Replace Conditional with Polymorphism

練習用的 REPL

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

function plumage(bird) {
  switch (bird.type) {              // 一堆 switch，而且在一些地方重複了
  case 'EuropeanSwallow':
    return "average";
  case 'AfricanSwallow':
    return (bird.numberOfCoconuts > 2) ? "tired" : "average";
  case 'NorwegianBlueParrot':
    return (bird.voltage > 100) ? "scorched" : "beautiful";
  default:
    return "unknown";
  }
}

function airSpeedVelocity(bird) {
  switch (bird.type) {
  case 'EuropeanSwallow':
    return 35;
  case 'AfricanSwallow':
    return 40 - 2 * bird.numberOfCoconuts;
  case 'NorwegianBlueParrot':
    return (bird.isNailed) ? 0 : 10 + bird.voltage / 10;
  default:
    return null;
  }
}

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

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

function plumage(bird) {
    // switch (bird.type) {
    //     case 'EuropeanSwallow':
    //       return "average";
    //     case 'AfricanSwallow':
    //       return (bird.numberOfCoconuts > 2) ? "tired" : "average";
    //     case 'NorwegianBlueParrot':
    //       return (bird.voltage > 100) ? "scorched" : "beautiful";
    //     default:
    //       return "unknown";
    // }
    return new Bird(bird).plumage;          // 只是委派行為
}

function airSpeedVelocity(bird) {
    // switch (bird.type) {
    //     case 'EuropeanSwallow':
    //       return 35;
    //     case 'AfricanSwallow':
    //       return 40 - 2 * bird.numberOfCoconuts;
    //     case 'NorwegianBlueParrot':
    //       return (bird.isNailed) ? 0 : 10 + bird.voltage / 10;
    //     default:
    //       return null;
    // }
    return new Bird(bird).airSpeedVelocity; // 只是委派行為
}

class Bird {                    //  建立起 parnet class
    constructor(birdObject) {
        Object.assign(this, birdObject);
    }
    get plumage() {
        switch (this.type) {
            case 'EuropeanSwallow':
                return "average";
            case 'AfricanSwallow':
                return (this.numberOfCoconuts > 2) ? "tired" : "average";
            case 'NorwegianBlueParrot':
                return (this.voltage > 100) ? "scorched" : "beautiful";
            default:
                return "unknown";
        }
    }
    get airSpeedVelocity() {
        switch (this.type) {
            case 'EuropeanSwallow':
                return 35;
            case 'AfricanSwallow':
                return 40 - 2 * this.numberOfCoconuts;
            case 'NorwegianBlueParrot':
                return (this.isNailed) ? 0 : 10 + this.voltage / 10;
            default:
                return null;
        }
    }
}

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

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

function plumage(bird) {
    // return new Bird(bird).plumage;
    return createBird(bird).plumage;            // 改使用 factory
}

function airSpeedVelocity(bird) {
    // return new Bird(bird).airSpeedVelocity;
    return createBird(bird).airSpeedVelocity;   // 改使用 factory
}

class Bird {
    constructor(birdObject) {
        Object.assign(this, birdObject);
    }
    get plumage() {
        switch (this.type) {
            case 'EuropeanSwallow':
                return "average";
            case 'AfricanSwallow':
                return (this.numberOfCoconuts > 2) ? "tired" : "average";
            case 'NorwegianBlueParrot':
                return (this.voltage > 100) ? "scorched" : "beautiful";
            default:
                return "unknown";
        }
    }
    get airSpeedVelocity() {
        switch (this.type) {
            case 'EuropeanSwallow':
                return 35;
            case 'AfricanSwallow':
                return 40 - 2 * this.numberOfCoconuts;
            case 'NorwegianBlueParrot':
                return (this.isNailed) ? 0 : 10 + this.voltage / 10;
            default:
                return null;
        }
    }
}

function createBird(bird) {     // 將 switch 的部分改成 factory
    switch (bird.type) {
        case 'EuropeanSwallow':
            return new EuropeanSwallow(bird);
        case 'AfricanSwallow':
            return new AfricanSwallow(bird);
        case 'NorweigianBlueParrot':
            return new NorwegianBlueParrot(bird);
        default:
            return new Bird(bird);
    }
}

class EuropeanSwallow extends Bird {}       // 為每個 switch 做出子類別
class AfricanSwallow extends Bird {}        // 為每個 switch 做出子類別
class NorwegianBlueParrot extends Bird {}   

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

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

function plumage(bird) {
    return createBird(bird).plumage;
}

function airSpeedVelocity(bird) {
    return createBird(bird).airSpeedVelocity;
}

class Bird {
    constructor(birdObject) {
        Object.assign(this, birdObject);
    }
    get plumage() {
        switch (this.type) {
            case 'EuropeanSwallow':
                throw "oops";       // 原本的地方就讓它出錯
            case 'AfricanSwallow':
                return (this.numberOfCoconuts > 2) ? "tired" : "average";
            case 'NorwegianBlueParrot':
                return (this.voltage > 100) ? "scorched" : "beautiful";
            default:
                return "unknown";
        }
    }
    get airSpeedVelocity() {
        switch (this.type) {
            case 'EuropeanSwallow':
                return 35;
            case 'AfricanSwallow':
                return 40 - 2 * this.numberOfCoconuts;
            case 'NorwegianBlueParrot':
                return (this.isNailed) ? 0 : 10 + this.voltage / 10;
            default:
                return null;
        }
    }
}

function createBird(bird) {
    switch (bird.type) {
        case 'EuropeanSwallow':
            return new EuropeanSwallow(bird);
        case 'AfricanSwallow':
            return new AfricanSwallow(bird);
        case 'NorweigianBlueParrot':
            return new NorwegianBlueParrot(bird);
        default:
            return new Bird(bird);
    }
}

class EuropeanSwallow extends Bird {
    get plumage() {
        return "average";           // 先把 plumage 的邏輯移到子類別
    }
}

class AfricanSwallow extends Bird {}

class NorwegianBlueParrot extends Bird {}

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

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

function plumage(bird) {
    return createBird(bird).plumage;
}

function airSpeedVelocity(bird) {
    return createBird(bird).airSpeedVelocity;
}

class Bird {
    constructor(birdObject) {
        Object.assign(this, birdObject);
    }
    get plumage() {
        return "unknown";
    }
    get airSpeedVelocity() {
        switch (this.type) {
            case 'EuropeanSwallow':
                return 35;
            case 'AfricanSwallow':
                return 40 - 2 * this.numberOfCoconuts;
            case 'NorwegianBlueParrot':
                return (this.isNailed) ? 0 : 10 + this.voltage / 10;
            default:
                return null;
        }
    }
}

function createBird(bird) {
    switch (bird.type) {
        case 'EuropeanSwallow':
            return new EuropeanSwallow(bird);
        case 'AfricanSwallow':
            return new AfricanSwallow(bird);
        case 'NorweigianBlueParrot':
            return new NorwegianBlueParrot(bird);
        default:
            return new Bird(bird);
    }
}

class EuropeanSwallow extends Bird {        // 抽取邏輯出來
    get plumage() {
        return "average";
    }
}

class AfricanSwallow extends Bird {         // 抽取邏輯出來
    get plumage() {
        return (this.numberOfCoconuts > 2) ? "tired" : "average";
    }
}

class NorwegianBlueParrot extends Bird {    // 抽取邏輯出來
    get plumage() {
        return (this.voltage > 100) ? "scorched" : "beautiful";
    }
}

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

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

function createBird(bird) {
    switch (bird.type) {
        case 'EuropeanSwallow':
            return new EuropeanSwallow(bird);
        case 'AfricanSwallow':
            return new AfricanSwallow(bird);
        case 'NorwegianBlueParrot':
            return new NorwegianBlueParrot(bird);
        default:
            return new Bird(bird);
    }
}

class Bird {
    constructor(birdObject) {
        Object.assign(this, birdObject);
    }
    get plumage() {
        return "unknown";
    }
    get airSpeedVelocity() {
        return null;
    }
}
class EuropeanSwallow extends Bird {
    get plumage() {
        return "average";
    }
    get airSpeedVelocity() {
        return 35;
    }
}
class AfricanSwallow extends Bird {
    get plumage() {
        return (this.numberOfCoconuts > 2) ? "tired" : "average";
    }
    get airSpeedVelocity() {
        return 40 - 2 * this.numberOfCoconuts;
    }
}
class NorwegianBlueParrot extends Bird {
    get plumage() {
        return (this.voltage > 100) ? "scorched" : "beautiful";
    }
    get airSpeedVelocity() {
        return (this.isNailed) ? 0 : 10 + this.voltage / 10;
    }
}