Typescript or JS class extension

Question

Let's say I want class Warrior in my game. And my warriors can only walk.

class Warrior{

constructor(){}

walk(){
  alert ('I can walk!');
  }
}

x.walk(); //'I can walk!'
x.fight(); //error

Then I decided to create a weapon class so that when this class is active, my Warriors in the game can fight. Some pseudocode how I imagine it:

class Weapon{

canFight(type:Warrior){
set warrior.fight=()=>{alert('I can fight!')};}

}

let x=new Warrior();

x.walk(); //'I can walk!'
x.fight() //'I can fight!'

So I need to extend the class itself and all its instances with the new methods and parameters when some sort of magic extention code is present. Thus I can incapsulate behaviour in the separate classes and extend it to other classes without need to have noticed them.

What I saw is mixins but the idea behind that is to explicit change my Warrior class to incapsulate new functions. I can't just say that since now my warriors can fight, I need to change the the cases when i use Warriors to some sort of new type - FighterWarriors and that can be a real pain if I need to quickly enchance the objects with the new behaviour.

This is working technique in c# and swift but I don't know about other languages.

So the questions are: how can I make this type of behaviour in Typescript? If can't, does the pure JS supports this? What can I read additionally for this theme?

Titian Cernicova-Dragomir · Accepted Answer · 2018-11-09 15:54:16Z

1

You can use interface merging and module augmentation to add members to a type. For the JS part, things are pretty simple you just need to add a new property on the Warrior prototype

// Warrior.ts
export class Warrior {

    constructor() { }

    walk() {
        alert('I can walk!');
    }
}
// Weapon.ts
import {Warrior } from './Warrior'

export class Weapon{
    canFight(type:Warrior){
    }
}

declare module './Warrior' {
    interface Warrior {
        fight(): void
    }
}
Warrior.prototype.fight = function (this: Warrior){
    console.log("FIGHT");
}

// Usage.ts

import {Warrior  } from './Warrior'
import './Weapon'


let x=new Warrior();
x.fight();

edited Nov 9, 2018 at 15:54

answered Nov 9, 2018 at 14:49

Titian Cernicova-Dragomir

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2 Comments

gleb.kudr Over a year ago

Thanks! One question, do you mean Warrior.ts or Weapon.ts in the second case? It seems it should be weapon.ts.

Titian Cernicova-Dragomir Over a year ago

@gleb.kudr yeah, sorry my bad

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