unfoldArray.js

let arr = [1,[6,7],3,[1,2,3,4, [2,3,4]]];

function unfoldArray(arr) {
  let arrNew = [];

  for (let i = 0; i < arr.length; i++) {
    if (arr[i].length === undefined) {
      arrNew.push(arr[i]);
    } else {
      arrNew = arrNew.concat(unfoldArray(arr[i]));
    }
  }
  return arrNew;
}

console.log(unfoldArray(arr));