Question

I have a module Vehicle that contains general vehicle info. I have another module Car, which adds more functionality to Vehicle object.

// Pseudo code only. The final functions do not have to resemble this
var vehicle = require('vehicle')
vehicle.terrain = 'Land'
var car = vehicle.createCar()
// car and anotherCar will have unique Car-related values,
// but will use the same Vehicle info
var anotherCar = vehicle.createCar()

I am looking at using Object.create for the Car module, but not sure where the Object.create calls should go.

• Should I have a constructor in the Car module that takes an instance of a Vehicle object and does an Object.create with the Vehicle instance as the prototype?
• Or should the Object.create happen in a function on the Vehicle object, like createCar? My issue with this way, is Car should care that it’s derived from Vehicle, Vehicle shouldn’t know Car requires that.
• Or even if Object.create is the right approach.

Please, any examples and best practices would be greatly appreciated.

Update:

I changed the example to better reflect the inheritance problem I’m trying to solve.

Answer 1

imo, you’re describing a builder pattern rather than inheritance I think — I wouldn’t use object.create for this. A VehicleBuilder is responsible for constructing an object that has certain properties associated with it.

var builder = new VehicleBuilder();
builder.terrain = 'Land';
builder.wheelCount = 2;
builder.color = "blue";
var motorcycle = builder.createVehicle();

It might use something like:

VehicleBuilder.prototype.createVehicle = function(){
    var me = this;
    return new Vehicle({
         color: me.color,
         terrain: me.terrain,
         wheelCount: me.wheelCount
    });
}

If you look at the typical inheritance pattern in js, its something much more well defined and uses two primary patterns in node. One is util.inherits. Its code is simple: https://github.com/joyent/node/blob/master/lib/util.js#L423-428

exports.inherits = function(ctor, superCtor) {
  ctor.super_ = superCtor;
  ctor.prototype = Object.create(superCtor.prototype, {
    constructor: { value: ctor, enumerable: false }
  });
};

And the second is calling the parent constructor in the child class constructor.

function ChildClass(){
    SuperClass.call(this); // here
}

Example: https://github.com/joyent/node/blob/master/lib/stream.js#L25-28

So instead of a vehicle taking a bunch of properties or another object in its constructor, you use the prototype chain and the constructor to define custom subclass behavior.