for循环迭代小改进

常见写法：

// original loops
for (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);

小改进：

//minimizing property lookups and reversing
for (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--);