Introduction to JavaScript

JavaScript Intro.

CITE – IT201 Timothy Bryan Marc

Overview

■ What is the relationship between: ■ SGML ■ HTML ■ XHTML ■ CSS ■ JavaScript

Recap

■ Markup ■ Annotations instructing how the document

should appear in print ■ Word processors use different markup

schemes ■ SGML

■ IBM - Standard Generalized Markup Language ■ Markup instructions stored with ASCII file

▪ Any computer could render document ▪ Lilly Example

Recap

■ HTML ■ Tim Berners-Lee created HTML – subset of

SGML ■ Not platform or application specific ■ Only server software needed to publish file s

on the net ■ Structure versus content

■ Browser parses HTML file into a tree

Recap

■ Sample HTML File ■ <FONT SIZE=14 FACE=Swing> ■ <B>Bryan Moore</B><BR> ■ </FONT> ■ <FONT SIZE=12 FACE=Textile> ■ 1234 Sunset Ave. <BR> ■ Walla Walla, WA 12345 <BR> ■ (123)123.4567<BR> ■ </FONT>

RecapBrowser parse tree

Recap

■ Problems with extracting data ■ Need to write a computer program to extract

the names and addresses by selecting text strings following font tags

■ Content and structure of document become intertwined

■ Not the intention of SGML and original HTML ■ Cascading Style Sheets

■ Attempt to separate content and style

Recap

■ CSS ■ Can alter the look of entire file with a simple

coding change ■ Can keep styles in an external file ■ Increases the knowledge needed to code

pages ■ Initial rationale of HTML was to appeal to the

masses

Recap

Parse tree using CSS

Still need to reference

Information based on

“second string after BR”

Information is not

meaningful

Recap

■ Extensible Markup Language XML ■ Extensible- can create own tags ■ Complete separation of content and style

Recap

■ Releases ■ HTML 4.0 1997 ■ XML 1.0 1998

■ XML and HTML need to better cooperate ■ XHTML 1.0 2000

■ XHTML ■ Contains elements and attributes of HTML ■ Elements must have closing tags ■ Lowercase ■ Attributes must have values ■ Attributes in single or double quotes

■ http://www.w3schools.com/xhtml/default.asp

HTML Forms and JavaScript

■ Client Server Model ■ Client Side Processing

■ JavaScript downloaded from web page and processed by the client – typically form checking

■ JavaScript can interact directly with form data ■ Server Side processing

■ Information from a form sent to server for processing ■ PHP Perl C++ ■ Server can interact directly with the form data

HTML Forms and JavaScript

■ JavaScript ■ HTML files are text files ■ JavaScript is interpreted not compiled ■ Object oriented

■ HTML forms are objects and can be manipulated via JavaScript

■ Dynamic HTML – merger of JavaScript and HTML ■ Different implementations of DHTML by

software companies

What is JavaScript■ Scripting language (object-oriented)

■ Lightweight programming language developed by Netscape ■ Interpreted, not compiled

■ Designed to be embedded in browsers ■ Ideal for adding interactivity to HTML pages ■ Detect browser versions ■ Work with info from user via HTML forms ■ Create cookies ■ Validate form data ■ Read and write HTML elements

■ Supported by all major browsers ■ Internet Explorer has JScript (started in IE3)

■ http://www.faqts.com/knowledge_base/view.phtml/aid/1380

■ It’s free, no license required

http://www.faqts.com/knowledge_base/view.phtml/aid/1380

What is JavaScript

■ Syntax is similar to Java, but it’s not Java per se

■ Usually JavaScript code is embedded within HTML code using the script tag:

■ Can have more than one pair of script tags in a page

■ Semicolons: C++ and JAVA require them but in JavaScript it’s optional

What is JavaScript

■ HelloWorld example program…

■ <html> ■ <head><title>JavaScript HelloWorld!</title></head> ■ <body> ■ <script language="JavaScript"> ■ ■ document.write('Javascript says "Hello World!"') ■ ■ </script> ■ </body>

■ </html>

■ Let’s try it in the browser

What is JavaScript

■ Javascript can be located in the head, body or external file ■ Head section

■ Ensures script is loaded before trigger event ■ Body section

■ Script executes when body loads ■ External

■ Allows scripts to run on several pages ■ Examples:

■ http://www.w3schools.com/

What is JavaScript

■ JavaScript statements in head write to the beginning of the body section but don’t violate HTML code already there.

■ JavaScript statements in body write based on their location

■ JavaScript interpreted first then HTML interpreted second ■ Document.write writes to the HTML document

not the web page

What is JavaScript■ <html> ■ <head> ■ <script language=“JavaScript”> ■ document.write (“<b> This is first </b>); ■ </script> ■ </head>

■ <body> ■ Now where does this print on the web page???? <br />

■ <script language=“JavaScript”> ■ document.write ( “This might be last?”) ■ </script>

■ </body> ■ </html> ■ Lets try.

What is JavaScript

■ Now, let JavaScript generate HTML for us…

■ <html> ■ <head><title>JavaScript HelloWorld!</title></head> ■ <body> ■ <script laguage="JavaScript"> ■ ■ document.write("<h2>Javascript-Generated output:</h2>

<p>This paragraph generated by JavaScript</p> ■ <p>It can even insert an image</p> ■ <img src='../assigns/RayWeb/images/cathedral.jpg' />") ■ ■ </script> ■ </body> ■ </html>

■ Let’s try it in the browser

Identifier Etiquette

■ Identifier– The name of a variable (or function) ■ Starts with a letter, can contains digits & underscores ■ Case Sensitive!! ■ Should be meaningful to someone reading your code

■ Good: accountBalance, amountDue

■ Bad: bal, due,

■ Just plain wrong: 2bOrNotToBe, +var, total-value

Variables

■ Must declare variables before they’re used in the program ■ Declare at the top of the program & terminate each

statement with ‘;’ ■ Intialize variables when appropriate ■ Local variables (declared within a function) destroyed

after function exit. ■ Can only be accessed within the function

■ Example – Note Assignments ■ var candyBarPrice = 2.50; ■ var taxRate = .075; ■ var candyBarsPurchased;

Assignment Operator

■ Assignment ‘looks like’ equal sign but does NOT behave like it

■ subTotal = subTotal + 1.50

■ subTotal ‘is assigned the value’ that is currently in subTotal plus the value of 1.50

Expressions

■ An expression is a statement that describes a computation

■ Usually look like algebra formulas ■ total = subTotal * taxRate

■ Operators (+, -, *, /, etc.) have different levels of precedence, similar to algebra ■ Don’t rely on it! For clarity, use parentheses

Conditional Statements--if

if (some boolean expression is true){

execute the statements within the curly braces, otherwise skip to the first statement after the closing brace

} Resume execution here in either case

Conditional Statements– if/else

if (some boolean expression is true){

execute these statements, otherwise skip to ‘else’ clause

} else { execute these statements if boolean expression is false

} Resume execution here in either case

Conditional Test Program

Diagnostic messages indicate flow of control var variable1 = 1; var variable2 = 2; if(variable1 >= 0){ document.write(“<p> 1 is greater than 0 </p>"); } document.write(“<p>Resuming execution after 'if' statement</p>");

if(variable1 > variable2){ document.write(“<p>1 is greater than 2</p>"); } else { document.write(“<p>2 is greater than 1</p>"); } document.write("Resuming execution after 'if/else‘ statement</p>");

http://www.sis.pitt.edu/~gray/INFSCI0010/Chp18/conditionalTest.html

Strings

■ Strings are sequences of keyboard characters enclosed in quotes ■ “Hello World” or ‘Hello World’

■ Variables can hold strings ■ var greeting = “Hello World”

■ String can be empty, i.e., contain no characters ■ var myAnswer = “”

■ Use ‘\’ (escape symbol) to ‘type’ prohibited characters ■ \b for backspace, \n for newline, \t for tab, \” for double

quote

JavaScript Functions – Basic Principles

■ Abstraction ■ Experience has taught us that much code is

duplicated throughout program

■ Functions let us write the functionality once, then reuse it


■ Encapsulation ■ Functions encapsulate a specific capability or feature

■ Function name allows us to access a function in our program

■ Parameterization ■ We can customize the function’s result by passing in different

values each time we use it

■ Must use functions to write serious software!


■ Reasons to use functions ■ Ease of communication ■ Problem simplification ■ Easier construction ■ Code readability ■ Reusability ■ Maintainability

■ In JS, functions are the primary encapsulation mechanism

JavaScript Functions – Syntax

■ JS function syntax

function myFunctionName (list of parameters) {

….JS code here…

}

JavaScript Functions -- Issues

■ Key issues about using functions ■ Naming functions ■ Passing in parameters ■ Using local variables within functions ■ How to call (i.e., invoke) functions ■ How to handle a function’s return value ■ Where to put JS functions in your webpage

JavaScript Functions – Naming

■ Function names ■ Name describes what function does ■ Name is an identifier, so follow JS identifier syntax rules

& course coding standards

■ Example, findMaxValue(‘put some parameters here’)

JavaScript Functions -- Parameters

■ Passing parameters to the function ■ Parameters provide functions with input ■ Implicitly declared variables that can be accessed by

code within function body ■ You provide actual input values when you call the

function ■ Parameters are named variables separated by commas

■ Example, function findMaxValue(num1, num2, num3)

JavaScript Functions – Where to put?

■ Put functions within <script>….</script> tags within the <head> section of the web page

<head> <script language=“JavaScript”>

declare functions here….

</script> </head>

JavaScript Functions – Local Variables

■ If needed, you can declare local variables within a function

■ local variable is visible only within the function body after it’s declared

■ Commonly used to store results of an intermediate calculation

JavaScript Functions – Local Variables

function findMaxValue(num1, num2,num3) { var tempMax; //local var

if (num1 >= num2) { tempMax = num1; }

else { tempMax = num2; }

if(num3 >= tempMax) { tempMax = num3; }

return tempMax;

} //end function

JavaScript Functions -- Calling

■ To call a function from your program, add a statement that contains the function name with values for the parameters the function requires

■ Example…somewhere in the <body>…., var x = 1, y = 4, z = 2;

findMaxValue(x, y, z);

■ What value does the function return? ■ What happens with the result?

JavaScript Functions -- Return

■ Return keyword tells function to return some value and exit immediately

■ Function can have multiple return statements but only 1 can be executed in any given function call

■ Normally used to return the final result of a calculation to the calling program

■ For an example, see findMaxValue function


■ Good Example ■ Uses var maxNumber in calling program ■ Function’s return value is assigned to maxNumber ■ Display of maxNumber has correct value for inputs

■ Code snippet ■ var x = 1, y = 4, z = 2;

var maxNumber = findMaxNumber(x, y, z);

document.write(“The maximum is: “ + maxNumber);

http://www.sis.pitt.edu/~gray/INFSCI0010/Chp20/findMaxNumberFunctionGood.html


■ The return must be handled properly or it will be ‘lost’

■ Usually, you want to assign the result of a function to a variable

JavaScript Functions – Parameter Sequence

■ When calling functions, must enter parameters in same order as specified in function argument list.

1. function calculateDensity(height, width, depth, mass){ 2. var volume = height * width * depth; 3. var density = mass / volume; 4. return density; 5. } 6. ………………………………………………. 7. var hth = 2, wth = 3, dth = 4, mass = 10;

8. var result = calculateDensity(2, 10, 3, 4); 9. //returns .07 but correct answer is .42!!!

JavaScript Functions – Global Variables

■ Global variables are those declared outside of functions

■ Global variables are ‘visible’ from anywhere in the program, including inside functions

var globalHello = “Hello!”;

function writeHello() { document.write(globalHello); } // outputs “Hello!”

Example program

http://www.sis.pitt.edu/~gray/INFSCI0010/Chp20/globalVariableInFunction.html

JavaScript Functions – Testing

■ Test each function thoroughly before proceeding with next programming task

■ Using multiple sets of inputs, be sure to test all possible outcomes

■ For each test, be sure calling program is properly handling return values

JavaScript Functions – Debugging

■ Use diagnostic output statements to trace program execution

■ Good places for diagnostic outputs ■ Just before entering function ■ Immediately upon entering function ■ Before/In/After complex logic or computation ■ Just before function return statement ■ Immediately after function returns to calling program

JavaScript Functions

■ Built-In Functions ■ Prompt ■ Alert ■ Confirm

JavaScript and HTML Forms

■ JavaScript Objects ■ Var truck = new Object(); ■ Expression on right is a constructor ■ Properties

■ truck.color=“white” ■ document.write(color);

■ Primitives ■ In JavaScript variable primitive types are

number, string and Boolean


■ Object Model for the Browser Window ■ Compound object structure is created when a

web page loads into a browser ■ Hierarchy

■ Window is an object, the HTML document is an object and its elements are objects

■ These objects have primitives associated with them


■ window [closed, location]

■ history [length]

■ document [bgColor, fgColor, URL, lastmodified,linkColor, vlinkColor] ▪ images [properties] ▪ links [properties] ▪ frames [properties] ▪ forms [properties]


■ Window object is parent of structure ■ window.closed is primitive that is Boolean ■ window.location primitive contains string of the URL of

the HTML file ■ window.document object is primary focus

■ When web page is loaded the HTML elements assign values to most of these window.document primitives

■ Often the word window is excluded as in document.write but need it if referring to multiple open windows

■ Properties can also be objects

JavaScript and HTML Forms■ <HTML> ■ <HEAD> ■ <TITLE>Document Properties</TITLE> ■ <SCRIPT LANGUAGE=JavaScript></SCRIPT> ■ </HEAD> ■ <BODY TEXT="#0000FF" BGCOLOR="#FFFFCC"> ■ <P>Blue text on a yellow background.<BR> ■ <SCRIPT LANGUAGE=JavaScript></SCRIPT> ■ </BODY> ■ </HTML>


■ false #ffffff#00000001/10/2001 17:18:30 Blue text on a yellow background.#ffffcc#0000ff


■ Methods ■ Behavior associated with an object ■ Essentially functions that perform an action ■ Some are built in and others user made

■ Built-In Methods of the window object ■ Confirm ■ Alert ■ Prompt


■ User Events ■ An event occurs when a user makes a change

to a form element ■ Ex. Click a button, mouseover an image

■ Detection of an event done by event handlers ■ Event handler is an attribute of the HTML

button ■ <form>

■ <input type=button value=“please click” onclick=“function name()”>

■ </form>

JavaScript and HTML Forms■ <HTML> ■ <HEAD> ■ <SCRIPT LANGUAGE=JavaScript></SCRIPT> ■ </HEAD> ■ <BODY> ■ <P><FORM > ■ <P><INPUT TYPE=button VALUE="Click Me"

onclick="changecolor()"> ■ </FORM> ■ </BODY> ■ </HTML>


■ Form Object ■ Window.document.form ■ A form is a property of the document but is also

an object ■ Form elements are properties of a form and are

also objects ■ Access to form’s properties is done through the

NAME attribute of the FORM tag ■ Access to the properties of the form elements

is done through the NAME attributes of the particular form element


JavaScript and HTML Forms■ <HTML> ■ <HEAD> ■ <SCRIPT LANGUAGE=JavaScript></SCRIPT> ■ </HEAD> ■ <BODY BGCOLOR="#FFFFCC"> ■ <P><FORM name=addmult> ■ <P>Enter a number in each field: ■ <INPUT TYPE=text NAME=num1 VALUE="" SIZE=5> ■ <INPUT TYPE=text NAME=num2 VALUE=""

SIZE=5><BR> ■ <INPUT TYPE=button VALUE="+" onclick="plus()"> ■ <INPUT TYPE=button VALUE="*"

onclick="times()"><BR> ■ <INPUT TYPE=reset VALUE="Reset Form"><BR> ■ <TEXTAREA NAME=result ROWS=3 COLS=27

WRAP=virtual></TEXTAREA> ■ </FORM> ■ </BODY> ■ </HTML>

JavaScript and HTML FormsForm for submitting info for server side processing

JavaScript and HTML Forms■ <HTML> ■ <HEAD> ■ <SCRIPT LANGUAGE=JavaScript></SCRIPT> ■ </HEAD> ■ <BODY BGCOLOR="#FFFFCC"> ■ <P><FORM name=infoform> ■ <P><TABLE BORDER=0> ■ <TR> ■ <TD WIDTH=83> ■ <P>Name: ■ </TD> ■ <TD> ■ <P><INPUT TYPE=text NAME=fullname VALUE=""

SIZE=32> ■ </TD> ■ </TR>

■ <TR> ■ <TD WIDTH=83> ■ <P>Address: ■ </TD> ■ <TD> ■ <P><INPUT TYPE=text NAME=address VALUE=""

SIZE=32> ■ </TD> ■ </TR> ■ <TR> ■ <TD WIDTH=83> ■ <P>E-mail: ■ </TD> ■ <TD> ■ <P><INPUT TYPE=text NAME=email VALUE="" SIZE=32> ■ </TD> ■ </TR> ■ <TR> ■ <TD WIDTH=83> ■ <P><INPUT TYPE=button VALUE="Submit"

onclick="verify()"> ■ </TD> ■ <TD> ■ <P><INPUT TYPE=reset VALUE="Clear Your

Information"> ■ </TD> ■ </TR> ■ <TR> ■ <TD COLSPAN=2> ■ <P><TEXTAREA NAME=display ROWS=5 COLS=41

WRAP=virtual></TEXTAREA> ■ </TD> ■ </TR> ■ </TABLE>

■ </FORM> ■ </BODY> ■ </HTML>

Software

Introduction to JavaScript