JavaScript代碼整潔之道

JavaScript代碼整潔之道

整潔的代碼不單單是讓人看起來舒服，更重要的是遵循一些規範可以讓你的代碼更容易維護，同時下降bug概率。

原文 clean-code-javascript，這裏總結摘錄出我的以爲有幫助的地方，也加入了一些本身的理解（有些文字我感受保留原文更好，因此直接copy了），其餘還有不少點沒有列出來，感興趣能夠去看原文。另外這不是強制的代碼規範，就像原文中說的， These are guidelines and nothing more。

1. 用命名的變量代替數組下標

// bad
const address = "One Infinite Loop, Cupertino 95014";
const cityZipCodeRegex = /^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$/;
saveCityZipCode(
  // 下標1,2不易於理解
  address.match(cityZipCodeRegex)[1],
  address.match(cityZipCodeRegex)[2]
);

// good
const address = "One Infinite Loop, Cupertino 95014";
const cityZipCodeRegex = /^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$/;
// 使用數組解構更好的命名變量
const [, city, zipCode] = address.match(cityZipCodeRegex) || [];
saveCityZipCode(city, zipCode);

2. 函數的參數最好<=2個，儘可能避免3個。

若是有不少參數就利用object傳遞，並使用解構。

注意 object array這些引用類型的值是 mutable的。

3. 一個函數只作一件事。

好處在於compose, test, and reason about。

4. 不要自行擴展原型

若是想擴展原型，能夠先繼承再添加方法，防止污染。

// bad
Array.prototype.diff = function diff(comparisonArray) {
  const hash = new Set(comparisonArray);
  return this.filter(elem => !hash.has(elem));
};

// good
class SuperArray extends Array {
  diff(comparisonArray) {
    const hash = new Set(comparisonArray);
    return this.filter(elem => !hash.has(elem));
  }
}

5. 用多態來代替條件語句

// bad
if (type === 'text') {
    // do something
} else if (type === 'select') {
    // do something else
}

我我的寫這種代碼的一種經常使用方式是：

const control = {
    text: {
        mapper() {},
        restore(){},
        name: 'this is a text field',
    },
    select: {
        mapper() {},
        restore(){},
        name: 'this is a select field',
    }
}

control[type].mapper();

實際上就是多態（polymorphism），也能夠考慮用class的方式，大概這樣：

class Field {
    ...
}

class TextField extends Field {
    mapper(){}
    restore(){}
    name = 'this is a text field';
}

class SelectField extends Field {
    mapper(){}
    restore(){}
    name = 'this is a select field';
}

6. 使用getter和setter函數。

// bad
function makeBankAccount() {
  // ...

  return {
    balance: 0
    // ...
  };
}

const account = makeBankAccount();
account.balance = 100;

// good
function makeBankAccount() {
  // this one is private
  let balance = 0;

  // a "getter", made public via the returned object below
  function getBalance() {
    return balance;
  }

  // a "setter", made public via the returned object below
  function setBalance(amount) {
    // ... validate before updating the balance
    balance = amount;
  }

  return {
    // ...
    getBalance,
    setBalance
  };
}

const account = makeBankAccount();
account.setBalance(100);

你能夠在getter和setter裏面作不少事情而不須要修改每個.balance的地方。

7. Prefer composition over inheritance

儘可能用組合來代替繼承，什麼狀況下用繼承：

• Your inheritance represents an "is-a" relationship and not a "has-a" relationship (Human->Animal vs. User->UserDetails).
• You can reuse code from the base classes (Humans can move like all animals).
• You want to make global changes to derived classes by changing a base class. (Change the caloric expenditure of all animals when they move).

8. SOLID

Single Responsibility Principle 單一職責原則

There should never be more than one reason for a class to change，一個類被改變的緣由數量應該儘量下降。若是一個類中功能太多，當你修改其中一點時會沒法估量任何引用該類的模塊所受到的影響。

Open/Closed Principle 開放封閉原則

用戶能夠在不修改內部實現的前提下自行擴展功能。例若有一個Http模塊，內部會根據環境判斷用哪一個adaptor。若是用戶要添加adaptor就必須修改Http模塊。

// bad
class AjaxAdapter extends Adapter {
  constructor() {
    super();
    this.name = "ajaxAdapter";
  }
}

class NodeAdapter extends Adapter {
  constructor() {
    super();
    this.name = "nodeAdapter";
  }
}

class HttpRequester {
  constructor(adapter) {
    this.adapter = adapter;
  }

  fetch(url) {
    if (this.adapter.name === "ajaxAdapter") {
      return makeAjaxCall(url).then(response => {
        // transform response and return
      });
    } else if (this.adapter.name === "nodeAdapter") {
      return makeHttpCall(url).then(response => {
        // transform response and return
      });
    }
  }
}

function makeAjaxCall(url) {
  // request and return promise
}

function makeHttpCall(url) {
  // request and return promise
}

// good
class AjaxAdapter extends Adapter {
  constructor() {
    super();
    this.name = "ajaxAdapter";
  }

  request(url) {
    // request and return promise
  }
}

class NodeAdapter extends Adapter {
  constructor() {
    super();
    this.name = "nodeAdapter";
  }

  request(url) {
    // request and return promise
  }
}

class HttpRequester {
  constructor(adapter) {
    this.adapter = adapter;
  }

  fetch(url) {
    return this.adapter.request(url).then(response => {
      // transform response and return
    });
  }
}

Liskov Substitution Principle 裏式替換原則

父類和子類應該能夠被交換使用而不會出錯。

// bad
class Rectangle {
  constructor() {
    this.width = 0;
    this.height = 0;
  }

  setColor(color) {
    // ...
  }

  render(area) {
    // ...
  }

  setWidth(width) {
    this.width = width;
  }

  setHeight(height) {
    this.height = height;
  }

  getArea() {
    return this.width * this.height;
  }
}

class Square extends Rectangle {
  setWidth(width) {
    this.width = width;
    this.height = width;
  }

  setHeight(height) {
    this.width = height;
    this.height = height;
  }
}

function renderLargeRectangles(rectangles) {
  rectangles.forEach(rectangle => {
    rectangle.setWidth(4);
    rectangle.setHeight(5);
    const area = rectangle.getArea(); // BAD: Returns 25 for Square. Should be 20.
    rectangle.render(area);
  });
}

const rectangles = [new Rectangle(), new Rectangle(), new Square()];
renderLargeRectangles(rectangles);

上面的Rectangle不能直接替換Square，由於會致使計算面積錯誤，考慮將計算面積的方法抽象出來：

class Shape {
  setColor(color) {
    // ...
  }

  render(area) {
    // ...
  }
}

class Rectangle extends Shape {
  constructor(width, height) {
    super();
    this.width = width;
    this.height = height;
  }

  getArea() {
    return this.width * this.height;
  }
}

class Square extends Shape {
  constructor(length) {
    super();
    this.length = length;
  }

  getArea() {
    return this.length * this.length;
  }
}

function renderLargeShapes(shapes) {
  shapes.forEach(shape => {
    const area = shape.getArea();
    shape.render(area);
  });
}

const shapes = [new Rectangle(4, 5), new Rectangle(4, 5), new Square(5)];
renderLargeShapes(shapes);

Interface Segregation Principle 接口隔離原則

Clients should not be forced to depend upon interfaces that they do not use。舉例來講，一個功能模塊須要設計必須傳的參數和可選參數，不該該強迫用戶使用可選參數。

Dependency Inversion Principle 依賴注入原則

原文：

1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
2. Abstractions should not depend upon details. Details should depend on abstractions.

// bad
class InventoryRequester {
  constructor() {
    this.REQ_METHODS = ["HTTP"];
  }

  requestItem(item) {
    // ...
  }
}

class InventoryTracker {
  constructor(items) {
    this.items = items;

    // BAD: We have created a dependency on a specific request implementation.
    // We should just have requestItems depend on a request method: `request`
    this.requester = new InventoryRequester();
  }

  requestItems() {
    this.items.forEach(item => {
      this.requester.requestItem(item);
    });
  }
}

const inventoryTracker = new InventoryTracker(["apples", "bananas"]);
inventoryTracker.requestItems();

上面例子在於，InventoryTracker內部實例化了InventoryRequester，也就意味着high-level的模塊須要知道low-level模塊的細節（好比實例化InventoryRequester須要知道它的構造參數等，或者說須要import該模塊，形成耦合）。

// good
class InventoryTracker {
  constructor(items, requester) {
    this.items = items;
    this.requester = requester;
  }

  requestItems() {
    this.items.forEach(item => {
      this.requester.requestItem(item);
    });
  }
}

class InventoryRequesterV1 {
  constructor() {
    this.REQ_METHODS = ["HTTP"];
  }

  requestItem(item) {
    // ...
  }
}

class InventoryRequesterV2 {
  constructor() {
    this.REQ_METHODS = ["WS"];
  }

  requestItem(item) {
    // ...
  }
}

// By constructing our dependencies externally and injecting them, we can easily
// substitute our request module for a fancy new one that uses WebSockets.
const inventoryTracker = new InventoryTracker(
  ["apples", "bananas"],
  new InventoryRequesterV2()
);
inventoryTracker.requestItems();

直接傳入low-level的實例而不須要考慮它是如何被實例化的，high-level只須要依賴抽象的接口就能夠完成對子模塊的調用。

9. 註釋

Comments are an apology, not a requirement. Good code mostly documents itself. 好的代碼是自解釋的。

參考：clean-code-javascript