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JavaScript Part 1

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JavaScript Part 1. Lecture Overview. JavaScript background The purpose of JavaScript JavaScript syntax. History Lesson. JavaScript is NOT Java Started at Netscape in 1995 Microsoft released its own version ' JScript " in 1996

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JavaScript Part 1

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  1. JavaScriptPart 1

  2. Lecture Overview • JavaScript background • The purpose of JavaScript • JavaScript syntax

  3. History Lesson • JavaScript is NOT Java • Started at Netscape in 1995 • Microsoft released its own version 'JScript" in 1996 • JavaScript is the default scripting language in .NET (VBScript has pretty much faded away)

  4. What do we do with JavaScript? • A starter list • Adds sizzle to pages (Animation) • Dynamic HTML (DHTML) • Client side data entry validation • Client side CGI • Reduce the load on overburdened servers • Process and manage cookies • Some servers are beginning to support JavaScript • AJAX to load parts of Web pages

  5. What is JavaScript? (1) • It’s a programming language that ‘closely’ resembles Java • Implementations • European Computer Manufacturers Association created ECMAScript to standardize different scripting versions See ECMA-262 • I’ll try to conform • There are others • It’s a “C ish” language

  6. What is JavaScript (2) • Most of what we do is access the object model supported by the underlying browser • The W3C defines the Document Object Model (DOM) • Differences do exist between browsers • I will try, where possible, to point out these differences • Most support the common ECMA standards though

  7. Creating a First Script • <script> tag appears in a <head> or <body> tag • type argument denotes that it’s JavaScript • Example: <script type=“text/javascript”> document.write(“hello”); </script>

  8. Creating a First Script script tag Script language <html> <body> <script type=“text/javascript"> document.write(“hello") </script> </body> </html> Script tag

  9. Script Placement (1) • A document may have multiple <script>blocks • Scripts in a <head> block • Create procedures here • Before or after the <style> section is fine • Scripts in a <body> block • Code executes as the page is rendered • Importing external script files • This is the recommended place to put generic functions

  10. Script Placement (2) • Scripts appearing in a <head> tag but outside a procedure execute first in the order they appear • More about procedures later • Code in a procedure is not executed unless explicitly called • Scripts appearing in a <body> tag execute as they are encountered • The placement has an effect on page rendering

  11. Handling Java Incapable Browsers • Include the <noscript>directive to display a message when JavaScript is disabled <html> <body> <script language="javascript"> document.write("Greetings") </script> <noscript> <p>JavaScript is not enabled.</p> </noscript> </body> </html>

  12. JavaScript IS CASE SENSITIVE

  13. JavaScript Semantics • Semicolons need not terminate statements although they do no harm • Unless two statements are placed on the same line • Variables • var data type is generic • JavaScript is not strongly typed like Java • Type conversion happens on the fly • Other types • number, boolean, string, function, object

  14. Creating a First Script (Example) • See JavaScriptExample1.htm • Pay particular attention to the order in which the script code executes

  15. Comments • Comments appear differently inside of JavaScript block that outside of a JavaScript block • The characters // on a line mark the line as a comment • The strings /* and */ mark the begin and end of a multi-line comment

  16. Adding Comments <html> <body> <script language="javascript"> // This is a comment. /* This is a two line comment */ document.write("Greetings") </script> </body> </html>

  17. Variables (1) • JavaScript is “loosely typed’ • data types change dynamically as the program runs • Declare variables with the var statement

  18. Variables (2) • Examples varx // Now x is undefinedvar x = 5; // Now x is a Numbervar x = "John"; // Now x is a String

  19. Variables (3) • Like VB, there are local and global variables • Local variables are declared inside of a procedure • Global variables are declared in a <script> block but outside of a procedure

  20. Functions (Introduction) • They are the same thing as a VB function or any other programming language function • Functions execute • When called by another procedure • When implemented as an event handler • Event handlers are discussed later

  21. Declaring a Function • function declarations typically appear in the <head> section • They are only executed when explicitly called • Syntax: function name(parameter –list) { statements: }

  22. Declaring a Function (Example) • Declare a function named printMessage <head> <script type="text/javascript"> function printMessage(msg) { alert(msg); } </script> </head>

  23. Calling a Function • Functions execute when called • Call functions explicitly from other JavaScript code • Call functions implicitly from an event handler

  24. Calling a Function (Exampple) • From another function or from within another JavaScript block, call the function with it’s name an parameters • Example: <script type="text/javascript"> printMessage(); </script>

  25. Returning a Value from a Function • Call the return statement with an argument as inreturn 0;

  26. Operators • They are about the same as VB • % is the modulus operator though • http://www.w3schools.com/js/js_operators.asp

  27. Comparisons • Similar to VB • == is the test for equality • != is the test for inequality • http://www.w3schools.com/js/js_comparisons.asp

  28. Decision-Making • Again, conceptually similar to VB • {} mark blocks instead of EndIf • http://www.w3schools.com/js/js_if_else.asp

  29. Loops • While VB we have for loops and while loops

  30. JavaScript Events (Introduction) • Conceptually, Java Script events work just like .NET (VB) events • They fire as a result of some user or other action • Code that executes in response to an event is called an event handler • The syntax and event names differ between VB and JavaScript

  31. Common Events (1) • Mouse Motion • mouseover – mouse enters the region of an element • mouseout – mouse leaves the region of an element • focus – an element becomes active • blur – an element because inactive • http://www.w3schools.com/tags/ref_eventattributes.asp

  32. Common Events (2) • Document related events • load – document loads • unload – fires just before the document is unloaded • Button related • click – user clocks a button (or other element) • submit

  33. Creating Event Handlers • There are two ways to create event handlers • Inline event handlers have code in the event argument • Create functions and wire them to the event

  34. Inline Event Handler (Example) • The alert dialog appears when the user clicks the button <body> <input type="button" value="Click Me" onclick= "alert('you clicked me');" /> </body>

  35. Calling a Function from an Event handler • Event handlers are functions with some special characteristics • The following statement calls the procedure btnShowEventClick when the button is clicked: <input id="btnShowEvent" type="button" value="Execute event handler" onclick="btnShowEventClick()" />

  36. The Document Object Model • There really is not that much to the JavaScript language itself • It’s just a subset of Java • JavaScript uses the Document Object Model (DOM) objects to get at the browser and its windows • This is where the real power comes in • Standard defined by W3C and European Computer Manufactures Association (ECMA) hence ECMAScript • I’ll introduce the DOM here and go into more detail later

  37. What is the DOM? • It permits access to the contents, structure, and style of an XHTML document • An XML document too • The DOM can communicate with multiple browser instances • It’s formally defined by the W3C • Use the DOM to dynamically create new document elements • Use the DOM to move document elements and remove them too

  38. The Basic DOM Hierarchy

  39. We will talk about • navigator (the browser itself) • window (a window in the browser) • document (the currently loaded document in the browser)

  40. The write() and writeln() Methods • Both write their argument to the output stream (HTML document) • Both apply to the document object • More in a moment • write() does not write a terminating carriage return • writeln()does write a carriage return

  41. Determining the Browser • Use the navigator object to get info about the browser • appVersion gets the major version • Netscape • Microsoft Internet Explorer • appMinorVersion gets the minor version • Supported by IE only • appPlatform gets the OS version • cookieEnabled gets cookie status

  42. Determining the Browser (Example) document.write(navigator.appName)document.write(navigator.appVersion)document.write(navigator.appMinorVersion)document.write(navigator.platform)document.write(navigator.cookieEnabled) See JavaScriptNavigatorExample.htm

  43. The window object • The window object provides a reference to the open browser window • Through the window object, you can • Reference the elements on the page through the DOM • Create new windows and destroy them

  44. The window Object (Introduction) • It’s the root of the IE object hierarchy • It refers to the IE Browser windows • The documentproperty contains a reference to the open document • More about the document object in a moment • window.open() creates a new browser window

  45. window.open (Semantics) • window – refers to the browser window • We can also use the keyword self • window.open (url, name, features) • url contains URI or file name • Second argument contains the nameof the window • Features allows browser window configuration • It’s a comma-separated list of key=valuepairs

  46. The window Object (Attributes 1) • fullscreen - defines whether window fills the desktop • toolbar – enable or disable the toolbar • menubar – enable or disable the menu bar • resizable – allow or disallow window resizing

  47. The window Object (Attributes 2) • alwaysRaised – browser floats on top of other windows regardless of whether it is active • height and width define the window size • scrollbars defines whether scroll bars appear when necessary • Unspecified attributes will have false values

  48. The window Object (Attributes – Example) • Create a blank Web page without a toolbar or a menu bar • Note attributes appear as a comma separated list • 1 and yes are equivalent • 0 and no are equivalent newWindow = window.open("","foo", "toolbar=no,menubar=no") newWindow = window.open("","foo", "toolbar=no,menubar=no")

  49. The window Object (Best practices) • Do not use to create those dreaded banner ads • Do not use to trap the user by handling onClose and displaying the window again • Do not hide the title bar

  50. The window Object (Example) • Display a very annoying window that’s hard to get rid of window1 = window.open("","Annoy", "height=300,width=300,titlebar=no") window1.document.write("Annoying") See JavaScriptWindowMaker.htm

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