Declaring method inside class

Question

I am new to javascript object oriented programming. I wish to write a JS class which has some variable and methods.

I am trying this way:

function B(){
    function makeB()
    {
        alert('make B');
    }
}
var b = new B();
b.makeB();

It shows: Uncaught TypeError: Object #<B> has no method 'makeB'

Can't I declare a function like this? But by simply adding this with the same, I am able to access it. Is it a local function within the variable.?

Apart from this, I tried this:

function B(){
    function call()
    {
        alert('B call');
    }
    this.makeB = function(){
        call();
        alert('makeB'); 
    }   
}
var b=new B();
b.makeB();

Using this, makeB can internally call call but if I try to access this using prototype it doesn't

function B(){
    function call()
    {
        alert('B call');
    }
}
B.prototype.makeB = function()
{
    call();
    alert('callB');
}

Here again I am not able to call call from makeB as if call is block specific function.

And what to do if I want any function to be private to a specific class and not to inherited by the child class. A method which can only be used by its object and not by object of inherited class.

function C(){}
C.prototype = new B();
C.prototype.constructor = C;

Say, I want object of C to call makeB but not call. In this case what should I do.?

JavaScript is prototype based and doesn't have a private modifier. Answers provided do not mention that using them makes your code harder to test re use and maintain. More on constructor function, prototype and why not to use closures to simulate private can be found here:http://stackoverflow.com/a/16063711/1641941 — HMR, Feb 05 '14 at 07:31

score 1 · Accepted Answer · answered Feb 05 '14 at 06:52

in order to declare publicly accessible function inside JavaScript object you have to use this. By applying this you actually expose this function as a property of an object

function Person(first,last) {
   this.firstname = first;
   this.lastname = last;

   //private variable available only for person internal use
   var age = 25;

   //private function available only for person internal use
   var returnAge = function() {
      return age;
   };

   // public function available as person propert
   this.askAge = function()
   {
       return returnAge ;
   }
}

var john = new Person('John','Smith');
console.log(john.returnAge); // will return undefined
var johnsAge = john.askAge(); // will return 25

score 1 · Answer 2 · answered Feb 05 '14 at 06:57

I'll try to answer in the order of your questions...

In your first example, the function call is considered private and will not be accessible outside of the object scope itself.

For your second question, it will be public if you assign the function to this or what I like to do, create a "private" variable called self or sometimes on late nights I might call it that. That way within other function calls and callbacks you always have scope of the object context.

 function B(){
   var self = this;
   function call()
   {
       alert('B call');
   }
   self.makeB = function() {
     call();
     alert('makeB'); 
   }   
 }
 var b=new B();
 b.makeB();

For the last I would say look at your inheritance chain and build your base with only the functions that will be shared by inheriting objects.

Hope this helps.

Declaring method inside class

2 Answers2