sets.js

class FiniteCardinality {
    constructor (initialValue=0) {
        this.value = 0;
    }
    increaseBy(val) {
        this.value += Number(val);
    }
}

class InfiniteCardinality {
    constructor () {
        this.value = Infinity;
    }
    increaseBy(/*val*/) {
        // no-op
    }
}

export class EmptySet {
    constructor() {
        this.elements = [];
        this._cardinality = new FiniteCardinality();
    }

    get size() {
        return this.cardinality;
    }

    get cardinality() {
        return this._cardinality;
    }

    addElement(el) {
        this.elements.push(el);
        this.cardinality.increaseBy(1);
    }

    contains(el) {
        return this.elements.indexOf(el) > -1;
    }
}

export class Set extends EmptySet {
    constructor(initialElements=[]) {
        super();

        initialElements.forEach(el => this.elements.addElement(el));
    }
}

export class NaturalNumbersSet extends EmptySet {
    constructor() {
        super();
        this._cardinality = new InfiniteCardinality();
    }

    contains(el) {
        return Number.isInteger(el) && el > 0;
    }
}

export class IntegersSet extends EmptySet {
    constructor() {
        super();
        this._cardinality = new InfiniteCardinality();
    }

    contains(el) {
        return Number.isInteger(el);
    }
}