为什么数组的“元素索引”方法不起作用？

Question

我正在学习循环，我使用loopCounter变量创建了循环程序。 喜欢这个

for (loopCounter = 0; loopCounter >= 0; loopCounter++)

终于有效了！但我走得更远，想要将loopCounter更改为elementIndex方式。它不起作用。

你能告诉我为什么吗？代码会是什么？谢谢！

<script type="text/javascript">
var userAge = new Array ();
var elementIndex;

for (elementIndex in userAge) {
userAge[elementIndex] = prompt("Enter your age please! ","0" );

   if (isNaN(userAge[elementIndex]))  { 
   alert ("Enter ONLY Number value please! "); 
   }  else   {   
      if (userAge[elementIndex] == 0) {
          alert ("You are a baby");
          } else if ((userAge[elementIndex] <=0) || (userAge[elementIndex] >=130))
            {
             alert ("I think you are lying about your age!");  }
   else {
        alert ("That\'s a good age!");
        }
   }
}
</script>

有效的代码

<script type="text/javascript">
var userAge = new Array ();
var loopCounter = 0;

while (loopCounter >= 0) {
userAge[loopCounter] = prompt("Enter your age please! "+ loopCounter,"0" );
   if (isNaN(userAge[loopCounter]))  { 
   alert ("Enter ONLY Number value please! "); 
   }  else   {   
      if (userAge[loopCounter] == 0) {
          alert ("You are a baby");
          } else if ((userAge[loopCounter] <=0) || (userAge[loopCounter] >=130))
            {
             alert ("I think you are lying about your age!");  }
   else {
        alert ("That\'s a good age!");
        }
   }
loopCounter++;}
</script>