Declaring private static member variable in JavaScript

Question

I am stuck with a design pattern in JavaScript where I would like to create a private static member variable for keeping object count. Something along these lines:

var person = function(name){
    //How to persist this value across multiple calls avoiding Globals
    var _personCount = _personCount || 0;
    _personCount++;
    var _name = name;
    getPerson = function(){
          return "Person is: " + _name;
    }
};

person("foo");//_personCount should be 1 after this call
person("bar");//_personCount should be 2 after this call

The idea is something to similar to private static variables, accessible only to the class's internal variables

First, use an actual object instead of just calling a function without new. Those variables are local to the function, and don't live past the function invocation. — Dave Newton
– Dave Newton, Commented Oct 15, 2015 at 20:38

Josh Beam · Accepted Answer · 2015-10-15 20:40:56Z

5

You can use the revealing module pattern:

var something = (function() {
  var myThing = 0;

  return {
    doSomething: function() {
      myThing++;
    }
  };
})();

something.doSomething(); // myThing is now 1

The variable is within what's called an IIFE (immediately invoked function expression) will remain throughout the lifecycle of the program.

If you're going to instantiate the module more than once in different places, you'll need to also create a singleton.

answered Oct 15, 2015 at 20:40

Josh Beam

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1 Comment

Furhan S. Over a year ago

Thanks a lot for making this one clear. I think, my problem was closer to singleton pattern

Barmar · Accepted Answer · 2015-10-15 20:40:14Z

2

If you want the variable to be global to all the persons, you need to define them within the scope of a containing function. This can be an IIFE, since it only need to run once.

var person = (function() {
    var _personCount = _perconCount || 0;
    return function(name) {
        _personCount++;
        var _name = name;
        getPerson = function() {
            return "Person is: " + _name;
        };
    };
})();

answered Oct 15, 2015 at 20:40

Barmar

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Comments

rajuGT · Accepted Answer · 2015-10-15 21:41:18Z

2

You must create the private static variable in the closure context.

var person = (function() {
    //private variable
    var _personCount = 1;

    return function(name) {
        this.name = name;
        this.id = _personCount++;
    }

}());

var foo = new person('foo');
console.log(foo.id + ' ' + foo.name);//1 foo

var boo = new person('boo');
console.log(boo.id + ' ' + boo.name);//2 boo

Here the enclosing anonymous function can never be called again. It gets executed while JS engine parses your code, and creates _personCount variable which can be accessed only by inner function function(name) making it like private static.

edited Oct 15, 2015 at 21:41

answered Oct 15, 2015 at 20:43

rajuGT

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Comments

Hitmands · Accepted Answer · 2015-10-15 20:52:07Z

I think that this could help you! note: the prototype property for having instance method! note: count, instead, is static!

var Person = (function() {
  var counter = 0;
  function Person(name) {
    this.name = name;
  }
  
  Person.prototype.getName = function() {
    counter += 1;
    return this.name;
  };

  Person.count = function() { return counter; };
return Person;
})();

var superman = new Person('SuperMan');
var batman = new Person('BatMan');

var el = function() { return window.document.getElementById('counter'); }
function p1() {
  var name = superman.getName();
  el().innerText = name + ' - ' + Person.count();
}
function p2() {
  var name = batman.getName();
  el().innerText = name + ' - ' + Person.count();
}

<h1 id="counter">0</h1>

<button onclick="p1()">Person1</button>
<button onclick="p2()">Person2</button>

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