jsfunctions.js

function flatten(arr) {
  if (arr.length == 0) {
    return [];
  }

  const first = arr[0];
  const flatRest = flatten(arr.slice(1));

  if (!Array.isArray(first)) {
    return [first].concat(flatRest);
  }

  return flatten(first).concat(flatRest);
}

const tests = [
  [],
  [1],
  [1, 2],
  [1, 2, [3]],
  [1, 2, [3], [4, [5, [6]]]],
  [[[1], 2, 3], 4, 5, [[[6, 7], 8]]],
];

tests.forEach(arr => {
  console.log(arr, " --> ", flatten(arr));
});

function sReverse(s) {
  if (s.length < 2) {
    return s;
  }
  return sReverse(s.substring(1)) + s[0];
}

["", "1", "12", "123"].forEach(s => {
  console.log(s, " --> ", sReverse(s));
});

flatten a list (recursively)
reverse a string (recursively)

in javascript