3-4中的js定时任务为什么会越来越快？

function byId(id){

return typeof(id) === "string"?document.getElementById(id):id;

}

var index=0,

    timer=null,

    pics=byId("banner").getElementsByTagName("div");

    len=pics.length;

   console.log(index);

function slideImg(){

// console.log(index);

var main =byId("main");

main.onmouseover=function(){

}

main.onmouseout=function(){

// console.log(index);

timer=setInterval( function(){

index++;

if(index>=len){

index=0;

}

changeImg();

console.log(index);

},3000);

}

main.onmouseout();

}

slideImg();

function changeImg(){

for(var i=0;i<len;i++){

pics[i].style.display="none";

}

pics[index].style.display='block';

}