this.index == oBtn.length - 1具体意思？

Question

var changeStyle = function (elem, attr, value)
{
elem.style[attr] = value
};
window.onload = function ()
{
var oBtn = document.getElementsByTagName("input");
var oDiv = document.getElementById("down");
var oAtt = ["width","height","background","display","display"];
var oVal = ["200px","200px","red","none","block"];

for (var i = 0; i < oBtn.length; i++)
{
    oBtn[i].index = i;
    oBtn[i].onclick = function ()
    {
        this.index == oBtn.length - 1 && (oDiv.style.cssText = "");
        changeStyle(oDiv, oAtt[this.index], oVal[this.index])
    }   
}

};

Answer 1

this.index == oBtn.length - 1 && (oDiv.style.cssText = "");
相当于
if (this.index != oBtn.length - 1)
oDiv.style.cssText = "";
利用了&&的短路原理（当前面表达式不满足，不对后面表达式求值）