How to define a local function in Constructor.prototype in Javascript

Question

Hi Guys I am facing difficulty in one of scenario where in I m not able to define local function in my Manager.prototype. Please find below detail..

I have a Constructor Function Employee.

function Employee(id){
   this.id = id;
}
Employee.prototype.getID = function(){
    return this.id;
}
var mark = new Employee(123);

Again I have a Manager Constructor

function Manager(managerOf){
   this.managerOf = managerOf;
}
Manager.prototype = Object.create(Employee.prototype);
Manager.prototype.getManagerOf = function(){
    return this.managerOf;
}
var john = new Manager(mark);

Now I want to define a function calcSalary() which is only accessible from getManagerOf() method & not from outside. [john.calcSalary() should not work]

Just declare it within the scope of `getManagerOf`. It won't be accessible from outside the function. — haim770, Nov 08 '16 at 12:56
No I wanted it be more generic, so that tomorrow if I add one more prototype function it can be accessible from that one as well, any thoughts? — Gautam, Nov 08 '16 at 13:03
There are patterns used to achieve "private" methods in JS. Typically using closures. And the example provided in the answer of @Daniel A. White is a good one. — haim770, Nov 08 '16 at 13:04
If you want your managers to inherit from employee, then you need an `Employee.call(this, id)` call in the `Manager` constructor - currently `john.getID()` wouldn't work. — Bergi, Nov 08 '16 at 13:05
@Bergi you are absolutely right, I can do that , but right now my concern is how can we add a generic local function to a prototype which can be accessed by all protoypal functions. — Gautam, Nov 08 '16 at 13:12

Daniel A. White · Accepted Answer · 2016-11-08T13:06:12.497

3

You could hide it with a self executing function.

var Manager = (function() {
   function calcSalary() {}
   function Manager(managerOf){
      this.managerOf = managerOf;
   }
   Manager.prototype = Object.create(Employee.prototype);
   Manager.prototype.getManagerOf = function(){
      // call calcSalary
      return this.managerOf;
   }
   return Manager;
}());

var john = new Manager(mark);

edited Nov 08 '16 at 13:06

answered Nov 08 '16 at 12:54

Daniel A. White

187,200
47
362
445

This doesn't seems right, I won't be able to use new keywork on my constructor anymore. Also If I wish to inherit my Manager/Manager.prototype object, then how would I do that? – Gautam Nov 08 '16 at 13:05
1

@RanjeetGautam i forgot to export it. – Daniel A. White Nov 08 '16 at 13:06
@RanjeetGautam the same way you did for Employee to Manager. – Daniel A. White Nov 08 '16 at 13:09
@DanielA.White, What if `calcSalary()` needs access to the `this` (stateful)? – haim770 Nov 08 '16 at 13:14
@haim770 just pass a reference like `calcSalary(this)`. – Daniel A. White Nov 08 '16 at 13:15
Thanks @Daniel A. White ! This works, I am able to acheive what I wanted. Thanks a ton. – Gautam Nov 08 '16 at 13:17
@DanielA.White, Effectively making it a static / non-instance method, as opposed to a real "private" method that is stateful. – haim770 Nov 08 '16 at 13:17
1

there's no `private` in javascript @haim770 – Daniel A. White Nov 08 '16 at 13:17
@DanielA.White, Obviously, Yet, I was under the impression that's what the OP is trying to achieve. – haim770 Nov 08 '16 at 13:18
1

@DanielA.White FYI: This JS construct you re advising is called “immediately-invoked function expression” (IIFE) – nothing is executing itself. (It would be a perpetuum mobile if it were self-executing.) See: https://en.wikipedia.org/wiki/Immediately-invoked_function_expression and http://stackoverflow.com/questions/8228281/what-is-the-function-construct-in-javascript – feeela Nov 08 '16 at 13:25
@RanjeetGautam it should - that sounds like a new question. – Daniel A. White Nov 08 '16 at 13:30

How to define a local function in Constructor.prototype in Javascript

1 Answers1