//Recursion to problem-solving. The ability to divide and coquer problems
//until they're small enough to be managed directly.

const flatten = ([first, ...rest]) => {
  if (first === undefined) {
    return [];
  }
  else if (!Array.isArray(first)) {
    return [first, ...flatten(rest)];
  }
  else {
    return [...flatten(first), ...flatten(rest)];
  }
};

console.log(flatten([[1,2,[3]],4]));
console.log(flatten([1, 3, [4, 5, [9, 3], [14, 13, [9, [2]]]]]));