flatten.js

function flatten = (arr = [], result = [], depth) => {
   depth--;
   for (var i = 0, length = arr.length; i < length; i++) {
       var value = arr[i];
       if (depth > -1 && Array.isArray(value))
           flatten(value, result, depth);
       else
           result.push(value);
   }

   return result;
}