Uncaught TypeError: Cannot read property 'getElementsByTagName' of null
    at script.js:7 

index.html

<div class="banner" id="banner">
   <a href="">
    <div class="banner-slide slide1 slide-active"></div>
   </a>
   <a href="">
    <div class="banner-slide slide2"></div>
   </a>
   <a href="">
    <div class="banner-slide slide3"></div>
   </a>
  </div>

scrpt.js

function byId(id){
   return typeof(id) === "string"?document.getElementById(id):id;
}

var index = 0,
    timer = null,
    pics = byId("banner").getElementsByTagName("div"),
    len = pics.length;
function slideImg(){
 var main  = byId("main");

 main.onmousemove = function(){

       
 }
    main.onmousemove = function(){
        timer = setInterval(function(){
         index++;
         console.log(index);
        },1000);
    }
}