js的变量

 var i = 0;
        function addFileItems(){
                 i++;
              var div = document.getElementById("d1");
        var fileItem = document.createElement("input");
        var br = document.createElement("br");
        var removeButton = document.createElement("input");
        fileItem.type = "file";
        removeButton.type = "button";
        removeButton.value = "remove" + i;
        removeButton.onclick = function(){
                  div.removeChild(fileItem);      //div.removeChild(this.previousSibling);        
             div.removeChild(br);            //div.removeChild(this.nextSibling);
             div.removeChild(removeButton);  //div.removeChild(this);
                     //请问这儿的fileItem，br，removeButton不是在不断变化吗？为什么动态添加完成后，
                     //单击remove按钮删除，还能用这些变量？没有后面那种写法好理解，不明白，请问这是怎么理解？
        };
        div.appendChild(fileItem);
        div.appendChild(removeButton);
        div.appendChild(br);
    }

<div id="d1">
    <input type="button" value="add.." id="b1" onclick="addFileItems()"/><br/>
</div>