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JavaScript 5. Overview A "scripting" language for HTML pages - a scripting language is a lightweight programming language Embed code in HTML pages so.

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Presentation on theme: "JavaScript 5. Overview A "scripting" language for HTML pages - a scripting language is a lightweight programming language Embed code in HTML pages so."— Presentation transcript:

1 JavaScript 5

2 Overview A "scripting" language for HTML pages - a scripting language is a lightweight programming language Embed code in HTML pages so they are downloaded directly to browser The browser interprets and executes the script (it is not compiled) Was designed to add interactivity to HTML pages Everyone can use JavaScript without purchasing a license Supported by all major browsers

3 … Overview Do not declare data types for variables (loose typing) Dynamic binding – object references checked at runtime Scripts can manipulate "browser objects:"  HTML form elements  Images  Frames  etc. For security – cannot write to disk (when run on a client)

4 Abilities Generating HTML content dynamically Monitoring and responding to user events Validate forms before submission Manipulate HTTP cookies Interact with the frames and windows of the browser Customize pages to suit users

5 It is not Java JavaScript is not Java, or even related to Java  The original name for JavaScript was “LiveScript”  The name was changed when Java became popular  Released in the Fall of 1995 Statements in JavaScript resemble statements in Java, because both languages borrowed heavily from the C language  JavaScript should be fairly easy for Java programmers to learn JavaScript is seldom used to write complete “programs”  Instead, small bits of JavaScript are used to add functionality to HTML pages  JavaScript is often used in conjunction with HTML “forms” JavaScript is reasonably platform-independent

6 … It is not Java JavaScript has some features that resemble features in Java:  JavaScript has Objects and primitive data types  JavaScript has qualified names; for example, document.write("Hello World");  JavaScript has Events and event handlers  Exception handling in JavaScript is almost the same as in Java JavaScript has some features unlike anything in Java:  Variable names are untyped: the type of a variable depends on the value it is currently holding  Objects and arrays are defined in quite a different way  JavaScript has with statements and a new kind of for statement

7 Scripting The entire script is stored in memory as plain text When requested by the user the applicable portion of the script is executed by fetching the associated machine instructions from a library  Tends to be a bit slower than compiling programs  However, there is no burden on the author to compile anything  Errors are not obvious when scripting; only rigorous testing will find errors

8 History Built into Netscape Navigator since v2.0 (early 1996) Developed independently of Java Proprietary, but submitted as standard and built into Microsoft IE 3.0 and later Standardized by ECMA (European Computer Manufacture’s Association) into ECMAscript EMCAscript joins JavaScript and Jscript to one standard

9 Javascript has many names In Netscape it’s Javascript In Internet Explorer it’s JScript  IE also supports it’s own VB Script, a Visual Basic scripting language  VB Script won’t work in Netscape There is also ECMAscript  A variation of Javascript 1.1  Open Standard  Promoted by European Computer Manufacturers Association (ECMA)  JScript is essentially ECMAscript in IE 4.0+

10 Javascript Versions 1.0 – Original version, largely obsolete  Supported in Navigator 2.0  Buggy version of it supported in IE 3.0 as JScript 1.1  Improved array processing  Supported in Navigator 3.0 as JScript, some discrepancies 1.2  Supports regular expressions, new statements  Supported in Navigator  Fixed some problems with dates, introduced in Navigator 4.06 ECMAScript  First supported in IE 4.0, also in Navigator 4.06  Largely the same as Javascript 1.1

11 Dynamic HTML HTML CSS Java Script HTML CSS Java Script

12 Web browser HTML Page: …code..… Desktop access Internet HTML/HTTP TCP/IP HTML/HTTP TCP/IP Web (HTTP) Server HTML pages w/ embedded script Remote host built-in JavaScript interpreter Web Architecture for JavaScript "CLIENT""SERVER"

13 Client and Server JavaScript can be used  On the client side  On the server More lightweight and reliable on clients than Java (Applets) Useful for developing interactive interface (Dynamic HTML)

14 Example JavaScript code is included within tags:  document.write(" Hello World! ") ; Notes:  The type attribute is to allow you to use other scripting languages (but JavaScript is the default)  This simple code does the same thing as just putting Hello World! in the same place in the HTML document  The semicolon at the end of the JavaScript statement is optional  You need semicolons if you put two or more statements on the same line  It’s probably a good idea to keep using semicolons

15 Dealing with old browsers Some old browsers do not recognize script tags  These browsers will ignore the script tags but will display the included JavaScript  To get old browsers to ignore the whole thing, use:  The , the // starts a JavaScript comment, which extends to the end of the line

16 Where to put JavaScript JavaScript can be put in the or in the of an HTML document  JavaScript functions should be defined in the  This ensures that the function is loaded before it is needed  JavaScript in the will be executed as the page loads JavaScript can be put in a separate.js file   Put this HTML wherever you would put the actual JavaScript code  An external.js file lets you use the same JavaScript on multiple HTML pages  The external.js file cannot itself contain a tag JavaScript can be put in HTML form object, such as a button  This JavaScript will be executed when the form object is used

17 Primitive data types JavaScript has three “primitive” types: number, string, and boolean  Everything else is an object Numbers are always stored as floating-point values  Hexadecimal numbers begin with 0x  Some platforms treat 0123 as octal, others treat it as decimal Strings may be enclosed in single quotes or double quotes  Strings can contains \n (newline), \" (double quote), etc. Booleans are either true or false  0, "0", empty strings, undefined, null, and NaN are false, other values are true

18 Variables Variables are declared with a var statement:  var pi = , x, y, name = "Dr. ABC" ;  Variables names must begin with a letter or underscore  Variable names are case-sensitive  Variables are untyped (they can hold values of any type)  The word var is optional (but it’s good style to use it) Variables declared within a function are local to that function (accessible only within that function) Variables declared outside a function are global (accessible from anywhere on the page)

19 Operators, I Because most JavaScript syntax is borrowed from C (and is therefore just like Java), we won’t spend much time on it Arithmetic operators: + - * / % Comparison operators: = > Logical operators: && || ! ( && and || are short-circuit operators) Bitwise operators: & | ^ ~ > >>> Assignment operators: += -= *= /= %= >= >>>= &= ^= |=

20 Operators, II String operator: + The conditional operator: condition ? value_if_true : value_if_false Special equality tests:  == and != try to convert their operands to the same type before performing the test  === and !== consider their operands unequal if they are of different types Additional operators: new typeof void delete

21 Comments Comments are as in C or Java:  Between // and the end of the line  Between /* and */

22 Statements, I Most JavaScript statements are also borrowed from C  Assignment: greeting = "Hello, " + name;  Compound statement: { statement;...; statement }  If statements: if (condition) statement; if (condition) statement; else statement;  Familiar loop statements: while (condition) statement; do statement while (condition); for (initialization; condition; increment) statement;

23 Statements, II The switch statement: switch (expression){ case label : statement; break; case label : statement; break;... default : statement; } Other familiar statements:  break;  continue;  The empty statement, as in ;; or { }

24 Exception handling, I Exception handling in JavaScript is almost the same as in Java throw expression creates and throws an exception  The expression is the value of the exception, and can be of any type (often, it's a literal String) try { statements to try } catch (e) { // Notice: no type declaration for e exception-handling statements } finally { // optional, as usual code that is always executed }  With this form, there is only one catch clause

25 Exception handling, II try { statements to try } catch (e if test1) { exception-handling for the case that test1 is true } catch (e if test2) { exception-handling for when test1 is false and test2 is true } catch (e) { exception-handling for when both test1 and test2 are false } finally { // optional, as usual code that is always executed } Typically, the test would be something like e == "InvalidNameException"

26 Object literals You don’t declare the types of variables in JavaScript JavaScript has object literals, written with this syntax:  { name1 : value1,..., nameN : valueN } Example:  car = {myCar: "Toyota", 7: "Mazda", getCar: CarTypes("Honda"), special: Sales}  The fields are myCar, getCar, 7 (this is a legal field name), and special  "Toyota" and "Mazda" are Strings  CarTypes is a function call  Sales is a variable you defined earlier  Example use: document.write("I own a " + car.myCar);

27 Three ways to create an object You can use an object literal:  var course = { number: “CS450", teacher="Dr. ABC" } You can use new to create a “blank” object, and add fields to it later:  var course = new Object(); course.number = “CS450"; course.teacher = "Dr. ABC"; You can write and use a constructor:  function Course(n, t) { // best placed in this.number = n; this.teacher = t; }  var course = new Course(“CS450", "Dr. ABC");

28 Array literals You don’t declare the types of variables in JavaScript JavaScript has array literals, written with brackets and commas  Example: color = ["red", "yellow", "green", "blue"];  Arrays are zero-based: color[0] is "red" If you put two commas in a row, the array has an “empty” element in that location  Example: color = ["red",,, "green", "blue"];  color has 5 elements  However, a single comma at the end is ignored  Example: color = ["red",,, "green", "blue”,]; still has 5 elements

29 Four ways to create an array You can use an array literal: var colors = ["red", "green", "blue"]; You can use new Array() to create an empty array:  var colors = new Array();  You can add elements to the array later: colors[0] = "red"; colors[2] = "blue"; colors[1]="green"; You can use new Array(n) with a single numeric argument to create an array of that size  var colors = new Array(3); You can use new Array(…) with two or more arguments to create an array containing those values:  var colors = new Array("red","green", "blue");

30 The length of an array If myArray is an array, its length is given by myArray.length Array length can be changed by assignment beyond the current length  Example: var myArray = new Array(5); myArray[10] = 3; Arrays are sparse, that is, space is only allocated for elements that have been assigned a value  Example: myArray[50000] = 3; is perfectly OK  But indices must be between 0 and As in C and Java, there are no two-dimensional arrays; but you can have an array of arrays: myArray[5][3]

31 Arrays and objects Arrays are objects car = { myCar: “Toyota", 7: "Mazda" }  car[7] is the same as car.7  car.myCar is the same as car["myCar"] If you know the name of a property, you can use dot notation: car.myCar If you don’t know the name of a property, but you have it in a variable (or can compute it), you must use array notation: car.["my" + "Car"]

32 Array functions If myArray is an array,  myArray.sort() sorts the array alphabetically  myArray.sort(function(a, b) { return a - b; }) sorts numerically  myArray.reverse() reverses the array elements  myArray.push(…) adds any number of new elements to the end of the array, and increases the array’s length  myArray.pop() removes and returns the last element of the array, and decrements the array’s length  myArray.toString() returns a string containing the values of the array elements, separated by commas

33 The for…in statement You can loop through all the properties of an object with for (variable in object) statement;  Example: for (var prop in course) { document.write(prop + ": " + course[prop]); }  Possible output: teacher: Dr. ABC number: CS450  The properties are accessed in an undefined order  If you add or delete properties of the object within the loop, it is undefined whether the loop will visit those properties  Arrays are objects; applied to an array, for…in will visit the “properties” 0, 1, 2, …  Notice that course["teacher"] is equivalent to course.teacher  You must use brackets if the property name is in a variable

34 Functions Functions should be defined in the of an HTML page, to ensure that they are loaded first The syntax for defining a function is: function name(arg1, …, argN) { statements }  The function may contain return value ; statements  Any variables declared within the function are local to it The syntax for calling a function is just name(arg1, …, argN) Simple parameters are passed by value, objects are passed by reference

35 Regular expressions A regular expression can be written in either of two ways:  Within slashes, such as re = /ab+c/  With a constructor, such as re = new RegExp("ab+c") Regular expressions are almost the same as in Perl or Java (only a few unusual features are missing) string.match(regexp) searches string for an occurrence of regexp  It returns null if nothing is found  If regexp has the g (global search) flag set, match returns an array of matched substrings  If g is not set, match returns an array whose 0 th element is the matched text, extra elements are the parenthesized subexpressions, and the index property is the start position of the matched substring

36 Debugging If you mess up on the syntax you will get a Javascript Error  Netscape  You will see a notification of an error on the status bar in the bottom left corner  You type “javascript:” in the URL field to pinpoint the error  Internet Explorer  By default a tiny little Javascript error message appears at the bottom left corner of the browser in yellow. Usually you won’t see it.  Can be explicitly disabled under Tools/Internet Options  Recommend under Tools/Internet Options/Advanced/Browsing to uncheck “Disable Script Debugging” and to check “Display a Notification about every script error” while doing development

37 Fixing Javascript Errors If possible use the debugging tool to locate the line containing the error Errors can be hard to find and fix  “code a little, test a little” strategy Often errors are due to things that are easy to overlook, like not closing a quote

38 Numbers In JavaScript, all numbers are floating point Special predefined numbers:  Infinity, Number.POSITIVE_INFINITY -- the result of dividing a positive number by zero  Number.NEGATIVE_INFINITY -- the result of dividing a negative number by zero  NaN, Number.NaN (Not a Number) -- the result of dividing 0/0  NaN is unequal to everything, even itself  There is a global isNaN() function  Number.MAX_VALUE -- the largest representable number  Number.MIN_VALUE -- the smallest (closest to zero) representable number

39 Strings and characters In JavaScript, string is a primitive type Strings are surrounded by either single quotes or double quotes There is no “character” type Special characters are: \0 NUL \b backspace \f form feed \n newline \r carriage return \t horizontal tab \v vertical tab \' single quote \" double quote \\ backslash \xDD Unicode hex DD \xDDDD Unicode hex DDDD

40 Some string methods charAt(n)  Returns the n th character of a string concat(string1,..., stringN)  Concatenates the string arguments to the recipient string indexOf(substring)  Returns the position of the first character of substring in the recipient string, or -1 if not found indexOf(substring, start)  Returns the position of the first character of substring in the given string that begins at or after position start, or -1 if not found lastIndexOf(substring), lastIndexOf(substring, start)  Like indexOf, but searching starts from the end of the recipient string

41 More string methods match(regexp)  Returns an array containing the results, or null if no match is found  On a successful match:  If g (global) is set, the array contains the matched substrings  If g is not set:  Array location 0 contains the matched text  Locations 1... contain text matched by parenthesized groups  The array index property gives the first matched position replace(regexp, replacement)  Returns a new string that has the matched substring replaced with the replacement search(regexp)  Returns the position of the first matched substring in the given string, or -1 if not found.

42 boolean The boolean values are true and false When converted to a boolean, the following values are also false : 00  "0" and '0'  the empty string, '' or ""  undefined  null  NaN

43 Arrays As in C and Java, there are no “true” multidimensional arrays  However, an array can contain arrays  The syntax for array reference is as in C and Java Example:  var a = [ ["red", 255], ["green", 128] ];  var b = a[1][0]; // b is now "green"  var c = a[1]; // c is now ["green", 128]  var d = c[1]; // d is now 128

44 Input Programming languages need to start with some data and manipulate it Confirm asks a yes or no question in a dialog box Prompt prompts the user to type in some information into a text field inside the dialog box Sources of data can include:  Files  Databases  User (keyboard & mouse typically)  Variable assignments (ex: pi= )  Javascript objects  Example: date object Example:  User_name = prompt(“What is your name?”, “Enter your name here”);

45 Output After a program manipulates the input data with various statements it usually creates an output of some kind Source of output may include:  Files  Database  Display or Printer  Devices (sound card, modems etc)  Javascript Objects  Via Object Methods

46 Simple Output document is an object (not a class) representing the current document write is a method on the document object that let’s you write any text to the browser window at the current location of the cursor Warning: if invoked as part of a form action output will appear in a new window Example:  document.write(“Hello world!”);

47 alert method A dialog box containing information can be written by using the window.alert method Example:  alert(“This brings up an annoying non- modal dialog box. The user can’t do anything until they click OK.”);

48 HTML names in JavaScript In HTML the window is the global object  It is assumed that all variables are properties of this object, or of some object decended from this object  The most important window property is document HTML form elements can be referred to by document.forms[formNumber].elements[elementNumber] Every HTML form element has a name attribute  The name can be used in place of the array reference  Hence, if   Then instead of document.forms[0].elements[0]  you can say document.myForm.myButton

49 Document Object Model (DOM) Document Object Model  The Dynamic HTML Document Object Model in this case  There are other DOM’s It allows a document (in this case the web browser) to be manipulated as multiple objects For example, the document with focus in the browser is the document object You manipulate properties of the objects by setting object properties and calling methods of objects

50 DOM Object Hierarchy

51 Navigating through the DOM 5 objects have predefined names that can’t be changed and are available to any page  window, document, history, location, navigator Any object of a type that can be replicated is accessed by the HTML name or ID attribute.  Ex:  Accessed by: window.document.form1 If an object is named uniquely it is not necessary to give a fully qualified name  Ex: use form1 instead of window.document.form1 Objects that are not named may be accessed indirectly through an array. Element 0 indicates the number of objects  Ex: document.forms[1] or forms[1]

52 Javascript object example Document  Properties  fgcolor : specifies color of document text  same as  Methods  clear : erases contents of current document  write : sends text to the browser

53 Math Object Can be accessed as, ex:  x=Math.pow(3,3); // x=27 Allows many common mathematical calculations including:  abs(x) : absolute value  ceil(x) and floor(x) : smallest integer not less than x and largest integer not greater than x  cos(x), exp(x), log(x), sin(x), tan(x) : trigonometric and log rhythmic functions  min(x,y) or max(x,y) : returns the minimum or maximum of values x and y  pow(x,y) : raises x to the power y  round(x) : rounds to nearest integer  sqrt(x) : Square root

54 Date Object By default creates an object with the computer’s current date and time, ex:  now = new Date(); // variable now contains current date and time  Note: months are expressed 0-11, 0 being January, 11 being December Dates are actually stored as an integer representing the number of milliseconds since January 1st, 1970  Negative values indicate dates before this date Once you have a date object you can set the date, or read the date in a number of useful formats  now.setFullYear(2003, 0, 31); /* Jan 31st, 2003 */  Now.setHours(13, 13, 13); /* 1:13:13 PM, local time zone */

55 Date Properties Some of the more useful properties of the date class include:  now.getDay(); /* returns 0-6 for the day of the week, 0=Sunday, 6=Saturday */  now.getFullYear(); /* year of this date object */  now.getMonth(); /* 0-11 returned for the current month */  now.getDate(); /* 1-31 for the day in the month */  now.getHours(); /* 0-23, may need to translate to PM */  now.getMinutes(); /* 1-60 */  Now.getSeconds(); /* 1-60 */

56 Window Object The window object is the “master” DOM object at the top of the DOM hierarchy Useful properties:  length : number of frames in window  frames : an array of window objects, one for each frame  parent : Since frames are window objects, sometimes parent window is needed Examples:  window.document : if frameless, accesses the top level document. If frames, accesses the top frame’s document  window.frame[1 ]. document : Access the document contained in the first frame  frame[1].parent.document : Access the document contained in the parent frame

57 Window Object Methods alert, confirm and prompt are actually methods of the window object, ex: window.alert; /* opens a window */ window.close(); /* closes window */

58 Navigator Object Contains information about the browser Can be accessed as window.navigator or just navigator Useful properties:  appName : name of browser used (can be deceiving; more on this in a later class)  appVersion : version of browser used (can be deceiving; more on this in a later class)  platform : operating system in use  cookieEnabled : can the browser store cookies?

59 Location Object Contains information about the current URL Can be accessed as window.location or just location Useful properties:  href : retrieves entire URL  host : retrieves just the domain name (ex:  pathname : retrieves just the path inside the domain (page name is at end)  hash : retrieves the anchor

60 History Object Contains information on the URLs that the browser has visited in this session within a window Can be accessed as window.history or just history Useful properties: next, previous (tells you the URL, but won’t direct you there) Useful methods:  back : same as pressing the back arrow button  forward : same as pressing the forward arrow button  go : go back or forward a given number of pages; to go back 3 pages:  history.go(-3);

61 Document Object This is the typically the most accessed object You can access all items in the document window through the document object  Forms, tables, paragraphs, lists, images, etc.  Consult a reference for properties and methods Frameless document: Access as window.document or document Document contained in a frame: window.frame[x].document, where x is the number or name of the frame

62 DOM Collections Inside objects may be one or more collections of child objects  Syntax: object.collection  object is either a standard object (like document) or the HTML ID attribute that uniquely identifies an item on a page  collection is a group of objects all of the same type  Ex: document.links  document is the DOM object  links is a collection of link objects inside the document object  The document object may have many links embedded inside of it (just as a page may have many links in it)  The collection of links can be thought of as an array of link objects, i.e. links[0], links[1] etc.

63 Addressing DOM Collections Collection objects: anchors, applets, embeds, forms, frames, images, links, plugins, scripts, styleSheets Addressing an element in a collection can be done in a number of ways. Pick the way easiest for you:  document.collection[i]  i is a number, 0 thru n where n is the last element in the collection array and 0 the first  Ex: document.links[0] references the first link on the page  Use this if there was no HTML ID attribute assigned to an element   id is the HTML ID attribute of the object  HTML: Link  DOM reference (using IE): document.all.B  document.collection[“id”]  HTML: Link  DOM reference: document.all[“B”]

64 Addressing blocks by ID Assume tag … IE:   Ex: document.all.X  or just  id  Ex: X Mozilla/Netscape 6+: find by ID, ex:  handle = document.getElementById("X");  // Now use the handle variable to access properties

65 Cross platform code Sometimes you have to write one set of code that will work for both browsers Here is a sample technique: var isNS = false; var isIE = false; if (!(document.all)) isNS=true; if (document.all) isIE=true; if (isNS) { el = document.getElementById("X"); el.setAttribute("style", "background- color:pink;"); } if (isIE)"pink";

66 Events Most browser DOM objects have “events” associated with them  Recall Javascript objects include windows, frames, forms, form fields, links, etc. An event is Javascript code that can be triggered when something happens to a Javascript object  Example: clicking on a hyperlink  Example: leaving a form field

67 Events Event code is usually invoked through HTML “Javascript only attributes”  They are not “HTML” attributes  They are not in the HTML specification  However, if Javascript is enabled and the event is valid for the Javascript object then associated code is executed.  Example:

68 Events Some events will behave differently depending on whether the code associated with the event returns a true or a false value Example: If the form onsubmit event returns false, the form is not submitted All events start with “ on ”, ex: onclick, onsubmit, etc. This makes them easy to distinguish

69 this “ this ” is a shortcut that can be used to refer to the current object It is useful in event handlers to pass properties of the object to generic functions This example passes the entire text object named “month” to the function checkrange() Otherwise would have to code something like document.form1.month!

70 this Example  This function embedded in the tag processes the example on the previous page. Note that field can be any Javascript object which has a numeric value attribute  function checkrange (field, min_value, max_value) { if ((field.value max_value)) { alert("The field value must be at least " + min_value + " and may not be greater than " + max_value + "."); return false; } else return true; }

71 onabort Event Action to occur when the user aborts loading an image Occurs when the browser STOP button is pressed or clicks on an image before it is loaded By default nothing happens which is out of the ordinary Example: 

72 onblur Event Occurs when the user leaves a form field (either by clicking outside the form field or pressing the tab key) onblur and its cousin onchange are very popular for validating form fields Example: 

73 onchange Event Occurs when the value of a form field is changed by the user and loses focus, or when a new choice is made in a select element Example: 

74 onclick Event Occurs when user clicks on a clickable form control or hyperlink with a mouse Example:  Go to Yahoo!

75 onerror Event Occurs when a document or image fails to load properly Example: 

76 onfocus Event Occurs when a window or form field is made active by moving the cursor into the field or clicking on the object Example: 

77 onload Event Action to take when the page has finished loading  onunload is similar and occurs when a document is unloaded Example: 

78 onmouseover Event Occurs once each time the mouse pointer moves over an object or area from outside that object or area Example:  Click me

79 onmouseout Event Occurs each time the mouse pointer leaves an area (client-side image map) or link from inside that area or link Example:  Click me

80 onsubmit Event Can be used to prevent a form from being submitted Commonly used with a form validation logic Must put the word “ return ” in front of your code. Anything other than a false value will let the form submit A common mistake is to instead attach logic to the onClick event of a submit button. This won’t keep the form from submitting! Example: 

81 onreset Event Occurs when a reset button on the form is pressed Event only works in the form tag A common mistake is to attach logic to the onclick event of a reset button. This won’t keep the form from resetting! Example:  onreset="alert(‘Defaults have been restored.');”>

82 onselect Event Occurs when a user selects some of the text within a text or textarea field Example: 

83 Object Event Handlers Button - onclick Check Box - onclick Document - onload, onunload, onerror Form - onsubmit, onreset Frames - onblur, onfocus Hyperlink - onclick, onmouseover, onmouseout Image - onload, onerror, onabort

84 Object Event Handlers Image Hot Spot - onmouseover, onmouseout Input Box - onblur, onchange, onfocus, onselect Radio Button - onclick Reset Button - onclick Selection List - onblur, onchange, onfocus Submit Button - onclick Text Area Box - onblur, onchange, onfocus, onselect Window - onload, onunload, onblur, onfocus

85 Emulating Events There are a number of methods for Javascript objects that let you pretend to be executing an event:  click(), clear(), reset(), submit(), blur(), close(), focus() and select() These can be useful, for example, to force the focus back into a particular field, use the focus() event  Use with care, particularly focus() events because they can cause loops that will hang your browser!

86 Warnings JavaScript is a big, complex language  We’ve only scratched the surface  It’s easy to get started in JavaScript, but if you need to use it heavily, plan to invest time in learning it well  Write and test your programs a little bit at a time JavaScript is not totally platform independent  Expect different browsers to behave differently  Write and test your programs a little bit at a time Browsers aren’t designed to report errors  Don’t expect to get any helpful error messages  Write and test your programs a little bit at a time

87 References W3 Schools JavaScript Tutorial  Several Online Presentations

88 Reading List W3 Schools JavaScript Tutorial  Voodoo's Introduction to JavaScript 

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