How do I make a generic factory in javascript?

Question

A factory class isn't the end product I am looking for but it is basically my problem boiled down. I am looking for a class we'll call foo that can be passed in another class bar as a parameter to its constructor such that when later calling fooInstance.create() it returns a new instance of bar. I can easily figure out how to make a fooInstance.create(bar) create an instance of bar. But that isn't what I need. I need each instance of the factory class to create a specific type of object. so:

    fooCar = new foo(Car);
    fooTruck = new foo(Truck);
    myCar = fooCar.create();
    myTruck = fooTruck.create();

This would be something easily handled by Generics in c#. I either get errors or I end up screwing with the prototype of foo which then changes the type created by all instances of foo.

What extra things is your foo class supposed to do. Javascript won't give you the static type safety you are looking for so perhaps you could instead opt for something simpler, such as assigning fooCar = Car and ignoring the create method completely: myCar = new fooCar. — hugomg
– hugomg, Commented Jan 18, 2013 at 4:28
Check this out addyosmani.com/resources/essentialjsdesignpatterns/book/… — SangYeob Bono Yu
– SangYeob Bono Yu, Commented Jan 18, 2013 at 4:29

Joseph Silber · Accepted Answer · 2013-01-18 04:20:15Z

1

Just return a regular object from your factory, with just one create method:

function foo (Constructor) {
    return {
        create: function () {
            return new Constructor();
        }
    };
}

Check out the fiddle: http://jsfiddle.net/fR5Gz/1/

edited Jan 18, 2013 at 4:20

answered Jan 18, 2013 at 4:13

Joseph Silber

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2 Comments

user1989150 Over a year ago

Thanks!! That was so deceptively short and simple that I thought it was a plain factory at first.

Joseph Silber Over a year ago

@user1989150 - It is a plain factory. Isn't that what you wanted?

user1989150 · Accepted Answer · 2013-01-18 15:11:22Z

0

Thanks to everyone for your help. @missingno I thought the details of what I was trying to do would cloud the issue but now I think it was the other way around. I'll know better for next time. I wasn't looking for type safety but without it maybe there is no reason to do it this way. Maybe I'm stuck in classical thinking.

I actually figured it out myself this morning because I didn't get any email alerts on this thread like I was supposed to so I thought there weren't any replies yet. This is my solution (relevant code only):

    ORM.Repository = (function (){
    function Repository(ob){
        this.construct = ob;
    };
    //bunch of other generic repository stuff deleted
    Repository.prototype.make = function () { return new this.construct();}
    return Repository;
})();

Usage:

    var carRepository = new ORM.Repository(car);
    var personRepository = new ORM.Repository(person);
    var myCar = carRepository.make();
    var user = personRepository.make();

answered Jan 18, 2013 at 15:11

user1989150

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1 Comment

Richard Marr Over a year ago

Have you looked at Resourceful? It's part of the Flatiron project and provides a mechanism for generating ORM models. github.com/flatiron/resourceful

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