overview javascript. overview 2 about basics objects dom

Overview

JavaScript

Overview

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AboutBasicsObjectsDOM

About JavaScript

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JavaScript is not Java, The original name for JavaScript was “LiveScript”The name was changed when Java became popularNow that Microsoft no longer likes Java, its name for their

JavaScript dialect is “Active Script”Statements in JavaScript resemble statements in

Java, because both languages borrowed heavily from the C languageJavaScript should be fairly easy for Java programmers to learnHowever, JavaScript is a complete, full-featured, complex

languageJavaScript is seldom used to write complete

“programs”Instead, small bits of JavaScript are used to add functionality

to HTML pagesJavaScript is often used in conjunction with HTML “forms”

JavaScript is reasonably platform-independent

Javascript --- 2 kinds???

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JavaScriptRuns on client

NodeJS (not the topic of this lecture)NEWRuns on server

JavaScript (client side) Allows InteractivityImprove appearance

Especially graphicsVisual feedback

Site navigationPerform calculationsValidation of inputOther technologies

javascript.internet.com

Why talk about JavaScript?Very widely used, and growing

Web pages, AJAX, Web 2.0Increasing number of web-related applications

Some interesting and unusual featuresFirst-class functions - interestingObjects without classes - slightly unusualPowerful modification capabilities - very unusual

Add new method to object, redefine prototype, access caller …Many security, correctness issues

Not statically typed – type of variable may change …Difficult to predict program properties in advance

Using JavaScript in a browser

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JavaScript code is included within <script> tags:<script type="text/javascript">

document.write("<h1>Hello World!</h1>") ;</script>

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

<h1>Hello World!</h1> 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

JavaScript isn’t always available

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Some old browsers do not recognize script tagsThese browsers will ignore the script tags but will display the

included JavaScriptTo get old browsers to ignore the whole thing, use:

<script type="text/javascript">  </script>

The ,

the // starts a JavaScript comment, which extends to the end of the line

Some users turn off JavaScriptUse the <noscript>message</noscript> to display a

message in place of whatever the JavaScript would put there

Where to put JavaScript

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JavaScript can be put in the <head> or in the <body> of an HTML documentJavaScript functions should be defined in the <head>

This ensures that the function is loaded before it is neededJavaScript in the <body> will be executed as the page loads

JavaScript can be put in a separate .js file<script src="myJavaScriptFile.js"></script>Put this HTML wherever you would put the actual JavaScript

codeAn external .js file lets you use the same JavaScript on multiple

HTML pagesThe external .js file cannot itself contain a <script> tag

JavaScript can be put in an HTML form object, such as a buttonThis JavaScript will be executed when the form object is used

welcome.html(1 of 1)

1 <?xml version = "1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 4 5  6  7 8 <html xmlns = "http://www.w3.org/1999/xhtml"> 9 <head> 10 <title>A First Program in JavaScript</title> 11 12 <script type = "text/javascript"> 13  17 </script> 18 19 </head><body></body> 20 </html>

HTML comment tags willresult in skipping of the script by those browsers that do not support scripting

welcome2.html(1 of 1)

1 <?xml version = "1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 4 5  6  7 8 <html xmlns = "http://www.w3.org/1999/xhtml"> 9 <head> 10 <title>Printing a Line with Multiple Statements</title> 11 12 <script type = "text/javascript"> 13  18 </script> 19 20 </head><body></body> 21 </html>

Escape character in combination with quotation mark: \” will result in insertion of a quotation mark inthe string that is actually writtenby JavaScript

welcome3.html1 of 1

1 <?xml version = "1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 4 5  6  7 8 <html xmlns = "http://www.w3.org/1999/xhtml"> 9 <head><title>Printing Multiple Lines</title> 10 11 <script type = "text/javascript"> 12  16 </script> 17 18 </head><body></body> 19 </html>

New line of the html document in a browser is determined by an html <br /> element

JavaScript

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Variables are not typed ---you simply use them –no need to declare type

show_info = new Boolean(true); Year = "2002";var Name = "Butch"; //instance variablevar Age = 6; //instance variable..........Age ="Six Years"; //this is legal to change.........

Instance Variables:•only accessible in the current function (or if not inside a function, the current script)•use the special keyword: var•Variables not declared as instance variables (preceeded by var), are accessible potentially anywhere

Literals --- data types

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JavaScript has three “primitive” types: number, string, and booleanEverything else is an object

Numbers are always stored as floating-point valuesHexadecimal numbers begin with 0xSome platforms treat 0123 as octal, others treat it as decimal

Since you can’t be sure, avoid octal altogether!Strings may be enclosed in single quotes or double

quotesStrings can contains \n (newline), \" (double quote), etc.

Booleans are either true or false0, "0", empty strings, undefined, null, and NaN are

false , other values are true

Variables

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Variables names must begin with a letter or underscorenames are case-sensitive Variables are untyped (they can hold values of

any type)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)

Operators, I

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Because most JavaScript syntax is borrowed from C (and is therefore just like Java), we won’t spend much time on it

Arithmetic operators (all numbers are floating-point): + - * / % ++ --

Comparison operators: < <= == != >= >

Logical operators: && || ! (&& and || are short-circuit operators)

Bitwise operators: & | ^ ~ << >> >>>

Assignment operators: += -= *= /= %= <<= >>= >>>= &= ^= |=

Operators, II

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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 (to be discussed):

new typeof void delete

OperatorsJavaScript operation

Arithmetic operator

Algebraic expression

JavaScript expression

Addition + f + 7 f + 7

Subtraction - p – c p - c

Multiplication * bm b * m

Division / x / y or or x y x / y

Remainder % r mod s r % s

Operator(s) Operation(s) Order of evaluation (precedence) *, / or % Multiplication

Division Modulus

Evaluated first. If there are several such operations, they are evaluated from left to right.

+ or - Addition Subtraction

Evaluated last. If there are several such operations, they are evaluated from left to right.

Precedence of arithmetic operators.

xy--

Always use parentheses to ensure desired order of evaluation: (a + b) / 6

Comments

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Comments are as in C or Java:Between // and the end of the lineBetween /* and */

Java’s javadoc comments, /** ... */, are treated just the same as /* ... */ comments; they have no special meaning in JavaScript

Example: simple calculation<html> … <p> … </p><script>

var num1, num2, sumnum1 = prompt("Enter first number")num2 = prompt("Enter second number")sum = parseInt(num1) + parseInt(num2)alert("Sum = " + sum)

</script>…

</html>

Statements, I

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Most JavaScript statements are also borrowed from CAssignment: 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;

Statements, II

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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 { }

JavaScript is not Java

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By now you should have realized that you already know a great deal of JavaScriptSo far we have talked about things that are the same as in Java

JavaScript has some features that resemble features in Java:JavaScript has Objects and primitive data typesJavaScript has qualified names; for example,

document.write("Hello World");JavaScript has Events and event handlersException 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 holdingObjects and arrays are defined in quite a different wayJavaScript has with statements and a new kind of for statement

Array literals

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You don’t declare the types of variables in JavaScript

JavaScript has array literals, written with brackets and commasExample: 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 locationExample: color = ["red", , , "green", "blue"];

color has 5 elementsHowever, a single comma at the end is ignored

Example: color = ["red", , , "green", "blue”,]; still has 5 elements

Arrays created

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var Names = new Array(3); //says there will 3 elements in the arrayfor(var i=0; i<Names.length; i++) //initially all names to a dummy string Names[i] = "Unknown";//Alternative declarationvar Names = new Array("Unknown","Unknown","Unknown");

Simply define a variable to be an array and call the built-in JavaScript class Array's constructor as follows:

Four ways to create an array

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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 sizevar 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");

The length of an array

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If myArray is an array, its length is given by myArray.length

Array length can be changed by assignment beyond the current lengthExample: var myArray = new Array(5); myArray[10] =

3;Arrays are sparse, that is, space is only allocated

for elements that have been assigned a valueExample: myArray[50000] = 3; is perfectly OKBut indices must be between 0 and 232-1

As in C and Java, there are no two-dimensional arrays; but you can have an array of arrays: myArray[5][3]

Arrays and objects

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Arrays are objectscar = { myCar: "Saturn", 7: "Mazda" }

car[7] is the same as car.7car.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"]

Array functions

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If myArray is an array,myArray.sort() sorts the array alphabeticallymyArray.sort(function(a, b) { return a - b; }) sorts

numericallymyArray.reverse() reverses the array elementsmyArray.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

The for…in statement

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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. Dave number: CIT597

The properties are accessed in an undefined orderIf you add or delete properties of the object within the loop, it

is undefined whether the loop will visit those propertiesArrays 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

The with statement

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with (object) statement ; uses the object as the default prefix for variables in the statement

For example, the following are equivalent:with (document.myForm) {

result.value = compute(myInput.value) ;}

document.myForm.result.value = compute(document.myForm.myInput.value);

One of my books hints at mysterious problems resulting from the use of with, and recommends against ever using it

Functions

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Functions should be defined in the <head> 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;

statementsAny variables declared within the function are local

to itThe syntax for calling a function is just

name(arg1, …, argN)Simple parameters are passed by value, objects

are passed by reference

More about functionsDeclarations can appear in function body

Local variables, “inner” functionsParameter passing

Basic types passed by value, objects by referenceCall can supply any number of arguments

functionname.length : # of arguments in definitionfunctionname.arguments.length : # args in call

“Anonymous” functions (expressions for functions)(function (x,y) {return x+y}) (2,3);

Function ExamplesAnonymous functions make great callbacks

setTimeout(function() { alert("done"); }, 10000)Variable number of arguments

function sumAll() { var total=0; for (var i=0; i< sumAll.arguments.length; i++) total+=sumAll.arguments[i]; return(total); }sumAll(3,5,3,5,3,2,6)

Exception handling, I

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Exception handling in JavaScript is almost the same as in Java

throw expression creates and throws an exceptionThe 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

Exception handling, II

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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 test1and test2 are false} finally { // optional, as usual code that is always executed}

Typically, the test would be something like e == "InvalidNameException"

Regular expressions

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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 regexpIt returns null if nothing is foundIf regexp has the g (global search) flag set, match returns

an array of matched substringsIf g is not set, match returns an array whose 0th element is

the matched text, extra elements are the parenthesized subexpressions, and the index property is the start position of the matched substring

Objects in JavaScript

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JavaScript: Object-Based Language

There are three object categories in JavaScript: Native Objects, Host Objects, and User-Defined Objects.Native objects: defined by JavaScript.

String, Number, Array, Image, Date, Math, etc. Host objects : supplied and always

available to JavaScript by the browser environment.window, document, forms, etc.

User-defined objects : defined by the author/programmer

User defined Classes/ObjectsHow to create your own

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Object literals

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You don’t declare the types of variables in JavaScriptJavaScript has object literals, written with this

syntax:{ name1 : value1 , ... , nameN : valueN }

Example (from Netscape’s documentation):car = {myCar: "Saturn", 7: "Mazda",

getCar: CarTypes("Honda"), special: Sales} The fields are myCar, getCar, 7 (this is a legal field name) ,

and special"Saturn" and "Mazda" are StringsCarTypes is a function callSales is a variable you defined earlier

Example use: document.write("I own a " + car.myCar);

Three ways to create an object

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You can use an object literal:var course = { number: "CIT597", teacher: "Dr. Dave" }

You can use new to create a “blank” object, and add fields to it later:var course = new Object();

course.number = "CIT597";course.teacher = "Dr. Dave";

You can write and use a constructor:function Course(n, t) { // best placed in <head>

this.number = n; // keyword "this" is required, not optional this.teacher = t;}

var course = new Course("CIT597", "Dr. Dave");

BOM, DOM and more“Host objects”

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Browser Object ModelThe BOM defines the components and hierarchy

of the collection of objects that define the browser window.

For the most part, we will only be working with the following components of the BOM.

• window object• location object • history object

• document object • navigator object • screen object

Window Object

Window object

History object

Location object

Navigator object

Screen object

Document object

Top level in the BOMAllows access to properties and method of:

display windowbrowser itselfmethods thing such as error message and alert boxesstatus bar

Document ObjectTop of the Document Object Model (DOM).This is probably the one you’ll use most.Allows access to elements of the displayed

document such as images and form inputs.The root that leads to the arrays of the

document: forms[ ] array, links[ ] array, and images[ ] array.

Least compliance to standard here – Netscape 6, Opera 6, and Mozilla 1.0 are the best.

Navigator ObjectProvides access to information and

methods regarding the client’s browser and operating system.

Commonly used to determine client’s browser capabilities so page can be modified real time for best viewing.

Example: A script may check the browser type in order to modify CSS styles.

History ObjectProvides access to the pages the client has

visited during the current browser session.Methods such as back() and forward() can

be used to move through the history. Can also be used to jump to any point in

the history.As with any browser history, it only allows

for a single path.

Other BOM ObjectsLocation Object – Provides access to and

manipulation of the URL of the loaded page.Screen Object – Provides access to information

about the client’s display properties such as screen resolution and color depth.

More information can be found at:http://javascript.about.com/library/bltut22.htm

http://www-128.ibm.com/developerworks/web/library/wa-jsdom/

http://javascript.about.com/library/bltut22.htm



Document Object ModelDocument is modeled as a tree.DOM Changes based on page displayed. Example:<html><head> <title>My Page</title></head><body> <h1>My Page</h1> <p name=“bob” id=“bob”> Here’s the first paragraph.</p> <p name=“jim” id=“jim”> Here’s the second paragraph.</p></body></html>

html

head body

title h1 p

bob jim

• Another example can be found at:http://oopweb.com/JavaScript/Documents/jsintro/Volume/part2/part2.htm

http://oopweb.com/JavaScript/Documents/jsintro/Volume/part2/part2.htm

DOM

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Modeled as a tree

welcome4.html1 of 1

1 <?xml version = "1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 4 5  6  7 8 <html xmlns = "http://www.w3.org/1999/xhtml"> 9 <head><title>Printing Multiple Lines in a Dialog Box</title> 10 11 <script type = "text/javascript"> 12  15 </script> 16 17 </head> 18 19 <body> 20 <p>Click Refresh (or Reload) to run this script again.</p> 21 </body> 22 </html>

alert method of the window object displays a Dialog box

Example using window object

welcome5.html(1 of 2)

1 <?xml version = "1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" 3 "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> 4 5  6  7 8 <html xmlns = "http://www.w3.org/1999/xhtml"> 9 <head> 10 <title>Using Prompt and Alert Boxes</title> 11 12 <script type = "text/javascript"> 13  22 </script>

JavaScript is a loosely typed language. Variables take on any data type depending on the value assigned.

Value returned by the prompt method of the window object is assigned to the variable name

“+” symbol can be used for text concatenation as well as arithmetic operator

More using window object -> prompt

23 24 </head> 25 26 <body> 27 <p>Click Refresh (or Reload) to run this script again.</p> 28 </body> 29 </html>

This is the promptto the user.

This is the default value that appears when the dialog opens.

This is the text field in which the user types the value.

When the user clicks OK, the value typed by the user is returned to the program as a string.

If the user clicks Cancel, the null value will be returned to the program and no value will be assigned to the variable.

Simple Script Example: Adding Integers The values of numbers to be added are obtained as user

inputs colleted through the window.prompt method parseInt

Converts its string argument to an integerWhat happens if the conversion is not done?

See example on our web site NaN (not a number): value returned if non-

numerical values are passed to the paresInt method

Another example using window object

Addition.html(1 of 2)

1 <?xml version = "1.0"?> 2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 3 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> 4 5  6  7 8 <html xmlns = "http://www.w3.org/1999/xhtml"> 9 <head> 10 <title>An Addition Program</title> 11 12 <script type = "text/javascript"> 13 <!-- 14 var firstNumber, // first string entered by user 15 secondNumber, // second string entered by user 16 number1, // first number to add 17 number2, // second number to add 18 sum; // sum of number1 and number2 19 20 // read in first number from user as a string 21 firstNumber = 22 window.prompt( "Enter first integer", "0" ); 23

Addition.html(2 of 2)

24 // read in second number from user as a string 25 secondNumber = 26 window.prompt( "Enter second integer", "0" ); 27 28 // convert numbers from strings to integers 29 number1 = parseInt( firstNumber ); 30 number2 = parseInt( secondNumber ); 31 32 // add the numbers 33 sum = number1 + number2; 34 35 // display the results 36 document.writeln( "<h1>The sum is " + sum + "</h1>" ); 37 // --> 38 </script> 39 40 </head> 41 <body> 42 <p>Click Refresh (or Reload) to run the script again</p> 43 </body> 44 </html>

JavaScript Alert window<html><head><title>My Page</title></head><body><p><a href="myfile.html">My Page</a><br /><a href="myfile.html" onMouseover="window.alert('Hello');">My Page</A></p></body></html>

JavaScript writteninside HTML

An Event

HTML Forms and JavaScript

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Using DOM

HTML Forms and JavaScriptJavaScript is very good at processing user

input in the web browserHTML <form> elements receive inputForms and form elements have unique

namesEach unique element can be identifiedUses JavaScript Document Object Model

(DOM)

Naming Form Elements in HTML

<form name="addressform">Name: <input name="yourname"><br />Phone: <input name="phone"><br />Email: <input name="email"><br /></form>

Forms and JavaScriptdocument.formname.elementname.valueThus:

document.addressform.yourname.valuedocument.addressform.phone.valuedocument.addressform.email.value

Using Form DataPersonalising an alert box

<form name="alertform">Enter your name:<input type="text" name="yourname"><input type="button" value= "Go" onClick="window.alert('Hello ' + document.alertform.yourname.value);">

</form>

DOM and dynamic manipulationAn example

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Events –you can based on triggered events run JavaScript (event handling) codeExamples of events for your HTML pages

include:onLoadonClickonMouseOveronMouseOut

Each of these events can execute a script.Events must be placed in a tag. For example:

<a href="somesite.htm" onClick = "javascript:function();">link text</a>

ALSO You can add or change elements on a pageSome possibilities

createElement(elementName)createTextNode(text)appendChild(newChild)removeChild(node)

Example: Add a new list item:

var list = document.getElementById(‘list1') var newitem = document.createElement('li') var newtext = document.createTextNode(text) list.appendChild(newitem) newitem.appendChild(newtext)

This example uses the browser Document Object Model (DOM).