Question

function Resource(){ 
    var id = '';

    this.getId = function(){
        return id;
    }

    this.setId = function(value){
        id = value;
    }
}

function Model(){ } 
Model.prototype = new Resource();

To test the above code, I have the following:

var m1 = new Model();
m1.setId("blah");
alert("m1 = " + m1.getId());

var m2 = new Model();
m2.setId("blahblah");
alert("m2 = " + m2.getId());

alert("m1 = " + m1.getId());

I would expect the following to be alerted: m1 = blah, m2 = blahblah, m1 = blah.

However, the following is being displayed: m1 = blah, m2 = blahblah, m1 = blahblah.

I have written the above code as an experiment so that I can begin to understand inheritance in JavaScript. Perhaps my expectation of what should be displayed is due to my Java/C# background. Clearly each new Model object created shares the same id variable defined in Resource. Why is this?

How can I write this so that each new Model can have its own unique id (other than the obvious – defining in within Model)?

BTW, I have purposely definted id as var id rather than this.id, as I am looking for the private-variable behavior.

Answer 1

A prototype object is shared among all instances.

Because the id variable belongs to the Resource constructor, and because the prototype of Model is that single instance of Resource, all the Model instances are using that single instance, and therefore the same variable.

For each Model instance to have its own id variable, you’ll need to create the id variable in the Model constructor, as well as the accessor methods.

function Model() {
    var id = '';

    this.getId = function(){
        return id;
    }

    this.setId = function(value){
        id = value;
    }
}

Another way to accomplish this is to invoke Resource using the Model instances as the context. This is done using .call or .apply like this:

function Model() {
    Resource.apply(this);
}

This will invoke the Resource function, which will create the id variable and its accessor methods, but the accessors will be assigned to the Model instance.