CatManager singleton

CatManager.ts

class Cat {
    public age: number = 0;
}

class CatManager {
    private static instance: CatManager;

    private catPool: Cat[] = [];

    constructor() {}

    public static getInstance(): CatManager {
        if(!CatManager.instance){
            CatManager.instance = new CatManager();
        }
        return CatManager.instance;
    }

    public generateNewCats(amount: number = 1) {
        for (let i: number = 0; i < amount; i++) {
            this.catPool.push(new Cat());
        }
    }

    public getCats(minAge?: number = 0, maxAge?: number = 999): Cat[] {
        return this.catPool.filter((cat: Cat) => {
            return cat.age > minAge && cat.age < maxAge;
        });
    }

    public incrementAges(years: number = 1): void {
        this.catPool.forEach((cat: Cat): void => {
            cat.age += years;
        });
    }
}
//State 1
let cm: CatManager = CatManager.getInstance();
cm.generateNewCats(5); //Create 5 cats
cm.incrementAges(2); //age all cats by 4 years

//State 2
let cm: CatManager = CatManager.getInstance();
cm.generateNewCats(3); //Create 3 cats
cm.incrementAges(4); //Now age all 8 cats by another 4 years
let cats: Cat[] = cm.getCats(5); //Fetch the cats who are at minimal 5 years old, should only fetch the cats created in state one
console.log(cats);//This logs: [Cat, Cat, Cat, Cat, Cat], the 5 cats we initialy created in the previous state