How to define a common variable in javascript Class that can be used across instances?

Question

I want to define a single variable used among all instances in the Class definition(which is a plain function constructor in a jQuery plugin).

Is there such a feature?

If there is,just a simple demo and I think I'll understand.

Answer 1

Your question is not very clear. Do you mean a global variable? A variable that can be accessed from any object?

EDIT (based on comments)

You can do something like this to make the variables scoped to just your code:

//This function will execute right away (it is basically the same as leaving it out,
//except that anything inside of it is scoped to it.
(function(){
    var globalToThisScope = "this is global ONLY from within this code section";


    window.object = {
        //Now any instance of this object can see that variable,
        // but anything outside of the outer empty function will not see anything
    };
})();

Answer 2

Javascript doen't really provide that kind of data hiding, but this may suit your purposes:

function A()
{
    if (!A.b) A.b = 1;//shared
    //this.b = 0;//not shared
}
A.prototype.getB = function()
{
    return A.b;//shared
    //return this.b;//not shared
}
A.prototype.setB = function(value)
{
    A.b = value;//shared
    //this.b = value//not shared
}
function load()
{
    var x = new A();
    var y = new A();
    y.setB(2);
    document.body.innerHTML += x.getB();
}

output: 2

Answer 3

What you're looking for is essentially a private static or protected static variable, which can't be 100% emulated in javascript (that I know of).

You can make public static or private, though.

tehMick's solution gives you public static with some convenience setters/getters, but it's really no different than if you replaced y.setB(2) with A.b = 2.

Here's how a private variable would work, but understand that this is still a per instance variable, and reflects the same value via the getter only because each instance sets it to the same literal string.

function SomeClass()
{
  var privateVariable = 'foo';
  this.publicVariable = 'bar';

  this.getPrivateVariable = function()
  {
    return privateVariable;
  }

  this.setPrivateVariable = function( value )
  {
    privateVariable = value;
  }
}
SomeClass.staticVariable = 'baz';

var a = new SomeClass();
var b = new SomeClass();

// Works...
alert( a.getPrivateVariable() );
alert( b.getPrivateVariable() );

// Until we try to set it...
a.setPrivateVariable( 'hello' );

// Then it breaks
alert( a.getPrivateVariable() );
alert( b.getPrivateVariable() );

Answer 4

I concur with Peter Bailey (+1). Douglas Crockford has a discussion of public and private instance variables; Peter suggests a nice way to add static public variables.