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--);