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Programming on the Client Side: Javascript/Dynamic HTML How Return to Client Side (The Browser) So far we have been developing Programming on the Client Side: Static Web pages JavaScript/Dynamic HTML • Recap —- Perl/CGI: 569 Static Pages: 570 Server Side • Very nice. Essential to do hard work processing on the Internet (PHP similar) • • Often Adequate for the task in hand – Search Engine • Often useful and – Log Usage • – Gather Data/Stats Often entertaining or informative. • – Interface to Databases – Many more suitable applications .... !! !! "" "" ! ! " " Back Back Close Close Making Web Pages Dynamic Other Typical Uses of Dynamic Web Page Can replace some (but not all) CGI functionality? Detecting which browser accesses page • Customising HTML for browsers E.g. HTML form: • User Customised HTML Users enter data and submit it to the server • • 571 Opening new browser windows for new links 572 a CGI script is used to verify and validate that data. • • Messages and confirmations, Alerts, Status Bar passes data across the network — slow. • • Writing to Frames • Rollover Buttons, Moving Images How much more interactive could a site • Multiple pages in a single Download be if data were checked by the browser and • ..... Many More any error messages originated locally? • !! !! "" "" ! ! " " Back Back Close Close Some More Definitions: Dynamic HTML and Document Object The Document Object Model (DOM) Model DOM: Before delving into the programming there are two more topics to The key element of DHTML is probably the document object introduce. • model. The Document Object Model (DOM) : 573 DOM makes everything on the page into an object 574 • Scripts can only manipulate objects on the page because of Elements can be manipulated by programmed scripts. • DOM • This was developed by the World Wide Web Consortium (W3C) • Limited Cross-browser Support — neither of the big players • yet adheres fully to it. Dynamic HTML (DHTML) : A combination of content formatted using HTML, cascading • style sheets, a scripting language and the DOM. !! !! "" "" ! ! " " Back Back Close Close DOM of an HTML Document Example HTML DOM The DOM model of an HTML document is a hierarchical structure The relationship between some HTML code and the DOM which might reasonably be represented as a tree. <html> Does NOT imply that DOM software must manipulate the <head> • document in a hierarchical manner, <title>TITLE OF PAGE</title> 575 576 </head> It is simply a representation. • <body> <p>Some text...</p> <ul> <li>First <li>Second </ul> </body> </html> !! !! "" "" ! ! " " Back Back Close Close Example HTML DOM (Cont.) JavaScript JavaScript is NOT JAVA: The only similarity between the two languages is in their names!!! • Basic idea : a client-side browser language not as complicated as • 577 Java. 578 JavaScript and JScript Meant to be implementations of the same thing. • Not exactly the SAME!!!! • The DOM models, but does not implement: Objects that comprise a document • !! !! Semantics, behaviour and attributes of those objects "" "" • Relationships between those objects ! ! • " " Back Back Close Close What is JavaScript and What is it Used For? Pros and Cons of JavaScript JavaScript is: Advantages: Wide browser support i.e. Most Browsers speak a dialect. A fairly simple language • • Easy access to document objects and their manipulation Only suitable for fairly simple tasks • • No long download times w.r.t java or graphic animations Best suited to tasks which run for a short time 579 • 580 • No Plug-in support required Most commonly used to manipulate the pieces of the document • • Relatively Secure object model. • Disadvantages: Not standard support for Javascript across browsers esp. DOM • Web page useless if script does not work!! • Javascript may be disabled by browser reading HTML file no • control over this !! !! JavaScripts can run slowly "" • "" ! The version of JavaScript that was used as the basis of the ECMA ! " Script standard was 1.1. Therefore everything that is written using " Back JavaScript 1.1 should comply with the standard. Back Close Close Learning JavaScript Borrowing Code First Steps in JavaScript As with HTML (and Perl): Running Javascript A lot of code on Web. • 581 JavaScript can be run on some file and Web servers 582 All the JavaScript that your browser encounters is freely available • • MOST COMMONLY: front-ends for Web pages. to you. • – Part of HTML document (as we will see very soon) Therefore: – So View Source, or You do not compile it like other programs, Java, C, Pascal – Save as ... HTML will give you access to lots of Examples. • It is an interpreted language: It is a script • To Use: Simply embed JavaScript in you HTML web page One of the best resources on the web is The JavaScript Source at • http://javascript.internet.com !! !! "" "" ! ! " " Back Back Close Close A Simple Example 1 — Hello World The key points of JavaScript Programs The script that follows could hardly be easier. The JavaScript program is contained between the <script> .... </script> We’ll list the code first then explain what’s going on. • • The language attribute must be set to "javascript" i.e <html> • <head> <script language="javascript"> <title> Hello World JavaScript</title> 583 584 </head> JavaScript programs contain variables, objects and functions. • <body > Each line of code is terminated by a semi-colon; <script language="javascript"> • Blocks of code must be surrounded by a pair of curly brackets. A block of code • var str = "Hello World"; is a set of instructions that are to be executed together as a unit. This might be because they are optional and dependent upon a boolean condition or because window.alert(str); they are to be executed repeatedly; Functions have parameters which are passed inside parenthesis; document.write("<h1>" + str + "</h1>"); • Variables are declared using the keyword var; </script> • !! Scripts require neither a main function nor an exit condition. !! • "" Functions may be used. "" </body> ! • ! </html> " " Back Back Close Close Our First Hello World Example A Simple Example — Hello World with a Function Specifically for the Hello World Program: Let us now see how we use use funtions in JavaScript We create a variable str and store the string "Hello World" in Here wee also note that • it. You may freely write JavaScript in any place in your Web page. • We create an alert window with the message "Hello World". • 585 However if you wish to trigger actions from certain events you 586 The window object is used and we call the alert method • may wish to put your JavaScript in functions in the Head section (function) of this object. of the document. !! !! "" "" Figure 29: Hello World JavaScript Example ! ! We write() to the document model some HTML. " " • Back Back Close Close Hello World with a Function Hello World with a Function — Explanation The Hello World program called from a function is as follows: The operation of the program is essentially the same as the first version except that <html> We write a function <head> • <script language="javascript"> 587 588 init() function init(){ var str = "Hello World"; to do the job. window.alert(str); We call the function when the Web page loads with the • document.write("<h1>" + str + "</h1>"); onLoad="init()" } </script> attribute in the body tag. </head> !! !! <body onLoad="init()"> "" "" ! ! </body> </html> " " Back Back Close Close More Functions Popup Browser Information Example We can add more functions that get called on different events: The program also introduces some more objects and some useful applications of JavaScript: We call a function popup() when the Web page loads • We call a function farewell() when the page is unloaded: The navigator contains • • 589 data about the browser being 590 – The onUnLoad="farewell()" attribute is set. used The program finds the • version of the application appVersion and the Safari Version userAgent(). This information is displayed • in a pop up alert When you leave the Web !! • !! "" page or Refresh the page an "" alert Explorer Version ! different is brought ! " up " Back Back Close Close Popup Browser Information Example Popup Browser Information Example Code The full code for this example is: <html> <head> <script language="javascript"> 591 function popup(){ 592 var major = parseInt(navigator.appVersion); var minor = parseFloat(navigator.appVersion); var agent = navigator.userAgent.toLowerCase(); document.write("<h1>Details in Popup</h1>"); window.alert(agent + " " + major); } Figure 30: Browser Version Page Exit JavaScript Example (Internet function farewell(){ window.alert("Farewell and thanks for visiting"); Explorer Display Shown) } </script> !! </head> !! "" "" ! <body onLoad="popup()" onUnLoad="farewell()"> ! </body> " </html> " Back Back Close Close More Browser Information A Second Browser Information Example So far our examples have done everything inside JavaScript The program achieves a similar task as the previous example but in environments: a completely different way: You can also mix HTML and JavaScript The program an HTML TABLE (see later) to a Browser with the • • Browser’s details: You can also access JavaScript variables in the HTML 593 594 • Figure 31: Browser Version 2 JavaScript Example (Explorer) !! !! "" "" ! ! " " Back Back Close Close Browser Information Example 2: Javascript and HTML Mix Browser Information Example 2: HTML HEAD <HEAD> <SCRIPT> <!-- This script and many more are available free online at --> 595 596 Figure 32: Browser Version 2 JavaScript Example (Safari) <!-- The JavaScript Source!! http://javascript.internet.com --> <!-- Begin The program use
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