• Second, user-defined objects are all objects defined by the programmer and cre-
ated during the execution of JavaScript code.
• The last group ar e host objects, which are defined by the host environment in
which the JavaScript interpreter is running. In our c ase, host objects will be the
browser objects because we run our programs within a browser.
11.3 Cl asses
Professor: The objects co mplex, conjugate, and englGerm, which we just create d,
were all unique, eac h having its own set of properties. It is, however, often useful to
define not just properties but also a behavior of an object. For complex numbers, for
instance, you could define arithmetic operations. Note tha t it wouldn’t make much
sense to define arithm etic op erations for each complex object separately as we did
for th e im and re property values, which must be unique for each object. In suc h
cases it is more useful to define a class o f objects. You may then cr eate members
or instances of th e defined class, which have their own unique property values that
specify their individual state, and methods that define their behavior and are shared by
all the members of the class.
The following picture shows an imaginary class named Dog and an object instance of
this class named myDog.
Class
Dog
Properties:
Methods:
sitting
length
sit()
bark()
quiet()
Property values:
sitting = false
length = 85
name
name = "Maxwell"
barking
barking = false
Object instance
myDog
come()
Methods:
sit()
bark()
quiet()
come()
Constructor:
Dog()
You can see how the o bject instance has the same methods as the class ob je ct, wh ile it
specifies its own values for the properties. The class also defines a constructor, which
is used for instantiating member obje cts.
To a certain extent, you are already familiar with these concepts. Date, for example,
is a class, and you create a Date instance using the Date() construc tor, like this:
var today = new Date(); //today is an instance of the Date class
Maria: I remember that you once said a class was a blueprint for creating o bjects.
11.3. Cla sses 209
Professor: Th at’s exactly what it is. I will now show you how to define your own
class.
Mike: But no complex numbers, please.
Professor: OK, let me think of something really simple. Are you familiar with ele-
mentary algebraic operations on two-dimensional vectors?
Maria: Can you please just briefly review the basics?
Professor: OK, let’s take vector addition a nd dot product. A ssume you h ave two
vectors, v
1
= (x
1
, y
1
) and v
2
= (x
2
, y
2
). You compute the sum of two vectors by
simply adding the corresp onding components, so that the x component of the resulting
vector is the sum of the x comp onents of both vectors, and the y componen t of the
resulting vector is the sum of the y components of both vectors:
v
1
+ v
2
= (x
1
+ x
2
, y
1
+ y
2
)
The dot product is the sum of the pro ducts of the corresponding compon ents of eac h
vector:
v
1
· v
2
= (x
1
x
2
+ y
1
y
2
)
Notice that the sum of two vectors is again a vector while the dot product of two
vectors is a single number.
We are going to define a class Vector, for which we first need a constructor. When-
ever y ou invoke a constructor using new, an object instance is automatically crea te d.
You d on’t need to worry about creating one by yourself. A constructor only needs to
set the initial state of the created object in stance b y settin g values o f its pro perties.
You can set those values using the th is keyword, which ac ts as a reference to the
object instance just being created. Here’s how a de finition of a Vector constructor
looks:
var Vector = function(x, y) {
//Create and initialize the properties x and y of the created
//instance of class Vector.
this.x = x;
this.y = y;
};
Notice that I capitalized th e name of the constructo r. Because constructors define
classes, it is a com mon convention that th ey have names that begin with capital letters.
In co ntrast, regular functions and variable names begin with lower-case letters.
When the body of a constructor starts to execute, a new object instance ha s already
been c reated and this is a reference to that obje ct instance. With the two assignment
statements inside the Vecto r() constructor, we created the properties x and y and
210 Meeting 11. Building Your Own Objects
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