Recursion

A recursive function solves a problem by solving a smaller version of it and combining. It needs a base case that is answered directly, or it recurses forever: factorial(0) is 1, factorial(n) is n * factorial(n - 1). Each call waits for the smaller one, so JavaScript limits the depth to some thousands of calls.

Recursion suits nested structures, like arrays that contain arrays: to flatten one, walk the items and flatten any item that is itself an array (Array.isArray tells you). The built-in arr.flat(Infinity) does this too; writing it once is the point.

function countDown(n) {
  if (n === 0) return [];
  return [n, ...countDown(n - 1)];
}
console.log(countDown(3));   // [3, 2, 1]

Your task

Write factorial(n), fib(n) returning the nth Fibonacci number with fib(0) = 0 and fib(1) = 1, and flatten(nested) turning arrays nested to any depth into one flat array, without using .flat().

function factorial(n) {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}

function fib(n) {
  if (n < 2) return n;
  return fib(n - 1) + fib(n - 2);
}

function flatten(nested) {
  const result = [];
  for (const item of nested) {
    if (Array.isArray(item)) result.push(...flatten(item));
    else result.push(item);
  }
  return result;
}