Convert nested array into a flat one

flatten.js

//ES6
var flatten = {

 makeFlat : function(arr) {
    var res = [];
    for (var i = 0; i < arr.length; i++) {
        if (arr[i] instanceof Array) { //if the current position is an array then we keep looking for a value.
            res.push.apply(res, this.makeFlat(arr[i])); //recursively calls makeFlat and concatenates with res.
        } else {
            res.push(arr[i]); //when is not an array, directly concatenate the value
        }
    }
    return res;
 },
 
 test : function(){
  console.log(this.makeFlat([1,2,3,4,[21,22,32,[55,66,[88]]]])))  
 }
};


module.exports = flatten;