修改全局變量基於對象屬性

Question

我努力學習面向對象的JavaScript跑進以下問題：

我有一個對象的構造函數（是正確的術語？）這樣的：

function itemCreator(itemName, itemType, itemPositionX, itemPositionY) { 
      this.itemName = itemName; 
      this.itemType = itemType; 
      this.itemPositionX = itemPositionX; 
      this.itemPositionY = itemPositionY; 
      allItems.push(this); //store all items in a global variable 
      }//end itemCreator; 


//then I use it to create an object 

megaRocket = new itemCreator (
      'megarocket', 
      'item_megarocket', 
      108, 
      475 
      ) 

現在我意識到我也需要映射這些對象來根據對象所具有的「itemType」來修改不同的全局變量。這是我卡住的地方。我怎樣才能使一個全局變量，只有具有特定itemType屬性的對象可以修改？

例如，我想創建一個對象，該對象增加一個名爲amountOfMegarockets的變量，但前提是該對象的itemType是「item_megarocket」。

我後來循環這些項目的數組，看看玩家物體接觸他們計劃（收集項）：

function checkForItems(){ 
       var itemLen =allItems.length; 
       for (i=0; i < itemLen; i++){ 
       var itemObject = allItems[i]; 

       if (//checking for "collisions" here 
       (ship.x < (itemObject.itemBitmap.x + itemObject.size) && (ship.x + shipWidth) > itemObject.itemBitmap.x) && 
       (ship.y < (itemObject.itemBitmap.y + itemObject.size) && (ship.y + shipWidth) > itemObject.itemBitmap.y) 
       ){ 
       itemObject.actor.y = -500; //just removing the item from canvas here (temporary solution) 
       // Here comes pseudo code for the part that I'm stuck with 
       variableBasedOnItemObject.itemType++; 

       } 

我希望我的解釋是有道理的人！

編輯：

BERGI的回答最有意義給我，但我不能讓語法正確。以下是我想要使用BERGI代碼：

var amounts = {}, 
allItems = []; 
function itemCreator(itemName, itemType, itemPositionX, itemPositionY) { 
this.itemName = itemName; 
this.itemType = itemType; 
this.itemPositionX = itemPositionX; 
this.itemPositionY = itemPositionY; 

(amounts[itemType]=2); // this is different from bergi's example because I need to set the initial value of the item to two 
//I also shouldn't increase the item amount on creation of the item, but only when it's specifically called from another function 
this.increaseCount = amounts[itemType]++; //this should IMO increase the itemType amount inside the amounts object when called, but it doesn't seem to work 
} 

//creating the object the way bergi suggested: 
allItems.push(new itemCreator('shootUp001', 'item_up', 108, 475)); 

現在，這裏的問題的一部分：

function checkForItems(){ 
       var itemLen =allItems.length; 
       for (i=0; i < itemLen; i++){ 
       var itemObject = allItems[i]; 

       if (my condition here) 
       ){ 

//code below is not increasing the value for the current itemType in the amounts object. 
//Probably a simple syntax mistake? 
       itemObject.itemType.increaseCount; 

       } 

      } 

      } 

爲什麼我itemObject.itemType.increaseCount的呼叫;不增加amount.itemType的值？

Answer 1

遞增一個名爲amountOfMegarockets全局變量，但前提是ITEMTYPE爲該對象是「item_megarocket」。

不要爲每個項目類型使用全局變量。請使用一個對象（在全局或本地作用域中）來計算其屬性上每種類型的數量。

var amounts = {}, 
    allItems = []; 
function Item(itemName, itemType, itemPositionX, itemPositionY) { 
    this.itemName = itemName; 
    this.itemType = itemType; 
    this.itemPositionX = itemPositionX; 
    this.itemPositionY = itemPositionY; 

    amounts[itemType]++ || (amounts[itemType]=1); // count by type 
    allItems.push(this); // store all items 
} 

請注意，我不會把所有Item情況下，在默認情況下的陣列，更好地省略線，讓它做來電：

allItems.push(new Item('megarocket', 'item_megarocket', 108, 475)); 

Answer 2

你可以做一些事情像

(function (global) { 
    // create a scope 
    var allItems = []; 
    global.itemCreator = function(itemName, itemType, itemPositionX, itemPositionY) { 
     this.itemName = itemName; 
     this.itemType = itemType; 
     this.itemPositionX = itemPositionX; 
     this.itemPositionY = itemPositionY; 
     allItems.push(this); //store all items in a global variable 
     }//end itemCreator; 
})(this); 

這會工作。但嚴重不要讓你的代碼變得太複雜，使私有變種。如果有人想用調試器作弊，他總會找到一種方法來做到這一點。

----編輯 如果您想探微訪問全局變量的基礎上ITEMTYPE你可以做一些喜歡的事情：

function checkForItems(){ 
    var itemLen =allItems.length; 
    for (i=0; i < itemLen; i++){ 
     var itemObject = allItems[i]; 

     if (//checking for "collisions" here 
      (ship.x < (itemObject.itemBitmap.x + itemObject.size) && (ship.x + shipWidth) > itemObject.itemBitmap.x) && 
      (ship.y < (itemObject.itemBitmap.y + itemObject.size) && (ship.y + shipWidth) > itemObject.itemBitmap.y) 
     ){ 
      itemObject.actor.y = -500; //just removing the item from canvas here (temporary solution) 
      // Here comes pseudo code for the part that I'm stuck with 
      if (window[itemObject.itemType + "Data"])) { 
       variableBasedOnItemObject.itemType++; 
      } else { 
       window[itemObject.itemType + "Data"] = { 
        itemType: 1 
       } 
      } 
     } 
    } 
}