Replace Conditional with Polymorphism

10_4_birds.js

function plumages(birds) {
    return new Map(birds
        .map(b => creaetBird(b))
        .map(bird => [bird.name, b.plumage])
    );
}

function speeds(birds) {
    return new Map(birds
        .map(b => creaetBird(b))
        .map(bird => [bird.name, bird.airSpeedVelocity])
    );
}

function creaetBird(bird) {
    switch (bird.type) {
        case '유럽 제비':
            return new EurpoeanSwallow(bird);
        case '아프리카 제비':
            return new AfricanSwallow(bird);
        case '노르웨이 파랑 앵무':
            return new NorwegianBlueParrot(bird);
        default:
            return new Bird(bird);
    }
}

class EurpoeanSwallow extends Bird {
    get plumage() {
        return "보통이다";
    }

    get airSpeedVelocity() {
        return 35;
    }
}
class AfricanSwallow extends Bird {
    get plumage() {
        return (bird.numberOfCoconuts > 2) ? "지쳤다" : "보통이다";
    }

    get airSpeedVelocity() {
        return 40 - 2 * bird.numberOfCoconuts;
    }
}
class NorwegianBlueParrot extends Bird {
    get plumage() {
        return (bird.voltage > 100) ? "그을렸다" : "예쁘다";
    }

    get airSpeedVelocity() {
        return (bird.isNailed) ? 0 : 10 + bird.voltage / 10;
    }
}

class Bird {
    constructor(birdObject) {
        Object.assign(this, birdObject);
    }

    get plumage() {
        return "알 수 없다";
    }


    get airSpeedVelocity() {
        return null;
    }
}