JS Bin[controlled _.flatten function]// source https://jsbin.com/hinawagohe

flatterCtrl.js

var arr =[1, [2,[3, [4, [5, [6]]]]]];

function flattenCtrl(list, level){
  
  do {
    
   list=_.flatten(list, true); 
   level--;
    
   } while(level>0); 
  
  
  return list;
  
}



console.log(flattenCtrl(arr, 10));

make use of underscore flatten function to control the flatten level.