Learn javascript easy steps

JavaScript - a Brief Introduction

IMI PRINCE
www.newsvelley.blogspot.com

What is JavaScript

• Object based (not object oriented)
programming language
– very limited object creation
– a set of pre-defined objects associated with
• HTML document structure
– many HTML tags constitute JS Objects
• Browser functionality
– provides a limited API to Browser functionality

Where did it come from
• Originally called LiveScript at Netscape
– started out to be a server side scripting language for
providing database connectivity and dynamic HTML
generation on Netscape Web Servers
– Netscape decided it would be a good thing for their
browsers and servers to speak the same language so it
got included in Navigator
– Netscape in alliance w/Sun jointly announced the
language and its new name Java Script
– Because of rapid acceptance by the web community
Microsoft forced to include in IE Browser

Browser compatibility

• For the most part Java Script runs the same
way in all popular browsers
• There are many areas where there are slight
differences in how Java Script will run
• there will be a separate set of slides
addressing these differences.

JavaScript…Java ?

• There is no relationship other than the fact
that Java and JavaScript resemble each
other (and C++) syntactically
• JavaScript is pretty much the de-facto
standard for client-side scripting (Internet
Explorer also provides VBScript & JScript)
• In Netscape browsers there is an API that
allows JavaScript and Java applets
embedded in the same page to converse

What can it be used for

• Some pretty amazing things….
– Text animation
– graphic animation
– simple browser based application
– HTML forms submission
– client-side forms data validation (relieving the
server of this task)
– web site navigation

What do I need to get started

• A web browser
– Netscape Navigator 4.x or later
– MS Internet Explorer 3.x or later
• A text Editor
– Wordpad/Notepad
– Vi, Emacs

Process

• Open your text editor
• create a file containing html and Javascript
• save as text file with file type .htm or .html
• open your browser
• click on file, open file
– locate the file you just created
• open file in browser

Putting JavaScript into your HTML

• in an external file
– collect commonly used functions together into
external function libraries on the server
• added benefit of privacy from curious users
• in-line with your HTML
• as an expression for an HTML tag attribute
• within some HTML tags as Event Handlers

<SCRIPT>…</SCRIPT>
• <SCRIPT language=…. src=……></SCRIPT>
• The <SCRIPT> tag indicates to the browser the
beginning of an embedded script; </SCRIPT>
indicates the end of an embedded script.
• the “language” attribute indicates the script
processor to be used
• the “src” attribute indicates the URL of a file on
the server containing the script to be embedded

Scripts

• Since scripts are placed in line with with
HTML older browsers will attempt to
render them as text.
• To preclude this hide them in side of an
HTML comment . 
• for JavaScript comments use // or /* */

<SCRIPT>
• <SCRIPT LANGUAGE=“JavaScript”>
• 
• </SCRIPT>

Object Hierarchy

window

history document location link

link anchor layer form applet image area

text radio button fileUpload select

textarea checkbox reset option

password submit

Objects
• window - the current browser window
• window.history - the Netscape history list
• window.document - the html document currently in the
browser client area
• window.location - the browser location field
• window.toolbar - the browser toolbar
• window.document.link - an array containing all of the
links in the document
• window.document.anchor - an array of all the anchor
points in the document

Objects (more…)
• Window.document.layer - a named document layer
• window.document.applet - a named java applet area
• window.document.image- a named image tag
• window.document.area - a named area
• window.document.form - a named form or the default form
• ect, ect

A few examples...
• window.location = “http://www.yahoo.com”
– will take you to the specified URL (like a goto)
• window.history.back()
– back() is a method on history
– will be like clicking the back button in Nav 3
– in Nav 4 will take you back to prev window
• window.history.goto(1)
– takes you back to first URL in history array

The Object Model

• It is very important to understand the object
model
• each object has its own properties, some of
which are read only some of which you can
set directly by assignment (as location)
• each object also has a set of behaviors
called methods

Object Model

= Text Object
HTML text tag
B l u r ()
defaultValue
form
name Select()

type
focus() value Red - gettable and settable

handleEvent

Object Event Handlers
• Most objects respond to asynchronous, user generated
events through predefined event handlers that handle the
event and transfer control to a user written event handling
function
• Each object has particular events that they will respond to
• the way you specify an event handler is by adding an
additional attribute to the HTML tag that specifies the
particular handler
• <input name=bt1 type=button value=ok onClick=“acb();”>

Events
• onAbort
• onBlur
• onChange
• onClick
• onError
• onFocus
• onLoad
• onMouseOut
• onMouseOver
• onReset
• onSelect
• onSubmit
• onUnload

onAbort

• Activated when a user aborts the loading of
an image

<img name=ball src=images/ball.gif onAbort=“alert(‘You missed a nice picture’)”>

onBlur

• Used with frame, select, text, textarea and
window objects
• invoked when an object loses the focus
• use with select, text and textarea for data
validation

onChange

• Used with select, text and textarea objects
• use instead of onBlur to validate only if a
value has changed

<form>
Color: <select onChange=“processSelection()”>
<option value=“R”>Red
<option value=“G”>Green
<option value=“B”>Blue
</select>
</form>

onClick

• Used with button, checkbox,link, radio,
reset, and submit objects.

<input type=button name=btn1 value=“Click Me” onClick=“alert(‘button was clicked’;” >

onError

• Used with image and window objects to
invoke a handler if an error occurs while an
image or window is loading.
• Setting window.onerror = null will prevent
users from seeing JavaScript generated
errors

onFocus

• Used with frame, select, text, textarea and
window objects.
• Just the opposite of onBlur; i.e. invoked
when the object gets focus.

<body bgcolor=“lightgrey” onBlur=“document.bgColor=‘black’ onFocus=“document.bgColor=‘white’” >

onLoad
• Used with window, frame and image objects (use
with <body ….><frameset ….> and <img ...>)

<img name=spinball src=images/spinball.gig onLoad=“startAnimation(this)”>

onMouseOut and onMouseOver

• Used with area and link objects
• user moves mouse off of an area or link
<map name=flower>
<area name=top coords=“0,0,200,300 href=“javascript:displayMessage()”
onMouseOver=“self.status=‘when you see this message click your left mouse button’ ;
return true”
onMouseOut=“self.status = ‘’ ; return true”>

onReset

• Used with form objects

<form onReset=“alert(‘the form has been reset’)” >

onSelect

• Used with text and textarea objects
• run some JavaScript whenever a user
selects a piece of text in a text or textarea
object
<input type=text name=line onSelect=“showHelp()” >

onSubmit

• Use with form objects to run a handler
whenever a form has been submitted.
• Useful to validate all fields prior to actual
submission

onUnload

• Just like onLoad but the handler is run
when the window/frame is exited

<body onUnload=“cleanup()” >

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