TypeScript Access Static Variables Using Instances

Question

So in most OOP languages static variables can also be called class variables, ie their value is shared among all instances of this class. For example, in my game I have a class Bullet which is extended by GreenBullet and PinkBullet. I want these subclasses to have a "class" or "static" variable called ammo so that I can keep track of the ammo count for that specific ammo type. But here is the catch: I want to be able to access this property through an instance of the subclass.

Example:

var bullet: GreenBullet = new GreenBullet()
if (bullet.ammo <= 0)
    return;
bullet.shoot();
bullet.ammo --;

I want ALL instances of GreenBullet to be aware of this change to their ammo count.

zlumer · Accepted Answer · 2015-05-23 08:24:55Z

18

First option is to create instance accessors to static variable:

class GreenBullet
{
   static ammo: number = 0;
   get ammo(): number { return GreenBullet.ammo; }
   set ammo(val: number) { GreenBullet.ammo = val; }
}
var b1 = new GreenBullet();
b1.ammo = 50;
var b2 = new GreenBullet();
console.log(b2.ammo); // 50

If you want all subclasses of Bullet (including itself) to have separate ammo count, you can make it that way:

class Bullet
{
   static ammo: number = 0;
   get ammo(): number { return this.constructor["ammo"]; }
   set ammo(val: number) { this.constructor["ammo"] = val; }
}
class GreenBullet extends Bullet { }
class PinkBullet extends Bullet { }

var b1 = new GreenBullet();
b1.ammo = 50;
var b2 = new GreenBullet();
console.log(b2.ammo); // 50
var b3 = new PinkBullet();
console.log(b3.ammo); // 0

On a side note, I'm fairly sure you should not store bullet count in a static variable.

edited May 23, 2015 at 8:24

answered May 23, 2015 at 1:26

zlumer

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3 Comments

MichaelS Over a year ago

Nice trick with the separate static members. How did you get "this.constructor.ammo" to compile? casting to Any?

David Sherret Over a year ago

@MichaelS that or do this.constructor["ammo"]. Also, I would suggest to make static ammo private since there's no need for anything outside the class to access it.

AnorZaken Over a year ago

You can take this one step further! If you want to check how much pink ammo is left, it's inconvenient to have access that from an instance. We can solve that by adding this method to the Bullet class: static getAmmo<T extends new (...args: any) => Bullet>(this: T) { return this['ammo']; } despite being declared on the base-class, this will in fact add this static method to the child-classes! So now we can call both GreenBullet.getAmmo(); and PinkBullet.getAmmo(); which is neat. Also in case you don't want it callable on the base-class, you can mark Bullet as an abstract class.

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