for循环迭代小改进
常见写法：
// original loopsfor (var i=0; i < items.length; i++){ process(items[i]);}var j=0;while (j < items.length){ process(items[j++]]);}var k=0;do { process(items[k++]);} while (k < items.length);//minimizing property lookups and reversingfor (var i=items.length; i--; ){ process(items[i]);}var j = items.length;while (j--){ process(items[j]]);}var k = items.length-1;do { process(items[k]);} while (k--);