function union(arra1, arra2) {  
    
  if ((arra1 == null) || (arra2==null))   
    return void 0;  
    
  var obj = {};  
   
  for (var i = arra1.length-1; i >= 0; -- i)  
     obj[arra1[i]] = arra1[i];  
   
  for (var i = arra2.length-1; i >= 0; -- i)  
     obj[arra2[i]] = arra2[i];  
   
  var res = [];  
   
  for (var n in obj)  
  {  
    
    if (obj.hasOwnProperty(n))   
      res.push(obj[n]);  
  }  
   
  return res;  
}  
console.log(union([1, 2, 3], [100, 2, 1, 10]));