为JavaScript对象编写枚举器

Question

我想了解如何编写JavaScript对象的枚举器。我遇到了一段Sharepoint code where items of a SP List were being obtained on the client side，在Sharepoint对象上正在执行类似于下面的代码。

var enumerator = list.getEnumerator(); 
while(enumerator.moveNext()) { 
    var currentItem = enumerator.getCurrent(); 
    // actions on currentItem 
} 

如果我可以写，可以在使用上述枚举迭代的自定义对象，如何将它看？这是我的尝试，但它不工作：

var list = { 
    items: ["A", "B", "C", "D", "E"], 
    counter: -1, 
    enumerator: { 
     moveNext: function() { 
      counter++; // outer object's variable not accessible 
      if(counter >= items.length) 
       return false; 
      else 
       return true; 
     }, 
     getCurrent: function() { 
      return items[counter]; // outer object's variable not accessible 
     } 
    }, 
    getEnumerator: function() { 
     return this.enumerator(); 
    } 
} 

var enumerator = list.getEnumerator(); 
while(enumerator.moveNext()) { 
    var currentItem = enumerator.getCurrent(); 
    alert(currentItem); 
} 

你能帮助我了解getEnumerator()如何能写成函数原型是适用于任何枚举对象？

Answer 1

承蒙@xdazz和@Juan门德斯三分球，我能够制定出枚举函数原型：

Array.prototype扩展：

Array.prototype.getEnumerator = Array.prototype.getEnumerator || 
function() { 
    var items = this; 
    var counter = -1; 
    var enumerator = { 
     moveNext: function() { 
      counter++; 
      return (counter < items.length); 
     }, 
     getCurrent: function() { 
      return items[counter]; 
     } 
    } 
    return enumerator; 
}; 

用法：

var list = ["A", "B", "C", "D", "E"]; 

var enumerator = list.getEnumerator(); // makes any array enumerable 
while(enumerator.moveNext()) { 
    var currentItem = enumerator.getCurrent(); 
    alert(currentItem); 
} 

JSFIDDLE DEMO

Answer 2

可以使用list.

var list = { 
    items: ["A", "B", "C", "D", "E"], 
    counter: -1, 
    enumerator: { 
     moveNext: function() { 
      list.counter++; // outer object's variable not accessible 
      if(list.counter >= list.items.length) 
       return false; 
      else 
       return true; 
     }, 
     getCurrent: function() { 
      return list.items[list.counter]; // outer object's variable not accessible 
     } 
    }, 
    getEnumerator: function() { 
     return list.enumerator; 
    } 
} 

访问的变量或者你可以做一个函数来创建它的实例

function makeList(items) { 
    var list = { 
     items: items 
     counter: -1, 
     enumerator: { 
      moveNext: function() { 
       list.counter++; // outer object's variable not accessible 
       if(list.counter >= list.items.length) 
        return false; 
       else 
        return true; 
      }, 
      getCurrent: function() { 
       return list.items[list.counter]; 
      } 
    }, 
    getEnumerator: function() { 
     return list.enumerator; 
    } 
    return list; 
} 


var enumerator = makeList(["A", "B", "C", "D", "E"]).getEnumerator(); 

Answer 3

尝试以下：

var list = function() { 
    // these local variables could be accessed by inner functions. 
    var items = ["A", "B", "C", "D", "E"], 
     counter = -1; 
    return { 
     enumerator: { 
      moveNext: function() { 
       counter++; 
       return counter < items.length; 
      }, 
      getCurrent: function() { 
       return items[counter]; 
      } 
     }, 
     getEnumerator: function() { 
      // here should not be return this.enumerator() but just this.enumerator 
      return this.enumerator; 
     } 
    }; 
}; 

var enumerator = list().getEnumerator(); 
while(enumerator.moveNext()) { 
    var currentItem = enumerator.getCurrent(); 
    alert(currentItem); 
} 

THE JSFIDDLE DEMO.