Replace Temp with Query

練習用的 REPL

class Order {
    constructor(quantity, item) {
    this._quantity = quantity;
    this._item = item;
  }
  
  get price() {
    var basePrice = this._quantity * this._item.price; // Temp Variable
    var discountFactor = 0.98;                         // Temp Variable
    if (basePrice > 1000) discountFactor -= 0.03;
    return basePrice * discountFactor;
  }
}

class Order {
    constructor(quantity, item) {
    this._quantity = quantity;
    this._item = item;
  }
  
  get price() {
    const basePrice = this._quantity * this._item.price; // Make it read-only
    var discountFactor = 0.98;
    if (basePrice > 1000) discountFactor -= 0.03;
    return basePrice * discountFactor;
  }
}

把計算 basePrice 的方法抽出來。

class Order {
    constructor(quantity, item) {
        this._quantity = quantity;
        this._item = item;
    }

    get price() {
        const basePrice = this.basePrice;
        var discountFactor = 0.98;
        if (basePrice > 1000) discountFactor -= 0.03;
        return basePrice * discountFactor;
    }

    get basePrice() { // Extract logic, make sure no side-effect
        return this._quantity * this._item.price;
    }
}

class Order {
    constructor(quantity, item) {
        this._quantity = quantity;
        this._item = item;
    }

    get price() {
        var discountFactor = 0.98;
        if (this.basePrice > 1000) discountFactor -= 0.03; // Replace temp variable with query
        return this.basePrice * discountFactor;            // Replace temp variable with query
    }

    get basePrice() {
        return this._quantity * this._item.price;
    }
}

把計算 discountFactor 的方法也抽出來。

class Order {
    constructor(quantity, item) {
        this._quantity = quantity;
        this._item = item;
    }

    get price() {
        const discountFactor = 0.98;                   // Make it read-only
        if (this.basePrice > 1000) discountFactor -= 0.03;
        return this.basePrice * discountFactor;
    }

    get basePrice() {
        return this._quantity * this._item.price;
    }

    get discountFactor() {                             // Extract logic, also make sure no side-effect
        var discountFactor = 0.98;
        if (this.basePrice > 1000) discountFactor -= 0.03;
        return discountFactor;
    }
}

這是最終結果。

class Order {
    constructor(quantity, item) {
        this._quantity = quantity;
        this._item = item;
    }

    get price() {
        return this.basePrice * this.discountFactor;  // Result!
    }

    get basePrice() {
        return this._quantity * this._item.price;
    }

    get discountFactor() {
        var discountFactor = 0.98;
        if (this.basePrice > 1000) {
            discountFactor -= 0.03;
        }
        return discountFactor;
    }
}