Java Script — a Brief Introduction Anupriya

Java. Script - a Brief Introduction Anupriya

What is Java. Script • 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 Live. Script 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.

Java. Script…Java ? • There is no relationship other than the fact that Java and Java. Script resemble each other (and C++) syntactically • Java. Script 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 Java. Script 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 Java. Script 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 HTML older browsers will attempt to render them as text. • To preclude this hide them in side of an HTML comment. <!---> • for Java. Script comments use // or /* */

<SCRIPT> • • <SCRIPT LANGUAGE=“Java. Script”>  </SCRIPT>

Object Hierarchy window history link anchor textarea password layer radio checkbox document form button reset submit location applet link image file. Upload select option area

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 () default. Value form name Select() type focus() value handle. Event Red - gettable and settable

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=bt 1 type=button value=ok on. Click=“acb(); ”>

Events • • • • on. Abort on. Blur on. Change on. Click on. Error on. Focus on. Load on. Mouse. Out on. Mouse. Over on. Reset on. Select on. Submit on. Unload

on. Abort • Activated when a user aborts the loading of an image <img name=ball src=images/ball. gif on. Abort=“alert(‘You missed a nice picture’)”>

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

on. Change • Used with select, text and textarea objects • use instead of on. Blur to validate only if a value has changed <form> Color: <select on. Change=“process. Selection()”> <option value=“R”>Red <option value=“G”>Green <option value=“B”>Blue </select> </form>

on. Click • Used with button, checkbox, link, radio, reset, and submit objects. <input type=button name=btn 1 value=“Click Me” on. Click=“alert(‘button was clicked’; ” >

on. Error • 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 Java. Script generated errors

on. Focus • Used with frame, select, textarea and window objects. • Just the opposite of on. Blur; i. e. invoked when the object gets focus. <body bgcolor=“lightgrey” on. Blur=“document. bg. Color=‘black’ on. Focus=“document. bg. Color=‘white’” >

on. Load • Used with window, frame and image objects (use with <body …. ><frameset …. > and <img. . . >) <img name=spinball src=images/spinball. gig on. Load=“start. Animation(this)”>

on. Mouse. Out and on. Mouse. Over • 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: display. Message()” on. Mouse. Over=“self. status=‘when you see this message click your left mouse button’ ; return true” on. Mouse. Out=“self. status = ‘’ ; return true”>

on. Reset • Used with form objects <form on. Reset=“alert(‘the form has been reset’)” >

on. Select • Used with text and textarea objects • run some Java. Script whenever a user selects a piece of text in a text or textarea object <input type=text name=line on. Select=“show. Help()” >

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

on. Unload • Just like on. Load but the handler is run when the window/frame is exited <body on. Unload=“cleanup()” >