var arr=[
{title:"first",submenu:
[{title:"ek"},{title:"do"},{title:"teen"}]},
{title:"second"},
{title:"third",submenu:
[{title:"do"},{title:"pal"}]}
];
var ul=document.createElement('ul');
document.body.appendChild(ul);
for(var i=0;i<arr.length;i++)
  {var li=document.createElement('li');
  li.innerHTML=arr[i].title;
  ul.appendChild(li);
    
    if (arr[i].submenu)
      {
        var ul2=document.createElement('ul');
        ul2.style.display="none";    
           
        
        li.appendChild(ul2);
        for(var j=0;j<arr[i].submenu.length;j++)
      
      {
         var li2=document.createElement('li');
       li2.innerHTML=arr[i].submenu[j].title;
         ul2.appendChild(li2);
        }
        li.onmouseover=function(){
        ul2.style.display="block";}
      }
  }