网络 表达式;定时器;毫秒
var id = setInterval(fn, delay); - Similar to setTimeout but continually calls the function (with a delay every time) until it is canceled.
var id = setInterval(fn delay); - 与setTimeout类似,只不过它会持续地调用指定的函数(每次都有一个延时),直到timer被取消为止。