPrevious Pagelt/Agt
Note must use ltA HREFjavascript
void(anyoldfunction())gt if you do not want link
to be invoked!
Use HREFjavascript void(0) if no action at
all -- used if real action from onclick or other
event handle for link
Navigator even has a mini-scripting environment
invoked by typing javascript into the browsers
location text field
This is used to display error messages as a
console instead of alternative (and clumsy)
stream of windows (one per error .)

18
Loading scripts from files

ltSCRIPT SRC"http//mywebsite.com/myfile.js"gt
alert("Error loading script file.")
lt/SCRIPTgt
Please note the alert function that only fires if
browser fails to execute the script
Good methodology to store reusable scripts
An external file must have a .js extension and
the server must be configured properly

19
The ltSCRIPTgt Tag

If you have two versions of a JavaScript script,
you can load one or the other
ltSCRIPT LANGUAGE"JavaScript1.1"gt
// Ignored by Navigator 2.0 and higher
location.replace("newVersion.html")
lt/SCRIPTgt
ltSCRIPT LANGUAGE"JavaScript"gt
// Read by Navigator 2.0 and above
lt/SCRIPTgt

20
The ltSCRIPTgt Tag (contd)

Multiple ltSCRIPTgt tags are allowed
Within a single ltSCRIPTgt, a function may be
called before it is defined
Within a single document, a function defined in
one ltSCRIPTgt may not be called from a previous
ltSCRIPTgt
In particular, a function defined in the ltBODYgt
may not be called from the ltHEADgt

21
Data types and declarations

Numbers, strings, true, false, null, and
undefined values are known as primitive values
Anything else is an object!
Assigning a primitive to a variable makes a copy,
assigning an object makes a reference!
Some objects return primitive values those that
dont, return the object string

22
Variables

Variable is an entity containing data
Variable name must start from a letter or _
JavaScript is case-sensitive! but has no fixed
type.
Variable scope
A variable declared inside a function definition
using the var keyword, is local to that function
Variables that are not declared inside a
function, or that are declared without the var
keyword, are global!
A helpful practice is to place all global
variable declarations in the ltHEADgt section of
the HTML
One can reference variables in other windows via
win.var syntax

23
Arrays

Array type is provided for ordered sets of data
Array indexes are represented by expressions in
square brackets, such as item50
The Array object provides a number of methods for
rearranging and converting arrays
a new Array(10)
regionNamenew Array("North", "Central", "South")
regionName.length 3
row1 new Array(3)
row2 new Array(3)
row3 new Array(3)
grid new Array(row1, row2, row3)

24
Arrays with String Indexes

var person new Array()
person"firstName" "Mary"
person"lastName" "Jones"
person"zipCode" 12345
Arrays with numerical and string indexes are
treated separately! person.length 0
personfirstName is equivalent to
person.firstname
Ergo, associative arrays are actually objects!

25
Objects, Properties Methods

In JavaScript, an object is a structured set of
data items, called properties, and code
(functions), called methods
var person new Array()
person.firstName "Mary"
person.lastName "Jones"
person.age 34
Internally, there is no difference between an
object and an associative array
However, it is rather inelegant to do it this way

26
Constructors

A clearer approach is to write a constructor
function for the object, and use the constructor
with the new operator
function person(first, last, yrs)
this.firstName first
this.lastName last
this.age yrs
var Mary new person("Mary", "Jones", 34)
var Bob new person("Bob", "Lee", 27)

Note use of this to access the object by which
function is called!
Name of the class this is as close as
JavaScript gets to the concept of classes
27
On the fly constructors

var girl firstName"Sally", lastName"Forth",
age16
These expressions can be nested, as in
var family
dadfirstName"George", lastName"Jones",
age58,
mom firstName"Martha", lastName"Forth",
age42,
numKids 2.7

28
Built-in object types (DOM!)
All these object types can be used in constructors
29
External Object

External object types cannot be used in
constructors
Browser provides these objects as needed,
either as a result of reflection process, or the
objects are simply there
Anchor, Applet, Area, Button, Checkbox,
FileUpload, Form, Frame, Hidden, Link, MimeType,
Password, Plugin, Radio, Reset, Select, Submit,
Text, Textarea
We will discuss some of these deeper

30
Values References

Unlike primitive values, objects are manipulated
by references
A reference is a pointer or label that specifies
where some actual values are stored
var p1 new person("George", "Smith", 42)
p2 p1
p2.firstName "Martha"
p2.age 30
p1 and p2 now refer to the same object, and both
now refer to information about Martha!
To create an actual new object, you must use the
new operator with a constructor function

31
More about objects

Objects behave differently from primitive values
when used as function arguments
if an argument is an object, a function can
modify its properties
function nextYear(p)
p.age
This code will actually increment person's age in
the object
Deleting Objects
An object can be deleted by setting any variables
that reference it to null
To delete specific properties of an object or
elements of an array, you can use the delete
operator

32
Properties as objects

A property of an object can itself be an object
The built-in document object contains a large
number of properties pertaining to contents of a
web page
Many of these properties are arrays or objects.
If your web page contains a form named info, you
could refer to it in a script as
document.forms.info
Hence, a property of an object can be either a
primitive value or another object, and the
objects can be nested to an arbitrary depth

33
Properties as objects

Constructor definition
function job(what, who, yr, mo, dy)
this.title what
this.person who
this.startDate new Date(yr, mo, dy)
Object creation
var Joe new person("Joe", "Zeelane", 46)
project1 new job("Report", Joe, 97, 06, 13)
Resulting object properties
project1.person, project1.startDate
And primitive values properties
project1.person.firstName

34
Adding properties to objects

One can add properties to existing objects by
using the special prototype property
available for all objects that are created with
the new operator, including built-in object types
as well as any constructors defined by programmer
person.prototype.gender ""
This statement adds a new property called gender
to the person object
All instances of this new property will be set to
the specified value
The gender property will be added to person
objects that have already been constructed, as
well as to any that are created later

35
Methods

JavaScript methods are functions that are
associated with an object. Examples
a.reverse()
reverses the order of the elements of an array
string s a.join(")
forms a string by combining an array's elements,
separated by vertical bars
Methods are always listed at the end of object
definition, and followed by (possibly empty) list
of arguments in parentheses

36
Creating a method

function personText()
var s
this.firstName" this.lastName
"("this.age")"
return s
To make this function a method of person, add a
line to your constructor function to assign it
function person(first, last, yrs)
this.firstName first
this.lastName last
this.age yrs
this.formatted personText
And use it as follows
document.write(Joe.formatted())

37
Universal Methods

There are three methods that are available for
all objects
eval evaluates a string as JavaScript code in the
context of the calling object, and returns its
value
toString returns a string representing a numeric
value in a specified radix
valueOf returns the primitive value of an object
Two other methods are available for large numbers
of objects
constructor() is available for all objects
created by constructor functions. It returns the
function that created an object
handleEvent() is available for all objects that
have event handlers. It calls an event handler
for a specific event

38
JavaScript Entities

One can parameterize HTML using
document.write(ln) commands such as
document.writeln(string1 Jsvariable
string2)
There is another way of achieving this without a
ltSCRIPTgt tag and no document.writeln -- namely
ltIMG widthJSvar1 srcJSvar2 gt
where JSvar1 and JSvar2 are JavaScript variables
holding width and URL of image
Syntax is modeled on that for special characters
where
GT is gt etc.
Such JavaScript Entities can only appear on right
hand side of attributes values and cannot appear
in HTML text such as titles
document.writeln approach has no such
restriction!

39
Javascript vs. windows

Every script runs in a window
The window object is the "outermost" object in
the JavaScript hierarchy
All variables and functions declared in scripts
are actually properties and methods of the window
a variable x is actually window.x
The symbol self is equivalent to window
Each window object corresponds to one browser
window on the host computer's screen
It is possible to communicate between windows be
referring to or setting other windows variables

40
Inter-window communication

To open additional windows
var win2 open("someFile.htm", "newWin",
features)
This opens a new browser window, and returns a
new window object in the variable win2
The new window can now be accessed
win2.document.write(someText) win2.toolbar.visibl
e false win2.document.close()
Scripts in win2 can access the original window
opener.win2status "ready
Or re-write original window contents
opener.location newtext.html

41
Javascript vs. documents

Every window has a document property which
manages all text, graphics, and other data
displayed in the window
A document object has properties that reflect the
HTML elements in the window, like the arrays
document.links
document.images
document.applets
document.embeds
document.forms

42
The Image Object

The JavaScript Image object allows scripts to
perform some basic operations on images stored in
file
Image objects are created automatically when
browser encounters ltIMGgt tags in a document
Scripts can create Image objects and assign them
to variables with the new Image() constructor
These objects are not displayed on the screen,
but they can be loaded with images from files and
later copied to other images that are in the
document
Allows you to change an image quickly in response
to user actions
You can create simple animation by rapidly
sequencing through a series of images

43
Input/Output

JavaScript provides several types of dialog boxes
that can be used with a single statement
These dialogs are all available through methods
of the window object
alert(mes) creates an alert box that displays the
specified string
confirm(mes) creates a confirm box that displays
the specified string and two buttons
Returns true if the user clicks OK, or false if
the user clicks Cancel
prompt(mes, default) creates a dialog box
displaying string message, a text box and two
buttons
The default string is initially displayed in text
box the user may enter other text
If the user clicks OK, prompt returns a string
containing the contents of the text box. If the
user clicks Cancel, prompt returns null

44
Forms

HTML forms are a very powerful way to interact
with the user of a web page
Originally intended for sending information to
servers, but use of JavaScript allows browser to
manipulate forms locally in browser
Browser creates Form objects corresponding to all
HTML ltFORMgt tags and stores them as elements of
the document.forms array
If the ltFORMgt tag has a NAME attribute, the Form
object can be referenced by its name as a
property of document
ltFORM NAME"userData"gt can be referenced as
document.userData

45
Forms 2

A form contains interactive elements such as
buttons and text boxes
Browser creates objects for each type of form
element and stores them in elements array
They can be referenced as elements0,
elements1, and so forth
If the elements' HTML tags have NAME attributes,
the objects can be referenced by their names as
properties of the form

46
Forms 3

ltFORM NAME"userData ACTIONprocessForm.cgigt
First nameltINPUT TYPE"text NAME"first"gt ltBRgt
Last nameltINPUT TYPE"text" NAME"last"gt ltBRgt
Address ltTEXTAREA NAME"addr" ROWS"4"
COLS"32"gtlt/TEXTAREAgt ltPgt
ltINPUT TYPE"button" NAME"btn" VALUE"Accept
data" ONCLICK"processData(this.form)"gt
ltINPUT TYPE"reset" NAME"rst" VALUE"Start
over"gt
ltINPUT TYPEhidden NAMEfngt lt/FORMgt
The first text box could be referenced by any of
this
document.userData.first
document.userData.elements0
document.forms0.first
document.forms0.elements0

47
Forms vs. JavaScript

Form objects and all their properties and
elements can be referenced by any statements in a
script
Some properties are read-only, but many can be
modified
Options can be added to or removed from a select
box
additional Option objects can be created with the
new Option() constructor

48
Event handlers in forms

processData has been defined as onclick event
handler for one of the buttons
processData() can manipulate the Form object and
its elements
function processData (fo)
fullName fo.first.value" "fo.last.value
fo.fn.value fullName fo.submit()
Before the form is submitted to server, the
function calculates full name and passes it on
along with values of other elements

49
Types of events

onsubmit onreset
onchange onclick ondblclick
onkeydown onkeypress onkeyup
onmousedown onmousemove onmouseout onmouseover
onmouseup
onabort onerror onload onunload
onmove onresize onblur onfocus ondragdrop
etc.

50
The event object

JavaScript can pass an event object to event
handlers
event object contains a number of properties that
provide detailed information about the event,
such as type of even, mouse button pressed, event
coordinates, window size after event, etc
document.links2.onclick reportEvent.
function reportEvent(ev)
alert("Event of type " ev.type
" occurred at X" ev.pageX
", Y" ev.pageY)

51
HTML Reflection

JavaScript built-in variables are initialized as
the web page is loaded
Many of these variables are copies, or
reflections, of the attribute values of various
HTML tags
Example document.bgColor is a reflection of the
bgcolor attribute of the ltBODYgt tag.
These built-in variables reflect much of the
content of the web page, allowing scripts to read
and, in some cases, modify this information
A script can change the web page background color
with a statement like
document.bgColor "red"

52
HTML Reflection

A more powerful example is
document.tags.H1.color "blue"
This changes the color of all ltH1gt headings on
the web page
This looks a little like what style sheets can do
is it the same thing?
No. It could be used as a stylesheet, but, in
addition to this, the statement above can be
executed at runtime to dynamically change all H1
headers to red.

53
HTML Reflection

ltFORMgt
Enter a number
ltINPUT TYPE"text" NAME"num1" gt ltPgt
Enter another number
ltINPUT TYPE"text" NAME"num2" gt ltPgt
ltINPUT type"button" value"Add the numbers"
onclick"addFields(this.form)"gt ltPgt
The sum is ltINPUT TYPE"text" NAME"result"
gt
lt/FORMgt
This is actually handled by
function addFields(form)
var sumeval(form.num1.value)
eval(form.num2.value)
form.result.value sum

54
Form example (again..)

This example is actually highly non-trivial!
this refers to the object being reflected, i.e,
the button!
form is property of every element of the form
that returns the Form object (note uppercase)
Form is an external object reflecting an HTML
form in a document
Hence, this.form can be passed to a function
which, in turn, can access all properties and
methods of the HTML form form which the function
was called

55
Some examples.
56
Hello World Example

ltHTMLgt
ltHEADgtltTITLEgtAn Examplelt/TITLEgtlt/HEADgt
ltBODYgt
lt!-- Insert HTML here --gt
ltSCRIPT LANGUAGE"JavaScript"gt
document.writeln("ltH1gtHello World!lt/H1gt")
lt/SCRIPTgt
lt!-- Insert more HTML here --gt
lt/BODYgt
lt/HTMLgt

57
Hello World Example (contd)

There is only one JavaScript statementdocument.w
riteln("ltH1gtHello World!lt/H1gt")
The writeln method writes its argument into the
current document
The script is in the ltBODYgt since it writes HTML
code into the document other scripts are written
in the ltHEADgt so they are processed and available
in body
In advanced applications, when JavaScript object
is actually defined becomes tricky and is an
example of how there are intrinsically undefined
features in the language
Scripts may be hidden from old browsers using
ugly constructltSCRIPT LANGUAGEJavaScript
gtlt!-- A comment to a dinosaur-- gt Bunch of
Exciting JavaScript Statements ..//script ends
here --gtlt/SCRIPTgt

58
Form Example

ltHTMLgt
ltHEADgtltTITLEgtJavaScript with Formslt/TITLEgt
ltSCRIPT LANGUAGE"JavaScript"gt
function compute()
if ( window.confirm("Is this what you want?") )
aForm.result.value eval(aForm.expr.value)
lt/SCRIPTgtlt/HEADgt
ltBODYgtltFORM NAME"aForm"gt Enter expression
ltINPUT TYPE"text" NAME"expr" SIZE15gt
ltINPUT TYPE"button" VALUE"Compute!"
onClick"compute()"gt ltBRgt Result
ltINPUT TYPE"text" NAME"result" SIZE15gt
lt/FORMgtlt/BODYgt
lt/HTMLgt

59
Form Example (contd)

confirm is a method of the Window object which
pops up a window requesting confirmation of
requested action
eval is a built-in JavaScript function
onClick is a JavaScript event handler and onClick
your stuff executes JavaScript code yourstuff
Note the user-defined names (aForm, expr, result)
referred to in the script

60
Example of Parameterized HTML

ltHTMLgtltHEADgtltTITLEgtJavaScript Used to
Parameterize HTMLlt/HTMLgt
ltSCRIPT LANGUAGEJavaScript gtvar
imagewidth600// These could be changed by form
var imagefilemy.gif //input or some
computation // based on size of window
(available to JavaScript)lt/SCRIPTgtlt/HEADgt
ltBODYgtlth2gtPlain old HTML of any typelt/h2gt
ltSCRIPT LANGUAGEJavaScript
gtdocument.writeln(ltimage aligntop width
imagewidth src imagefile
gt)lt/SCRIPTgt
ltbgtAnd the beat goes on lt/bgt
lt/BODYgtlt/HTMLgt
Note the horrible mix of JavaScript and HTML).