Replace Derived Variable with Query

練習用的 REPL

class ProductionPlan {
    get production() {
        return this._production;
    }
    applyAdjustment(anAdjustment) {
        this._adjustments.push(anAdjustment);
        // this._production 在這邊做加總，但其實可以從 this._adjustments 內容算出來
        this._production += anAdjustment.amount;
    }
}

為了保險起見，先不改既有程式，而是先做 assert

class ProductionPlan {
    get production() {
        assert(this._production === this.calculatedProduction);
        return this._production;
    }
    // 這個 function 計算出來的結果，應該會跟 this._production 一樣
    get calculatedProduction() {
        return this._adjustments.reduce((sum, a) => sum + a.amount, 0);
    }
    applyAdjustment(anAdjustment) {
        this._adjustments.push(anAdjustment);
        this._production += anAdjustment.amount;
    }
}

確認 OK 後，可以移除 assert。

class ProductionPlan {
    get production() {
        return this._adjustments.reduce((sum, a) => sum + a.amount, 0); // OK 後才移除 assert
    }
    applyAdjustment(anAdjustment) {
        this._adjustments.push(anAdjustment);
    }
}