var flatten = function(a, shallow,r){  
  if(!r){ r = []}  
     
if (shallow) {  
  return r.concat.apply(r,a);  
  }  
        
   for(var i=0; i<a.length; i++){  
        if(a[i].constructor == Array){  
            flatten(a[i],shallow,r);  
        }else{  
            r.push(a[i]);  
        }  
    }  
    return r;  
}  
  
console.log(flatten([1, [2], [3, [[4]]],[5,6]]));  
  
console.log(flatten([1, [2], [3, [[4]]],[5,6]], true));