Encapsulate Variable

練習用的 REPL。

let defaultOwner = {firstName: "Martin", lastName: "Fowler"};
let spaceshipOwner = defaultOwner;  // 只要有人這樣使用
defaultOwner.firstName = "Rebecca"; // 就會意外改到變數內容
defaultOwner.lastName = "Parsons";

把直接存取的邏輯封裝起來。

let defaultOwner = {firstName: "Martin", lastName: "Fowler"};

function getDefaultOwner()    {return defaultOwner;}
function setDefaultOwner(arg) {defaultOwner = arg;}

// 呼叫端就可以這樣用
spaceshipOwner = getDefaultOwner();
setDefaultOwner({firstName: "Rebecca", lastName: "Parsons"});

除了單純封裝之外，還可以做的更多。

// 把程式挪到另外獨立檔案，然後後只 export 存取方法出來
let defaultOwner = {firstName: "Martin", lastName: "Fowler"};
export function getDefaultOwner()    {return defaultOwner;}
export function setDefaultOwner(arg) {defaultOwner = arg;}

// 改個名字
let defaultOwnerData = {firstName: "Martin", lastName: "Fowler"};
export function defaultOwner()       {return defaultOwnerData;}
export function setDefaultOwner(arg) {defaultOwnerData = arg;}

// 回傳新的物件
let defaultOwnerData = {firstName: "Martin", lastName: "Fowler"};
export function defaultOwner()       {return Object.assign({}, defaultOwnerData);}
export function setDefaultOwner(arg) {defaultOwnerData = arg;}

// 沒有 setter，任何嘗試修改內容會出錯
let defaultOwnerData = {firstName: "Martin", lastName: "Fowler"};
export function defaultOwner()       {return new Person(defaultOwnerData);}
export function setDefaultOwner(arg) {defaultOwnerData = arg;}

class Person {
  constructor(data) {
    this._lastName = data.lastName;
    this._firstName = data.firstName
  }
  get lastName() {return this._lastName;}
  get firstName() {return this._firstName;}
  // and so on for other properties
}