In javascript, the subclassing is done this way:
[1] function Person(name, sex){ # Define the superclass
[2] this.name = name
[3] this.sex = sex
[4] this.prn = function(){ return this.name + ', '+ this.sex }
[5] }
[6] function Man(name){ this.sex = 'M' } # Define the subclass
[7] Man.prototype = new Person() # Connect Person and Man
And the instantiation:
[8] p = new Person('Jane','F')
[9] m = new Man('John')
Checking the function
prn (defined in [4]):
p.prn() ==> Jane, F
m.prn() ==> undefined, M
As shown, the name
John given to
Man when instantiating
m ([9]) is NOT transferred to
m. It is obvious 'cos we didn't instruct it to do so when defining
Man in [6].
Problem.1: Subclass Arguments Are Not Automatically Transferred To Subclass Instances
So, unless explicitly instructed, the arguments of a subclass will not be assigned to the instance of that subclass. That means, [6] needs to be re-defined as:
function Man(name){
this.sex = 'M'
this.name= name # <== new line
}
Note that the above highlighted line already exists in the superclass (line [2]). Why does it have to be re-done again in the subclass ? Didn't we already instruct
Person to be the super class ?
Problem.2: An Instance of Subclass Is an Instance of the Superclass too ?
Lets investigate the
instanceof operator in javascript:
[10] (p instanceof Person) ==> true
[11] (m instanceof Man) ==> true
[12] (m instanceof Person) ==> true
Line [10] and [11] are what we expect. But [12]? How come
m is an instance of
Person ? This doesn't sound right.
Problem.3: The Constructor of A Subclass' Instance Is The Superclass But Not the Subclass ?
Check the constructor of the instances of subclass and of superclass:
[13] getConstructor(p) ==> Person
[14] getConstructor(m) ==> Person
Shouldn't the constructor of
m be
Man?
Solution Proposal
Lets try a new way of defining a subclass. Instead of
function Man(name){
this.sex = 'M'
this.name= name # <== new line
}
We try this:
function Woman(name){ update(this, new Person(name, 'F'))}
w= new Woman("Jan")
See the following table for comparison:
| Classes | Instance | instanceof
Person | constructor |
function Person(name, sex){
this.name = name
this.sex = sex
this.prn = function(){
return this.name +','+this.sex}
} | p | true | Person |
function Man(name){
this.sex = 'M'
this.name= name }
Man.prototype = new Person | m | true | Person |
function Woman(name){
update(this, new Person(name,'F'))
} | w | false | Woman |
