11. session 11 functions and objects

Functions and Objects
Session 11

Objectives
 User-defined functions.
 User-defined objects.
 JavaScript objects.

What is a function?
 A function is an independent reusable block of
code that performs certain operations on variables
and expressions to fulfill a specific task.
 In JavaScript, a function is similar to method, but
there is a little difference between them. A
method is always associated with an object and is
executed by referencing that object. But a
function is executed independently.
 In JavaScript, a function is always created under
the Script element.

Function Definition
 Syntax:
function <functionName>([paraList])
{
//Body of function
}
- <functionName>: comply with identifier naming convention.
- [paraList]:is optional. If there are many parameters, separated by commas.
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script language="javascript" type="text/javascript">
function add(num1, num2)
{
var result = num1 + num2;
document.write("Result of " + num1 + " + " + num2 + " = " + result);
}
</script>
</head>
<body onLoad="add(5, 17)">
</body>
</html>

Invoking functions
 You can invoke or call a unction to execute it in the browser.
 You can call a function from another function in JavaScript. The
function that calls another function is called calling function,
whereas, the called function is referred to as the called function.
 Syntax to calling a function
<functionName>([paralist]);

Ways of passing parameters
 There are two ways of passing parameters to a function
namely, pass by value and pass by reference.
 Passing by value refers to passing variables as
parameters to a function. The called function do not
change the values of the parameters passed to it from
the calling method. Because, each parameter occupies
different memory locations.
 Passing by reference, the called can change the value of
parameters passed to it from the calling method.

Calling by value- Demo
<head>
<title>Calling function by value</title>
function swap(num1, num2)
{
var temp = num1; num1 = num2; num2 = temp;
document.write("Called method: num1="+num1+ " and num2=" + num2);
}

function calculate()
{
var num1=10, num2=20;
swap(num1, num2); //Invoking the swap mathod
document.write("<br>Calling method: num1 = " + num1 + " and num=
" + num2);
}
</script>
</head>
<body onLoad="calculate()">
</body>
</html>

Calling by reference - Demo
<head>
<title>Calling function by reference</title>
function modify(names)
{
names[0] = "Thanh Hung";
}
function initialize()
{
var names = new Array("James","Helene","John");
document.write("Before invoking method:<br>");
for(var i=0; i<names.length; i++)
document.write(names[i] + " - ");
modify(names); //Invoking method
document.write("<Br>After invoking method:<br>");
for(var i=0; i<names.length; i++)
document.write(names[i] + " - ");
}
</script>
</head>
<body onLoad="initialize()“></body></html>

return statement
 JavaScript allows you to send the result back to the
calling function by using return statement.
 Syntax:
return <expression>

Returning an array
 You can use the return statement to return a collection
of values stored in arrays.
 Syntax:
return <ArrayObject>

Object definition
 In JavaScript, object is a collection of properties and methods.
Properties specify the characteristics or attributes of an object, while
methods identify the behavior of an object.
 For example, car is an object in the real world.
 Car properties: brand, model, color, number, …
 Car methods: run, reverse, brake, …
 In JavaScript, objects have their own properties and methods.
JavaScript provides built-in objects and allows creating user-defined
objects.
 Pre-defined objects are those objects that are already defined and you
just need to use their properties and methods to perform a specific
task. (e.g. Array object).

Creating user-defined objects
 Can create custom objects to store related data in a script.
Example: to store the doctor details such as name, age,
hospital name …, can to create doctor object.
 There are two methods to create a custom object:
 By using Object object: This is known as the direct method.
var <objectName> = new Object();
 By defining a template and initializing it with the new keyword.

There are two steps to create an object by using this method.
 First: Declare the object type by using the constructor.
 Second: Specify the object of declared object type by using the new keyword.
function <objectType>(paraList)
{
//Body specifying properties and methods
}

Creating properties
 You can define properties of a custom object by specifying
the object name followed by a period and property name.
 To get value to property of custom object:
<objectName>.<property>
 To set value to property of custom object:
<objectName>.<property> = <value>

Creating properties in template
 You can specify the properties in the template, if the template
method is implemented to create a custom object.
 In the constructor to create the custom object, you want to
declare properties with the same names as that of parameters
to specify meaningful names of properties.
 Finally, you can set and get the value o properties easily.

Creating methods
There are two ways to define methods o custom object.
 First way: for the custom object created by Object object.
<objectName>.<MethodName> = function([paraList])
{
//Body of function
}

Creating methods in a template
 Second way: creating methods in a template. You can create the custom
method with the following steps:
1. Define a method function that implements a functionality.
2. Define a constructor function where the custom method is assigned
the name of the method function.
3. Create an object.
4. Invoke the custom method, which in turn, will invoke the method
function.

String object
 Strings in JavaScript are set of characters that are surrounded
by single or double quotes.
 The built-in String object represents such a set of characters
and allows you to perform different text operations on them.
 You can perform these text operations by creating an instance
(custom object) the String object.
 Syntax:
var <objectName> = new String(“strings”);
 Example:
var name = new String(“John Smith”);

Properties and methods of String object
 Properties:
 length: retrieves the number of characters in a string.

 Methods:
 chartAt(): retrieves a character from a particular position
within a string.
 concat(): concatenates two strings.
 indexOf(): retrieves the position at which the specified string
value first occurred in the string.
 lastIndexOf(): retrieves the position at which the specified
string value last occurred in the string.
 match(): matches a regular expression with the string and
replaces it with a new string.
 search(): searches for a match where the string is in the

same format as specified by a regular expression.

Properties and methods of String object
 Methods:
 split(): divides the string into substrings and defines an array
of these substrings.
 substring(): retrieves a part of a string between the specified
positions of a string.
 toLowerCase(): specifies the lowercase display of the string.
 toUpperCase(): specifies the uppercase display of the string.
 charCodeAt(): retrieves the Unicode of character located at a
particular position.
 fromCharCode(): retrieves the string representation of the
specified set of Unicode values.
 substr(): retrieves the particular number of a characters in a
string from the specified index until the specified length.

Math object
 The Math object allows you to perform mathematical operations
on numeric values. It is a pre-defined object that provides static
properties and methods to perform mathematical operations.
 Properties:
 E: retrieves the Euler’s constant that is approximately 2.7183.
 PI: retrieves the value of pi that is approximately 3.142.

 Syntax to use Math properties:
var <variableName> = Math.<Property>
 Example:
var Pi = Math.PI;

Methods of Math object
 Methods:
 abs(): retrieves the absolute value of a numeric value.
 ceil(): retrieves the integer greater than or equal to the
given numeric value.
 floor(): retrieves the integer less than or equal to the given
numeric value.
 exp(): returns E exponent of parameter value.
 max(): retrieves the greatest value from the list of values
passed as arguments.
 min(): retrieves the most lesser value from the list of values
passed as arguments.
 pow(): calculates and retrieves the value of a raised to the
power of b.

Methods of Math object
 Methods:
 random(): retrieves a random value between 0 to 1.
 round(): is used to round the number.
 sqrt(): retrieves the square root of a numeric value.

Date object
 The Date object allows you to define and manipulate the
date and time values programmatically.
 The Date object calculates dates in milliseconds from 01
January, 1970.
 You can specify date and time by creating an instance of the
Date object. This is done by instantiating the Date object
with the new keyword.
 Syntax:
var <objectName> = new Date();
 Example:
var today = new Date();

Methods of Date object
 Methods:
 getDate(): retrieves the day of month (1-31).
 getDay(): retrieves the day of week (0-6).
 getMonth(): retrieves the month of year (0-11).
 getFullYear(): retrieves the year.
 getHours():retrieves the hour value between 0 and 23.
 getMinutes(): retrieves the minute value (0-59).
 getSeconds(): retrieves the second value (0-59).
 getTime(): retrieves a numeric value, which indicates the
time passed in milliseconds since midnight 01/01/1070.
 setTime(): specifies the time based on the given
milliseconds, which have been elapsed since 01/01/1970.

with statement
 You need to use the object name every time when you
want to access its properties and methods. This affects the
readability of the code. To overcome this drawback, you
can use the with statement.
 The with statement allows you to remove the object
reference for each JavaScript statement. You can reference
directly properties and methods of the object after using
with statement with object.
 Syntax:
With(<objectName>)
{
//statements
}

Summary
 Javascript function is a block of code that can
be resused later in the script.
 Javascript Object is a set of properties and
methods, which allow store and manipulate
information about specific entity.
 Can create custom functions, custom objects
with custom properties and methods.

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11. session 11 functions and objects

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