Length Of Array

var grd=[2,4,6,8,10];
var sm=0;
var count =0;
if(grd.length>0)
  {
    for(index=0;index<grd.length;index++)
      if (grd[index]!=undefined){
         sm += grd[index];
    count =count+1;
      }
  }

document.write(sm/count);