current community

your communities

more stack exchange communities

Stack Exchange
tour help
stack overflow careers
Take the 2-minute tour ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free, no registration required.
up vote 11 down vote favorite
9

This question already has an answer here:

In the following code, I would like to have a counter to keep track of the number of Person objects created. This code is not doing so, how would I accomplish that?

function Person(){
    this.name = "Peter";
    this.counter = this.counter + 1;
    alert(this.counter);
}

Person.prototype.counter = 0;

var p1 = new Person;
var p2 = new Person;
share|improve this question

marked as duplicate by James Allardice, Derek Henderson, nsgulliver, cadrell0, SteveP Jul 3 '13 at 14:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
1  
Can somebody explain how this actually works. –  nepsdotin Sep 5 '11 at 11:35

5 Answers 5

up vote 26 down vote accepted 
function Person(){
    this.name = "Peter";
    Person.counter++;
    alert(Person.counter);
}

Person.counter = 0;

var p1 = new Person();
var p2 = new Person();

Make the "static" variable a property of the Person function, rather than the prototype, and use Person instead of this inside the constructor.

This is possible because JavaScript functions are first-class (i.e. they are objects), so can have properties of their own.

Here's a working example of the above code.

share|improve this answer
    
3rd answer, but the most complete/correct. –  John Green Sep 5 '11 at 11:22
up vote 7 down vote

You can also make your counter variable "private", declaring it as local to a closure. It's the best way to have something similar to private - static variables:

var Person = (function() {

    var counter = 0;

    return function() {
        counter++;
        this.name = "Peter";
        alert(counter);
    }
})();


var p1 = new Person();
var p2 = new Person();
share|improve this answer
up vote 3 down vote

There are no static properties. If you want you can store data on the Person function.

function Person(){
    this.name = "Peter";
    Person.counter++;
    alert(Person.counter);
}
share|improve this answer
up vote 1 down vote

For a static you can assign a property to the function object itself;

Person.counter = 0;

And within the constructor increment with;

Person.counter += 1;

You can also check-if-undefined and create Person.counter within the constructor

function Person(){
   if (typeof Person.counter === 'undefined')
      Person.counter = 0;
   else
      Person.counter += 1;
   ...
share|improve this answer
up vote 0 down vote

There is no such thing like static class variables/properties in js. The simplest approach is just us "class" function as namespace for static variables.

It means, just access in Person.count directly.

You can use closures as well, but actually in 90% cases it will be overkill. In modern browsers you also can redefine getter/setter function to wrap usage of Person.count and other "static" variables.

This snippet demonstrates the idea:

    function borrow(obj, borrowobj, fname) {
    obj.__defineGetter__(fname, function() {
         return borrowobj[fname]   
    })  

    obj.__defineSetter__(fname, function(val) {
             borrowobj[fname] = val      
    })
}

function Person() {
    borrow(this, Person, "count");
    this.count++
}

Person.count = 0;

new Person();
new Person();
var p = new Person();
alert(p.count);
share|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.