Question

I’m trying to implement very simple inheritance for some custom classes in node. I am doing something like this:

function MyClass() {
    this.myFunction = function(){
        //do something
    }
}

function MySubclass(){
    this.myOtherFunction = function(){
         //do something else
    }
}

util.inherits(MySubclass, MyClass)
console.log(MySubclass.super_ === MyClass); // true

var x = new MySubclass()
console.log(x instanceof MyClass); // true

x.myFunction()

And if I run this, I get the error:

TypeError: Object #<MySubclass> has no method 'myFunction'

This exact pattern works perfectly well for inheriting from events.EventEmitter. Does it just not work for custom classes, or is there something I’m missing?

Answer 1

util.inherits only sets up the prototype chain. What you’re missing is to call the super constructor so that it adds stuff to this. Here’s how to do it:

function MyClass() {
    this.myFunction = function() {
        // Do something
    };
}

MyClass.prototype.doFoo = function () {
};

function MySubclass() {
    // This is doing the same as MyClass.apply(this, arguments);
    MySubclass.super_.apply(this, arguments);

    this.myOtherFunction = function() {
        // Do some other thing
    };
}

util.inherits(MySubclass, MyClass);

MySubclass.prototype.doBar = function() {
};

var x = new MySubclass();
x.myFunction();
x.myOtherFunction();
x.doFoo();
x.doBar();

You may still want to move those methods to each prototype.