flatten a nested array

flatten.js

// function definition ES6
let flatten = (arr, res=[]) => {
  for(let itm of arr){
    Array.isArray(itm)? flatten(itm, res): res.push(itm);
  }
}

// calling the function
let result = [];
flatten([1,2, [3, 4], [5, [6, 7, [8, 9]]]], result);
console.log(result);


// PS: I think a better solution will be to use lodash.flatten though :p