JS實現30s倒計時

代碼如下

var timestart = 30;
var timestep = -1;
var timeID;
function timecount() {
    document.getElementById("timeout").innerHTML = timestart;
    timestart += timestep;
    if(timestart < 0){
      timestart = 30;
      //倒計時為0時執行xxxx方法   
    }
    timeID=setTimeout("timecount()",1000);
}