Javascript 闭包与 Javascript OO

Question

我正在阅读 Closures 并且它有以下闭包的示例。

var makeCounter = function() {
var privateCounter = 0;
  function changeBy(val) 
     {
       privateCounter += val;
     }
  return 
     {
       increment: function() 
     {
       changeBy(1);
     },
  decrement: function() 
     {
       changeBy(-1);
     },
  value: function() 
     {
       return privateCounter;
     }
   }
};

var counter1 = makeCounter();
var counter2 = makeCounter();
alert(counter1.value()); /* Alerts 0 */
counter1.increment();
counter1.increment();
alert(counter1.value()); /* Alerts 2 */
counter1.decrement();
alert(counter1.value()); /* Alerts 1 */
alert(counter2.value()); /* Alerts 0 */

我想知道这个闭包和下面产生相同结果的代码之间有什么区别 advantages/disadvantages。

var makeCounter = function() {
var privateCounter = 0;
  function changeBy(val) 
      {
        privateCounter += val;
      };
   this.increment= function() 
     {
       changeBy(1);
     };
   this.decrement= function()
     {
       changeBy(-1);
     };
   this.value= function() 
     {
       return privateCounter;
     };
  };

var counter1 = new makeCounter();
var counter2 = new makeCounter();
alert(counter1.value()); /* Alerts 0 */
counter1.increment();
counter1.increment();
alert(counter1.value()); /* Alerts 2 */
counter1.decrement();
alert(counter1.value()); /* Alerts 1 */
alert(counter2.value()); /* Alerts 0 */

Answer 1

你的第二个版本也使用了闭包，在这方面几乎没有区别。

重要的区别是你的第一个 makeCounter 是一个工厂，return 是一个对象文字（继承自 Object.prototype），而你的第二个 makeCounter 是一个构造函数（需要用 new 调用），它执行 return 一个继承自 makeCounter.prototype 的实例。您可以 gain some advantages 通过在原型上移动您的方法：

function Counter() {
    var privateCounter = 0;
    this.changeBy = function changeBy(val) {
        privateCounter += val;
    };
    this.value = function() {
        return privateCounter;
    };
}
Counter.prototype.increment = function() {
    this.changeBy(1);
};
Counter.prototype.decrement = function() {
    this.changeBy(-1);
};

您可以避免闭包（特权方法）并仅使用原型方法：

function Counter() {
    this.counter = 0; // no more private
}
Counter.prototype.valueOf = function() {
    return this.ounter;
};
Counter.prototype.increment = function() {
    this.counter += 1;
};
Counter.prototype.decrement = function() {
    this.counter -= 1;
};

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