# 重構 - Chapter 10 簡化條件邏輯
[TOC]
## 1. 分解條件邏輯 (Decompose Conditional)
**動機：** 凸顯意圖、降低閱讀複雜度
**作法：** 將分支中的條件邏輯抽成 函式 （Extract Function）

From

if (!aDate.isBefore(plan.summerStart) && !aDate.isAfter(plan.summerEnd)) 
    charge = quantity * plan.summerRate; 
else
    charge = quantity * plan.regularRate + plan.regularServiceCharge;

To

if (summer())
    charge = summerCharge();
else
    charge = regularCharge();

To

const charge = summer() ? summerCharge() : regularCharge()

## 2. 合併條件式（Consolidate Conditional Expression）
**動機：** 當檢查不同、底下卻做相同的事時，代表 1. 檢查可結合、 2. 可抽出函式 （Extract Function）凸顯意圖 （封裝、隱藏細節，從說 how 變成說 why）
**作法：**
1. 確保條件式中沒 Side Effect （否則請先執行 Separate Query from Modifier）
2. 將兩個條件式中，用邏輯運算子結合 （or -> 依序、 and -> 巢狀）
3. 測試
4. 將結合完的這個條件式與下個條件結合，直到只剩一個條件式
5. 考慮將此條件式抽成 Function

From

function disabilityAmount (anEmployee) {
    if (anEmployee.seniority < 2) return 0; 
    if (anEmployee .monthsDisabled > 12) return 0;
    if (anEmployee.isPartTime) return 0; 
    // 計算 disability amount
    ...
}

To

function disabilityAmount (anEmployee) {
    if (isNotEligableForDisability()) return 0;
    //計算 disability anount
    ...
    function isNotEligableForDisability() {
        return ((anEmployee.seniority < 2)
                || (anEmployee.monthsDisabled > 12)
                || (anEmployee.isPartTime));
    }
}

From

if(anEmployee.onVacation)
    if (anEmployee.seniority > 10)
        return 1;
return 0.5;

To

if ((anEmployee.onVacation) && (anEmployee.seniority > 10)) return 1;
return 0.5;

## 3. 將巢狀條件式換成防衛敘句（Replace Nested Conditional with Guard Clauses）
**動機：** 使入口和出口更“清晰”！
防衛敘句能指出某情況非功能“核心”，發生了就採取行動離開。有意識到思考你的條件分支是屬於正常 or 異常，(2組都正常，用 if/else <= 權重相同；有一組為異常 <= 提早離開)

**作法：**
1. 先將最外層需要替換的條件轉為 Guard Clauses
2. 測試
3. 重複該行為
4. 最後如果 Guard Clauses 都回傳相同結果，可使用二：合併條件式

From

function getPayAmount () {
    let result;
    if (isDead)
        result = deadAmount();
    else {
        if (isSeparated)
            result = separatedAmount();
        else {
            if (isRetired)
                result = retiredAmount();
            else
                result = normalayAmount();
        }
    }
    return result;
}

To

function getPayAmount () {
    if (isDead) return deadAmount() ;
    if (isSeparated) return separatedAmount();
    if (isRetired) return retiredAmount();
    return normalPayAmount ();
}

From

function adjustedCapital (anInstrument) {
    let result = 0;
    if (anInstrument.capital > 0) {
        if (anInstrument.interestRate > 0 && anInstrument.duration > 0){
            result = (anInstrument.income / anInstrument.duration) * anInstrument.adjustmentFactor;
        }
    }
    return result;
}

// 條件反轉、簡化過程
+ if (anInstrument.capital <= 0)  return result
+ if (!(anInstrument.interestRate > 0 && anInstrument.duration > 0)) return result
+ if (anInstrument.interestRate <= 0 || anInstrument.duration <= 0) return result
+ 合併 12,14 行
+ 移除多餘的變數 result

註解：狄摩根定律 `(!a) && (!b) === !(a || b)` 和 `(!a) || (!b) === !(a && b)`
To

function adjustedCapital (anInstrument) {
if ( anInstrument.capital <= 0
|| anInstrument.interestRate <= 0
|| anInstrument.duration<= 0 ) return 0;
return (anInstrument.income / anInstrument .duration) * anInstrument .adjustmentFactor

## 4. ~~將條件式換成多型（Replace Conditional with Polymorphism）~~

## 5. ~~加入特例（Introduce Special Case）~~

## 6. 加入斷言（Introduce Assertion）
**動機：** 凸顯意圖、寶貴的溝通工具、除錯的好幫手
**作法：** 當你見到一個條件為真的狀況時，加入斷言來說明此事

From

if(this.discountRate)
    base = base - (this.discountRate * base)

To

assert(this.discountRate >= 0) // 斷言失敗代表程式碼有錯
if(this.discountRate)
    base = base - (this.discountRate * base)


// assert 大致會長得像下面這樣，nodejs 有提供 API
function assert(condition, message) {
    if (!condition) {
        throw new Error(message || "Assertion failed");
    }
}

## Discucsion
1. 盡量使用正向表述，人腦比較好理解

disabled = !isEligible || !isActive
disabled = !(isEligible && isActive)
enabled = isEligible && isActive

2. 在單元測試、TypeScript 型別斷言都還滿常見 assert 的概念

expect(add(5, 5)).toBe(10);

let str: unknown = "geeksforgeeks";  
let len: number = (str as string).length;

3. IIFE 變數，來替代巢狀三元運算式

  const content = (() => {
    switch (status) {
      case 'loading':
        return <LoadingBar  />;
      case 'loaded':
        return <Table />;
      default:
        return null;
    }
  })();

  return (
    <Container>
      {content}
    </Container>

4. 用 switch case true 來做多重判斷
https://seanbarry.dev/posts/switch-true-pattern

const user = {
	firstName: "Seán",
	lastName: "Barry",
	email: "[e-mail]",
	number: "00447123456789",
};

switch (true) {
	case !user:
		throw new Error("User must be defined.");
	case !user.firstName:
		throw new Error("User's first name must be defined");
	case typeof user.firstName !== "string":
		throw new Error("User's first name must be a string");
	// ...lots more validation here
	default:
		return user;
}

5. 用 object literal 取代 switch case
https://ultimatecourses.com/blog/deprecating-the-switch-statement-for-object-literals

// from
  var drinks;
  switch(type) {
  case 'coke':
    drink = 'Coke';
    break;
  case 'pepsi':
    drink = 'Pepsi';
    break;
  default:
    drink = 'Unknown drink!';
  }

// to
function getDrink (type) {
  var drinks = {
    'coke': 'Coke',
    'pepsi': 'Pepsi',
    'lemonade': 'Lemonade',
    'default': 'Default item'
  };
  return 'The drink I chose was ' + (drinks[type] || drinks['default']);
}

## 參考資料
1. 重構：改善既有程式的設計
1. [Code の 斷捨離 — 重構 (Refactoring)-ch9](https://medium.com/@duidae/code-%E3%81%AE-%E6%96%B7%E6%8D%A8%E9%9B%A2-%E9%87%8D%E6%A7%8B-refactoring-ch9-6768dae8a6ba)
2. [What is “assert” in JavaScript?](https://stackoverflow.com/questions/15313418/what-is-assert-in-javascript#:~:text=There%20is%20no%20assert%20in%20JavaScript.%20However%2C%20there,line%20argument%20-enableassertions%20%28or%20its%20shorthand%20-ea%20%29%2C)