單例模式

singleton1.js

// 儲存在靜態屬性中的實體
function Universe() {
  
  // do we have an existing instance?
  if (typeof Universe.instance === 'object') {
    return Universe.instance;
  }
  
  // proceed as normal
  this.start_time = 0;
  this.bang = "Big";
  
  // cache
  Universe.instance = this;
  
  // implicit return:
  // return this;
}

// testing
var uni = new Universe();
var uni2 = new Universe();
uni === uni2; // true

// 缺點是instance是public

singleton2.js

/*** Instance in a Closure ***/
function Universe() {
  
  // the cached instance
  var instance = this;
  
  // proceed as normal
  this.start_time = 0;
  this.bang = "Big";
  
  // rewrite the contructor
  Universe = function() {
    return instance;
  };
}

// testing
var uni = new Universe();
var uni2 = new Universe();
uni === uni2; // true


// 缺點是重新定義後的函式會失去初次定義到重新定義之間加在它身上的屬性
// adding to the prototype
Universe.prototype.nothing = true;

var uni = new Universe();

// again adding to the prototype
// after the initial object is created
Universe.prototype.everything = true;

var uni2 = new Universe();

// only the original prototype was
// linked to the objects
uni.nothing; // true
uni2.nothing; // true
uni.everything; // undefined
uni2.everything; // undefined

// that sounds right:
uni.constructor.name; // "Universe"

// this looks weird
uni.constructor === Universe; // false

singleton3.js

// 這個實作讓原型和建構式指標正常運作
function Universe() {
  // the cached instance
  var instance;
  
  // rewrite the constructor
  Universe = function Universe() {
    return instance;
  };
  
  // carry over the prototype properties
  Universe.prototype = this;
  
  // the instance
  instance = new Universe();
  
  // reset the constructor pointer
  instance.constructor = Universe;
  
  // all the functionality
  instance.start_time = 0;
  instance.bang = "Big";
  
  return instance;
}

// testing
var uni = new Universe();
var uni2 = new Universe();
uni === uni2; // true

// adding to the prototype
Universe.prototype.nothing = true;
var uni = new Universe();

// again adding to the prototype
// after the initial object is created
Universe.prototype.everything = true;
var uni2 = new Universe();

// only the original prototype was
// linked to the objects
uni.nothing; // true
uni2.nothing; // true
uni.everything; // true
uni2.everything; // true

// that sounds right:
uni.constructor.name; // "Universe"
uni.constructor === Universe; // true

singleton4.js

// 另一種解法
var Universe;
(function() {
  var instance;
  
  Universe = function Universe() {
    if (instance) {
      return instance;
    }
    
    instance = this;
    
    // all the functionality
    this.start_time = 0;
    this.bang = "Big";
  };
}());