Functions and Objects

Functions and Objects

First Midterm exam

Date: October 8, 2008Content:

Two parts:

On-paper: Multiple choices

On-computer:

Write codes

Cover everything from Week 1 to Week 5

Working with Functions

Functions allow modular/structure programming Functions encapsulate multiple statements so that

these statements can be reused A function is a block of statements that performs

some tasks and/or returns a value A function is executed when called Function and method can be used interchangeably

Example A function to format text before it is written to the page could be

reused whenever you desired the same formatting

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

function boldText(incomingText) { var newText = "<b>" + incomingText + "</b>"; return newText; }

document.write(boldText("Important Information"));</script>

How to design a Function

Functions should perform only one job, this makes the function more useful

Functions can return a value to the calling statement In this way functions can be made more flexible

by working on many different pieces of data Functions are encapsulated (self contained) Functions must have unique names

Function Syntax Each function must have a unique name, and normally

declared in the <head> tag, must be defined before used.

The keyword “function” is used to begin a function definition

Use { } brackets enclose the statement block of a function

Syntax of creating a function:function <function name> (<parameters>) {

one or more statements}

Syntax of calling a function:<function name> (<arguments>)

Parameter Passing Parameters passed to

functions do not necessarily affect the original values of the variables

JavaScript passes variables by “value” and not by “reference to the variable”

Because of this passing by value, local copies of the variables can have different names than the original variables

Parameter Passing

Arguments passed

To receiving function

Arguments received and stored in local variables. They will disappear when

the function ends

Calling function Receiving function

function name (arg1, arg2..) function name (par1, par2..)

Creating a Function

In the head:

<script type="text/javascript">

function displayInterest ( ) {

var interest= simpleInterest( );

if (document.promissoryNote.time.value == 1) {

window.alert("Interest after one year is $"+interest+".");

}

else {

window.alert("Interest after "+document.promissoryNote.time.value+" years is $"+interest+".");

}

}

Calling function from a javascript

Creating a Function (continued)function simpleInterest() { return document.promissoryNote.principal.value *

document.promissoryNote.annualInterestRate.value/100 * document.promissoryNote.time.value;

} </script>

In the body: <form name="promissoryNote"> Principal: $<input type="text" name="principal"/><p> Annual Interest Rate:

<input type="text" name="annualInterestRate"/> %<p> Time: <input type="text" name="time"/> years<p> <input type="submit" value="Calculate Interest"

onClick="displayInterest();"/> </form>

Multiple Calls to the Same Function

In the head


function displayInterest( ) {

var msg="Cumulative interest. ";

var cumInterest=0;

for (i=0; i<document.promissoryNote.time.value; i++) {

var interest= simpleInterest();

cumInterest=cumInterest+interest;

msg=msg+" Yr "+(i+1)+": $"+cumInterest;

}

window.alert(msg);

}

Multiple Calls to the Same Function(continued)

function simpleInterest() { return document.promissoryNote.principal.value * document.promissoryNote.annualInterestRate.value/100; } </script>

In the body <form name="promissoryNote"> Principal: $<input type="text" name="principal"/><p> Annual Interest Rate: <input type="text" name="annualInterestRate"/>

%<p> Number of Years: <input type="text" name="time"/> years<p> <input type="submit" value="Calculate Interest"


Passing Parameters


function displayInterest()

{

var msg="Cumulative interest. ";

var cumInterest=0;

for (i=0; i<document.promissoryNote.time.value; i++) {

var interest= simpleInterest();

cumInterest=computeCumInterest(cumInterest,interest);

msg=msg+" Yr "+(i+1)+": $"+cumInterest;

}

window.alert(msg);

}

Passing Parameters(continued)

function computeCumInterest(a,b) { var _interest=a+b; a=a*1000; return _interest; }

function simpleInterest( ) { return document.promissoryNote.principal.value * document.promissoryNote.annualInterestRate.value/100; } </script>

Passing Parameters

<form name="promissoryNote"> Principal: $<input type="text"

name="principal"/><p> Annual Interest Rate: <input type="text"

name="annualInterestRate"/> %<p> Number of Years: <input type="text"

name="time"/> years<p> <input type="submit" value="Calculate Interest"


Calling function from an event

Calling functions from a link


function greetings() {

document.bgColor ="lightblue";

alert("Greetings to you!");

}

<a href="javascript:greetings()"> Click here for Salutation </A> </center>

Calling function from a link

Variable Scope in Functions

Variables in functions have meaning either globally or locally

Global variables are created outside any functions

Local variables are created inside a function Global or local variables can be referenced

or altered inside functions

Example

<script language=javascript>var name="William";var hometown ="Chico";

function greetme() {var name="Daniel";document.bgColor="cyan";document.write("<h2> In function, <em> name </em> is

"+name);document.write(" and <em> hometown </em> is

"+hometown);}greetme();document.write("<h2> Out of the function, <em> name </em> is "+name);document.write(" and <em> hometown </em> is "+hometown);

</script>

Global variables

Local variable

Return a value

A return can be used to send back the results of some tasks

The returned value then can be assigned to a variable if the call to the function is a part of an expression

Summary Functions allow for the re-use of code Functions must begin with the keyword “function” Parameters can be passed to functions to be acted

upon Functions can return (pass back) values to the

calling statement Functions are an excellent way of validating data in

an HTML form before sending the form to the server Functions can be called multiple times Variables have different scopes inside and outside

of functions

Advantages of Functions

Promote good application design Because they move discrete chunks of script

logic into re-usable modules Modules can be one or more times Provide a library of modules that can be used in

more than one program or page Reduces the amount of time required to create

scripts Reduces the amount of time required to test and

debug scripts

Lab

Step 1: Copy and paste (or type) this code<html> <head> <title> Practice debugging Javascript </title><script type="text/javascript">function addem() {

var n=2;var y=3;document.write(n+y,"<br>");}

</script></head><body bgcolor=red><a href="javascript.addem()"> Click here </a><H2> Hello </H2></body></html>

Lab

Step 2: Run this code. What is wrong? Please fix it.

Step 3: Modify the script by adding a function called changeColor ( ). This function will take one parameter: a color. Its function is to change the background color of the current document to the given color

Lab

Step 4: Modify the body of the existing HTML file to add a form, which looks like:

When a user click on the left button, the background is changed to yellow. When a user clicks on the right button, the background is changed to light green

Objects

Work with JavaScript objectsCreate user-defined objects

Working With Objects

JavaScript is an object based languageObjects encapsulate variables called properties

These properties can be changed by a script

Objects can contain methods as well as properties

Methods are used to extend the functionality of the object

Using the Object Constructor

Syntax:function objectname(parameters){

this.detail = reference;…..

}

Example:function customerInformation(custName, custAddress, custCity, custState, custZip) {

this.customerName = custName;this.customerAddress = custAddress;this.customerCity = custCity;this.customerState = custState;this.customerZip = custZip;

} Advantage

Used for compatibility with older browsers

Accessing Object Properties and Methods

Syntax:object.<property name>object.<method name> (<arguments>)

Example:window.name – returns the name of the current windowwindow.open() – opens a new browser window

Example of object properties

<script type=“text/javascript”>var.pet =new Object();

pet.cat = new Object();

pet.cat,name=“Friend”;

pet.cat.color=“yellow”;

pet.cat.size=“medium”;

</script>

petcat

FriendYellow

Medium

Example of object methods

<script type=“text/javascript”>var.pet =new Object();

pet.cat = new Object();

pet.cat,name=“Friend”;

pet.cat.color=“yellow”;

pet.cat.size=“medium”;

function changeSize() {

pet.cat.size=“fat”

}

</script>

petcat

FriendYellow

Medium

Creating a User-Defined Object

<script type="text/javascript"> function checkDetails() { customer=new Object(); customer.firstName =document.customerAccount.firstName.value; customer.lastName=document.customerAccount.lastName.value, customer.zip=document.customerAccount.zip.value;

window.alert("Dear "+customer.firstName+" "+customer.lastName+", you are eligible to enroll for Internet Banking.");

} </script>

Creating a User-Defined Object(continued)

<form name="customerAccount"> First Name: <input type="text" name="firstName"/><p> Last Name: <input type="text" name="lastName"/><p> Zip: <input type="text" name="zip"/><p> <input type="submit" value="Verify Details"

onClick="checkDetails();"/> </form></html>

Creating an Object Method

function checkDetails() {

customer=new Object();

customer.firstName =document.customerAccount.firstName.value;

customer.lastName=document.customerAccount.lastName.value,

customer.zip=document.customerAccount.zip.value;

if(verifyZip(customer.zip)) {

window.alert("Dear "+customer.firstName+" "+customer.lastName+", you are eligible to enroll for Internet Banking.");

} else {

window.alert("Dear "+customer.firstName+" "+customer.lastName+", you are not eligible to enroll for Internet Banking.");

}

}

Creating an Object Method(continued)

function verifyZip(zip) {

if (zip<23228)

{

return true;

}

else

{

return false;

}

}

</script>

Creating an Object Method(continued)

<form name="customerAccount">

First Name: <input type="text" name="firstName"/><p>

Last Name: <input type="text" name="lastName"/><p>

Zip: <input type="text" name="zip"/><p>

<input type="submit" value="Verify Details" onClick="checkDetails();"/>

</form>

</body>

</html>

Summary

Objects have properties and methods Object properties can be manipulated Objects can be created using an instantiation

process or by using the Object constructor JavaScript allows for User-Defined objects

Practice exercise

Which of the following is required in HTML to use JavaScript?

<head>

<body>

<script>

<java>

Matching

I.JavaScript A. Http://www.prenhall.com/

II. Java B. allows clients to request a page

III.URL C. object-based

IV. HTTP D. object-oriented

V. Variable E. Occur when something is happened

VI. Event-handling F. Data can be stored there

Choices

3. What does this mean: if (is_ie5up || is_nav4)?

A. If both, is_ie5up and is_nav4 are true.

B. If neither, is_ie5up and is_nav4 are true.

C. If, is_ie5up or is_nav4 are true

D. All of the above.

Choices

What type of variable is ourCustomer in the following code: var ourCustomer=”true”;?

a. Boolean

b. Null.

c. Number

d. String

Choices

An "if" statement without an "else" includes statements for which of the following?

a. Logic for neither the true or false piece.

b. Logic for only the true piece.

c. Logic for only the false piece.

d. Logic for both the true and false piece.

Choices

Which of the following is the CORRECT syntax for an if statement?

A. if annualIncome < 5000

B. if annualIncome < 5000;

C. if (annualIncome < 5000)

D. if (annualIncome < 5000);

Choices

What is the minimum number of values that can be returned in a function?

A. 0

B. 1

C. 2

D. 3

Choices

What is TRUE in the following statement: var interest = simpleInterest (int, rate); ?

A. The function has no return values or parameters.

B. The function has only a return value.

C. The function has only parameters.

D. The function has a return value and parameters.

Functions and Objects

Documents