text: function flatten(arr) {
      // 削藩
      return arr.reduce(function(prev, next) {
          console.log(prev, next);
           return prev.concat(Array.isArray(next)? flatten(next): next);
          //  return prev.concat(next);        
      }, []);
  }
  
  console.log(flatten([1,2,[3,4],[5,[6,7]]]));