Emerging Technology Fall Semester, 2004 JavaScript part 2
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CS 683 Emerging Technologies Fall Semester, 2004 Doc 5 JavaScript part 2

Contents

References

Exceptions

Objects

Properties

Methods

Prototypes

Inheritance

Copyright ©, All rights reserved. 2004 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA. OpenContent ( http://www.opencontent.org/opl.shtml) license defines the copyright on this document.

References

JavaScript: The Definitive Guide, 4th Ed., David Flanagan, O’Reilly, 2002

ECMAScript Language Specification v3, Dec 1999,

http://www.ecma-international.org/publications/files/ecma-st/ECMA-262.pdf

Remembering the Context

var a = 'global';
function test() 
   {
   print( a);
   }
   
function perform(aFunction )
   {
   var a = 'local';
   aFunction();
   }
   
perform(test);

Code prints global

More Context

var b = 'global';
   
function give( )
   {
   var b = 'local';
   function inner() 
      {
      print( b );
      }
   return inner;
   }
   
var test = give();
test();

Code prints local

Exceptions

try
   {
   var n = prompt( “Enter positive integer” , “”);
   var result = factorial(n);
   alert(‘Answer is: ‘ + result);
   }
catch (exception )
   {
   alert(exception);
   }
finally
   {
   alert( ‘Done’);
   }

Throwing an Exception

throw new Error(“a foobar occurred”);
   
throw 5;
   
throw ‘Cat’;

Objects

Creation

var anObject = new Object();
   
var now = new Date();      //now
   
var semesterStart = new Date(2004, 8, 30);
   
var circle = { x: 0, y:0, radius: 3 }
   
print( circle.x);
print( circle.y);
print( circle.radius);

Constructors

function Circle(x, y, radius)
   {
   this.x = x;
   this.y = y;
   this.radius = radius;
   }
   
var y = new Circle(1, 2, 3);

Properties

var circle = { x: 0, y:0, radius: 3 };

x, y, radius are properties of the object circle

circle.x = 12;         //set
   
var z = circle.y;      // read
   
circle.theta = 1.342;   // create new property
   
var w = {z: circle, name: ‘sam’ };
   
w.z.radius;         // nested properties
   
var a = circle[‘x’];       //read x
   
circle[‘x’] = 55;         //set x

Enumerating Properties

var circle = { x: 0, y:0, radius: 3 }
   
for (var name in circle)
   print( name);

Output

x
y
radius

for (var name in circle)
   print( circle[name]);

Output

0
0
3

Methods

function Circle(x, y, radius)
   {
   this.x = x;
   this.y = y;
   this.radius = radius;
   }
   
function computeArea()
   {
   return Math.PI * this.radius * this.radius;
   }
   
var circle = new Circle(0, 0, 3);
   
circle.area = computeArea;
   
circle.area();         //28.274

var x = new Circle(0, 0, 3);
   
x.area()            //raises an exception

Methods in Constructors

function computeArea()
   {
   return Math.PI * this.radius * this.radius;
   }
   
function Circle(x, y, radius)
   {
   this.x = x;
   this.y = y;
   this.radius = radius;
   this.area = computeArea;
   }
   
var x = new Circle(0, 0, 3);
   
x.area()            //28.274

Now every circle object will contain an area property. All of them will contain the same value. This is a waste of space

Prototypes

function computeArea()
   {
   return Math.PI * this.radius * this.radius;
   }
   
function Circle(x, y, radius)
   {
   this.x = x;
   this.y = y;
   this.radius = radius;
   }
   
Circle.prototype.area = computeArea;
Circle.prototype.pi = 3.1415;
   
var x = new Circle(0, 1, 3);
   
x.area();
x.pi;

Prototype Rules

Each object has a prototype

Constructors define new types of objects

An object’s prototype starts as a clone of Object’s prototype

Reading a property

circle.x;

If the object has the property return its value

If not check the object’s prototype for the property and return its value

If don’t find the property it is not defined

Writing a Property

circle.x = 23;

If the object has the property set the value

If not create the property in the object and set it.

Simulating Class Methods & Properties

function increase(n) {return n +1;};
   
function Circle(x, y, radius)
   {
   this.x = x;
   this.y = y;
   this.radius = radius;
   }
   
Circle.PI = 3.1425;
   
Circle.increase = increase;
   
   
Circle.PI;               //returns 3.1425
   
Circle.increase(3 );      //returns 4
   
var x = new Circle(1, 2, 3);
   
x.PI;               //not defined
   
x.increase(3);            //Exception thrown

Inheritance

function Rectangle(centerX, centerY, height, width)
   {
   this.x = centerX;
   this.y = centerY;
   this.height = height;
   this.width = width;
   }
   
Rectangle.prototype.area = function()
      {return this.height * this.width; }
   
Rectangle.prototype.isSquare = function()  {return false; }
   
function Square(centerX, centerY, height)
   {
   this.x = centerX;
   this.y = centerY;
   this.height = height;
   this.width = height;
   }
   
Square.prototype = new Rectangle(0,0,0,0);
Square.prototype.constructor = Square;
   
Square.prototype.isSquare = function()   {return true; }
   
var x = new Square(0,0,2);
x.area();

Object Properties & Methods

toString()

toLocaleString()

valueOf()

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