Question

How can I add data/functions to all instances of a javascript object created by a constructor so that all instances have the same reference and not a copy of it?

Basically implementing the equivalent of a static method in C#.

For example, given the following code which creates a Widget class.

(function() {

    var Widget = function() {
    };

    Widget.prototype.init = function(data) {
        this.data = data;
    };

    this.Widget = Widget;
}).call(this);
var instance1 = new Widget();
instance1.init('inst1');
var instance2 = new Widget();
instance2.init('inst2');

alert(instance1.data); // inst1
alert(instance2.data); // inst2

In the above case each instance has it’s own copy of the data property. However I want to add a function that sets data for all current and future instances.

My current solution is to add a function to the constructor function object, not to it’s prototype. See below for example. Is there any pitfalls to this and is there a better way?

(function() {

    var Widget = function() {
    };

    Widget.prototype.init = function(data) {
        this.data = data;
    };

    Widget.addStaticData = function(data) {
        this.staticData = data;
    };

    Widget.prototype.getStaticData = function() {
        return Widget.staticData;
    };

    this.Widget = Widget;
}).call(this);
var instance1 = new Widget();
instance1.init('inst1');

Widget.addStaticData('static');

var instance2 = new Widget();
instance2.init('inst2');

alert(instance1.data); // inst1
alert(instance2.data); // inst2
alert(instance1.getStaticData()); // static
alert(instance2.getStaticData()); // static

Answer 1

Three pitfalls that I can think of:

• methodological: the prototype is the place for shared, reused, inherited functionality/properties – utilise it as such

• performance: it is quicker to inherit than to set each time on an instance. John Resig (jQuery creator) did some benchmarking on this in a blog post that I appear unable to find at present.

• losing the split between inherited and own properties. If you apply everything to an instance via the constructor, everything is an instance property.

Everything via constructor:

function Dog() { this.legs = 4; }
var fido = new Dog();
fido.name = 'Fido';
for (var i in fido) if (fido.hasOwnProperty(i)) alert(i+' = '+fido[i]);

…alerts both properties as they are deemed the instance’s own.

Via prototype and constructor

function Dog2() { }
Dog2.prototype.legs = 4;
var fido = new Dog2();
fido.name = 'Fido';
for (var i in fido) if (fido.hasOwnProperty(i)) alert(i+' = '+fido[i]);

…alerts just name because that is the only instance property. (Nonetheless, fido.legs is retrievable – but it comes from the prototype).

[EDIT – in response to the OP’s commet below]

If you want a static method, then that should be added to the function after its declaration.

function Dog() {}
Dog.static = function() {}